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PROFESSOR: OK, let's
get started here.

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Go ahead and take 10 more
seconds on the clicker

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00:00:26,350 --> 00:00:29,100
question, which probably looks
all too familiar at this

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point, if you went to recitation
yesterday.

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All right, and let's
see how we do here.

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OK.

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So, let's talk about this
for one second.

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So what we're asking here, if we
can settle down and listen

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00:00:44,830 --> 00:00:47,440
up, is which equations can be
used if we're talking about

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converting wavelength to
energy for an electron.

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00:00:50,590 --> 00:00:53,260
Remember, the key word
here is electron.

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This might look familiar to the
first part of problem one

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on the exam, and problem one on
the exam is what tended to

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be the huge problem
on the exam.

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I think over 2/3 of you decided
on the exam to use

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00:01:06,310 --> 00:01:09,530
this first equation, e equals
h c over wavelength.

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So I just want to reiterate one
more time, why can we not

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00:01:13,250 --> 00:01:17,720
use this equation if we're
talking about an electron?

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00:01:17,720 --> 00:01:19,500
C. OK, good, good,
I'm hearing it.

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So the answer is c.

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What you need to do is you need
to ask yourself if you're

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trying to convert from
wavelength to energy for an

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electron, and you are tempted,
because we are all tempted to

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use this equation, and if you
were tempted, say, does an

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electron travel at the
speed of light?

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00:01:35,280 --> 00:01:38,150
And if the answer is no, an
electron does not travel at

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the speed of light, light
travels at the speed of light,

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then you want to stay away
from using this equation.

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00:01:43,290 --> 00:01:46,440
And I know how tempting it is to
do that, but we have other

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00:01:46,440 --> 00:01:49,320
equations we can use -- the
DeBroglie wavelength, and this

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is just a combination of energy
equals 1/2 m v squared,

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and the definition of momentum,
so we can combine

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those things to get it.

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You might be wondering why I'm
telling you this now, you've

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already -- if you've lost
points on that, lost the

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points on it, and what I'm
saying to you is if there are

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parts of exam 1 that you did not
do well on, you will have

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a chance to show us again that
you now understand that

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material on the final.

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One quarter of the final is
going to be exam 1 material,

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and what that means is when we
look at your grade at the end

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of the semester, and we take a
look at what you got on exam

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1, and you're right at that
borderline, and we say well,

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what happened, did they
understand more at the end of

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the semester, did the
concepts kind of

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solidify over the semester?

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And if they did and if you
showed us that they did, then

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you're going to get bumped up
into that next grade category.

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So keep that in mind as you're
reviewing the exam, sometimes

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if things don't go as well
as you want them to, the

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temptation is just to put that
exam away forever and ever.

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But the reality is that new
material builds on that

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material, and specifically exam
1 a, question 1 a that

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deals with converting
wavelength to

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energy for an electron.

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I really want you guys know this
and to understand it, so

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I can guarantee you that you
will see this on the final.

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Specifically, question
1, part a.

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You will see something
very, very similar

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to this on the final.

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If you are thinking about 1
thing to go back and study on

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exam 1, 1 a is a really
good choice for that.

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This is important to me,
so you're going to

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see it on the final.

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So if you have friends that
aren't here, you might want to

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mention it to them, or maybe
not, maybe this is your reward

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for coming to class, which
is fine with me as well.

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All right.

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So I want to talk a little
bit about exam 1.

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I know most you picked up your
examine in recitation.

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If you didn't, any extra exams
can be picked up in the

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Chemistry Education office,
that's room 2204.

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So, the class average for the
exam was a 68%, which is

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actually a strong, solid average
for an exam 1 grade in

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the fall semester of 511-1.

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What we typically see is
something right in this range,

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00:04:01,280 --> 00:04:05,060
either ranging from the 50's
for an exam 1 average to

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occasionally getting into the
70's, but most commonly what

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we've seen for exam 1 averages
is 60, 61 -- those low 60's.

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So in many ways, seeing this 68
here, this is a great sign

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that we are off to a good
start for this semester.

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And I do want to address,
because I know many of you,

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this is only your second exam
at MIT, and perhaps you've

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never gotten an exam back that
didn't start with a 90 or

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start with an 80 in terms
of the grades.

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00:04:33,570 --> 00:04:36,910
So one thing you need to keep in
mind is don't just look at

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the number grade.

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The reason that we give you
these letters grade categories

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is that you can understand what
it actually means, what

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your exam score actually says
in terms of how we perceive

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00:04:47,150 --> 00:04:49,360
you as understanding
the material.

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00:04:49,360 --> 00:04:53,030
So, for example, and this is the
same categories that were

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shared in recitation, so I
apologize for repeating, but I

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00:04:56,900 --> 00:04:59,770
know sometimes when you get an
exam back, no more information

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00:04:59,770 --> 00:05:02,180
comes into your head except
obsessing over the exam, so

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00:05:02,180 --> 00:05:05,130
I'm just going to say it one
more time, and that is between

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00:05:05,130 --> 00:05:08,540
88 and 100, so that's 20% of
you got an A. This is just

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00:05:08,540 --> 00:05:11,810
absolutely fantastic, you really
nailed this very hard

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00:05:11,810 --> 00:05:14,140
material and these hard
questions on the exam where

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you had to both use equations
and solve problems, but also

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understand the concept in order
to get yourself started

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on solving the problem.

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The same with the B, the B range
was between 69 and 87 --

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anywhere in between those
ranges, you've got a B, some

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00:05:30,280 --> 00:05:32,100
sort of B on the exam.

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00:05:32,100 --> 00:05:35,360
So again, if you're in the A or
the B category here, this

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00:05:35,360 --> 00:05:38,330
is really something to be proud
of, you really earned

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00:05:38,330 --> 00:05:39,130
these grades.

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You know these exams, our 511-1
exams, we're not giving

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you points here, there are no
give me, easy points, you

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earned every single one
of these points.

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So, A and B here, these are
refrigerator-worthy grades,

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hang those up in your dorm.

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This is something to
feel good about.

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All right.

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So, for those of you that got
between a 51 and a 68, this is

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somewhere in the C range.

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For some people, they feel
comfortable being in the C

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range, other people really do
not like being in this range.

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We understand that, there is
plenty of room up there with

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the A's and the B's.

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You are welcome to come up to
these higher ranges starting

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with the next exam.

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And what I want to tell you if
you are in the C range, and

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this is not a place that you
want to be in, anyone that's

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got below the class average,
so below a 68 -- or a 68 or

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00:06:26,910 --> 00:06:29,850
below, is eligible for free
tutoring, and I put the

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00:06:29,850 --> 00:06:32,250
website on the front
page of your notes.

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00:06:32,250 --> 00:06:34,700
This means you get a one-on-one
tutor paid for by

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00:06:34,700 --> 00:06:36,950
the Chemistry Department to
help you if it's concepts

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00:06:36,950 --> 00:06:40,315
you're not quite up on, if it's
exam strategy that you

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need to work on more.

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00:06:42,020 --> 00:06:44,520
Whatever it is that you need
to work on, we want to help

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you get there.

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00:06:45,350 --> 00:06:47,680
So, if you have a grade that
you're not happy with, that

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00:06:47,680 --> 00:06:52,040
you're feeling upset or
discouraged about, please, I'm

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00:06:52,040 --> 00:06:54,050
happy to talk to all of
you about your grades

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00:06:54,050 --> 00:06:54,940
individually.

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00:06:54,940 --> 00:06:58,070
You can come talk to me, bring
your exam, and we'll go over

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00:06:58,070 --> 00:06:59,810
what the strategy should
be in terms of you

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00:06:59,810 --> 00:07:01,070
succeeding on the next exam.

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00:07:01,070 --> 00:07:03,300
You can do the same thing with
all of your TAs are more than

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00:07:03,300 --> 00:07:05,370
happy to meet with each
and every one of you.

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00:07:05,370 --> 00:07:07,640
And then in addition to that, we
can set you up with a tutor

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00:07:07,640 --> 00:07:11,010
if you are in the C range
or below, in terms

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of this first exam.

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All right.

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00:07:13,570 --> 00:07:16,720
So 44 to 50, this is going
to be in the D range.

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And then anything below
a 44 is going to be

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failing on this exam.

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00:07:20,880 --> 00:07:22,860
And also keep in mind, for
those of you that are

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freshman, you need at least
a C to pass the class.

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00:07:25,470 --> 00:07:29,270
So, if you did get a D or an F
on the first exam, you are

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00:07:29,270 --> 00:07:32,980
going to need to really evaluate
why that happened and

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00:07:32,980 --> 00:07:35,100
make some changes, and
we're absolutely here

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00:07:35,100 --> 00:07:36,530
to help you do that.

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So the real key is identifying
where the problem is -- is it

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00:07:40,030 --> 00:07:42,240
with understanding the concepts,
are you in a study

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00:07:42,240 --> 00:07:44,190
group that's dragging you
along but you're not

167
00:07:44,190 --> 00:07:45,340
understanding?

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00:07:45,340 --> 00:07:47,300
Do you kind of panic when
you get in the exam?

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00:07:47,300 --> 00:07:49,270
There are all sorts of scenarios
we can talk about

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00:07:49,270 --> 00:07:51,270
and we want to talk about
them with you.

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Seriously, even if we had a huge
range in this exam from

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17 to 100, if you're sitting
there and you're the 17, and

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00:07:58,420 --> 00:08:01,505
actually there's more than 1 so
don't feel alone, if you're

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00:08:01,505 --> 00:08:05,210
a 17 or you're a 20, it's not
time to give up, it's not time

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00:08:05,210 --> 00:08:06,740
to drop the class and
say I'm no good at

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00:08:06,740 --> 00:08:08,070
chemistry, I can't do this.

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00:08:08,070 --> 00:08:11,840
You still can, this is your
first couple of exams,

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00:08:11,840 --> 00:08:14,400
certainly your first in this
class, potentially one of your

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00:08:14,400 --> 00:08:17,690
first at MIT, so there's
tons of room to improve

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00:08:17,690 --> 00:08:18,450
from here on out.

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This is only 100 points
out of 750.

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So, the same thing goes if you
did really well, you still

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00:08:23,610 --> 00:08:26,180
have 650 other points that
you need to deal with.

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00:08:26,180 --> 00:08:28,930
So, make sure you don't just
rest on your high score from

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this first exam.

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So, OK, so that's pretty much
what I wanted to say about the

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exam, and in terms of there's
tons of resources if things

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didn't work out quite
as you wanted.

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If you feel upset in any way,
please come and talk to me.

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We want you to love chemistry
and feel good about your

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ability to do it.

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Nobody get into MIT by mistake,
so you all deserve to

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be sitting here, and you all
can pass this class and do

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00:08:50,650 --> 00:08:53,650
well in it, so we can help you
get there no matter what.

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You all absolutely
can do this.

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And then one more time, to
reiterate, in case anyone

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missed it, 1 a, make sure you
understand that, I feel like

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00:09:01,990 --> 00:09:02,700
that's important.

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And actually all of 1 --

200
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I really feel like the
photoelectric effect is

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00:09:06,200 --> 00:09:08,710
important for understanding all
of these energy concepts.

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So, as you go on in this class,
make sure you don't go

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00:09:12,380 --> 00:09:14,030
on before you go back
and make sure you

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understand that problem.

205
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All right, so let's move on to
material for exam 2 now, and

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00:09:22,260 --> 00:09:25,550
we're already three lectures
into exam 2 material.

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And I do want to say that in
terms of 511-1, what tends to

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00:09:29,370 --> 00:09:33,140
happen is the exam scores go up
and up and up, in terms of

209
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as we go from exam 1, to
exam 2, to exam 3.

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One of these reasons is we are
building on material, the

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00:09:38,470 --> 00:09:40,510
other reason is you'll be
shocked at how much better you

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00:09:40,510 --> 00:09:43,830
are at taking an exam just
a few weeks from now.

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00:09:43,830 --> 00:09:47,660
So this will be on, starting
with the Lewis structures, so

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00:09:47,660 --> 00:09:50,300
go back in your notes -- if this
doesn't sound familiar,

215
00:09:50,300 --> 00:09:54,030
if you spent too much time -- or
not too much time, spent a

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00:09:54,030 --> 00:09:56,520
lot of time studying exam 1
and didn't move on here.

217
00:09:56,520 --> 00:09:58,400
Today we're going to talk about
the breakdown of the

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octet rule.

219
00:09:59,550 --> 00:10:02,310
Cases where we don't have eight
electrons around our

220
00:10:02,310 --> 00:10:04,600
Lewis structures, then
we'll move on to

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00:10:04,600 --> 00:10:06,260
talking about ionic bonds.

222
00:10:06,260 --> 00:10:08,590
We had already talked about
covalent bonds, and then we

223
00:10:08,590 --> 00:10:11,060
talked about Lewis structures,
which describe the electron

224
00:10:11,060 --> 00:10:13,060
configuration in
covalent bonds.

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So now let's think about the
other extreme of ionic bonds,

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00:10:16,160 --> 00:10:19,390
and then we'll talk about polar
covalent bonds to end,

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if we get there or will start
with that in class on Monday.

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Also, public service
announcement for all of you,

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00:10:26,330 --> 00:10:28,620
voter registration in
Massachusetts, which is where

230
00:10:28,620 --> 00:10:32,620
we are, is on Monday, the
deadline if you want to

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00:10:32,620 --> 00:10:33,800
register to vote.

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There's some websites up there
that can guide you through

233
00:10:36,750 --> 00:10:38,850
registering and also can guide
you through, if you need an

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00:10:38,850 --> 00:10:41,480
absentee ballot for
your home state.

235
00:10:41,480 --> 00:10:44,500
And I actually saw, and I saw
a 5.111 student manning,

236
00:10:44,500 --> 00:10:47,230
there's some booths around MIT
that will register you or get

237
00:10:47,230 --> 00:10:48,790
you an absentee ballot.

238
00:10:48,790 --> 00:10:52,120
So, the deadline's coming soon,
so patriotic duty, I

239
00:10:52,120 --> 00:10:54,580
need to remind you of that as
your chemistry teacher --

240
00:10:54,580 --> 00:10:56,940
chemistry issues are important
in politics as well.

241
00:10:56,940 --> 00:11:00,540
So make sure you get
registered to vote.

242
00:11:00,540 --> 00:11:02,330
I just remembered one more
announcement, too, that I did

243
00:11:02,330 --> 00:11:05,770
want to mention, some of you may
have friends in 511-2 and

244
00:11:05,770 --> 00:11:08,610
have heard their class
average for exam 1.

245
00:11:08,610 --> 00:11:11,100
And I want to tell you, this
happens every year, their

246
00:11:11,100 --> 00:11:14,040
average was 15 points higher
than our average.

247
00:11:14,040 --> 00:11:16,090
Last year, their average
was 15 points

248
00:11:16,090 --> 00:11:17,140
higher than our average.

249
00:11:17,140 --> 00:11:18,690
This is for exam 1.

250
00:11:18,690 --> 00:11:23,660
This is what tends to happen
to 511-2 grades as

251
00:11:23,660 --> 00:11:24,430
the exam goes on.

252
00:11:24,430 --> 00:11:25,800
This is what happens to 511-1.

253
00:11:25,800 --> 00:11:27,600
You guys are in a good spot.

254
00:11:27,600 --> 00:11:30,160
Also, I want to point out that
what's not important is just

255
00:11:30,160 --> 00:11:32,610
that number grade, but also the
letter that goes with it.

256
00:11:32,610 --> 00:11:36,990
So, for example, if you got a 69
in this class on this exam,

257
00:11:36,990 --> 00:11:38,260
that's a B minus.

258
00:11:38,260 --> 00:11:41,870
If you got a 69 on your exam
in 511-2, that's a D, you

259
00:11:41,870 --> 00:11:42,920
didn't pass the exam.

260
00:11:42,920 --> 00:11:45,970
So keep that in mind when your
friend might have gotten a

261
00:11:45,970 --> 00:11:49,670
higher number grade than you
and you know you understand

262
00:11:49,670 --> 00:11:51,490
the similar material
just as well.

263
00:11:51,490 --> 00:11:54,600
Similarly, an 80 in this class
on the exam was a B plus, a

264
00:11:54,600 --> 00:11:58,790
very high B. An 80 in that class
is going to be a C. So,

265
00:11:58,790 --> 00:12:01,890
just don't worry so much about
exactly where that average

266
00:12:01,890 --> 00:12:03,330
lies, you really want to
think about what the

267
00:12:03,330 --> 00:12:04,510
letter grade means.

268
00:12:04,510 --> 00:12:05,290
OK, I've said enough.

269
00:12:05,290 --> 00:12:05,650
I just --

270
00:12:05,650 --> 00:12:07,550
I hate to see people
discouraged, and I know that a

271
00:12:07,550 --> 00:12:10,030
few people have been feeling
discouraged, so that's my

272
00:12:10,030 --> 00:12:14,030
long-winded explanation
of exam 1 grades.

273
00:12:14,030 --> 00:12:14,390
All right.

274
00:12:14,390 --> 00:12:17,890
So, let's move on with life
though, so talking about the

275
00:12:17,890 --> 00:12:20,070
breakdown of the octet rule.

276
00:12:20,070 --> 00:12:22,400
The first example where we're
going to see a breakdown is

277
00:12:22,400 --> 00:12:25,470
any time we have an odd number
of valence electrons.

278
00:12:25,470 --> 00:12:28,900
This is probably the easiest to
explain and to think about,

279
00:12:28,900 --> 00:12:31,740
because if we have an odd number
that means that we

280
00:12:31,740 --> 00:12:34,440
can't have our octet rule,
because our octet rule works

281
00:12:34,440 --> 00:12:36,140
by pairing electrons.

282
00:12:36,140 --> 00:12:40,020
And if we have an odd number,
we automatically have an odd

283
00:12:40,020 --> 00:12:41,440
electron out.

284
00:12:41,440 --> 00:12:44,560
So, if we look at an example,
the methyl radical, we can

285
00:12:44,560 --> 00:12:47,120
first think about how we draw
the Lewis structure -- we draw

286
00:12:47,120 --> 00:12:49,150
the skeletal structure here.

287
00:12:49,150 --> 00:12:50,900
And then what we're going to
do is add up our valence

288
00:12:50,900 --> 00:12:55,580
electrons -- we have 3 times 1
for the hydrogen atoms, carbon

289
00:12:55,580 --> 00:12:58,730
has 4 valence electrons, so
we have a total of 7.

290
00:12:58,730 --> 00:13:02,000
If we want to fill all of our
valence shells in each of

291
00:13:02,000 --> 00:13:06,010
these atoms, we're going to need
a total of 14 electrons.

292
00:13:06,010 --> 00:13:07,770
So, what we see we're
left with is that we

293
00:13:07,770 --> 00:13:10,340
have 7 bonding electrons.

294
00:13:10,340 --> 00:13:14,020
So we can fill in 6 of those
straightforward here, because

295
00:13:14,020 --> 00:13:16,370
we know that we need to make
3 different bonds.

296
00:13:16,370 --> 00:13:18,670
And now we're left over
with 1 electron, we

297
00:13:18,670 --> 00:13:19,890
can't make a bond.

298
00:13:19,890 --> 00:13:24,190
So, what we'll do is carbon does
not have an octet yet.

299
00:13:24,190 --> 00:13:27,195
We can't get it one, but we
can do the best we can and

300
00:13:27,195 --> 00:13:30,890
help it out with adding that
extra electron onto the carbon

301
00:13:30,890 --> 00:13:33,420
atom, so that at least we're
getting as close as possible

302
00:13:33,420 --> 00:13:36,380
to filling our octets.

303
00:13:36,380 --> 00:13:40,280
This is what we call a radical
species or a free radical.

304
00:13:40,280 --> 00:13:43,090
Free radical or radical species
is essentially any

305
00:13:43,090 --> 00:13:46,580
type of a molecule that has this
unpaired electron on one

306
00:13:46,580 --> 00:13:49,890
of the atoms. This might look
really strange, we're used to

307
00:13:49,890 --> 00:13:51,180
seeing octets.

308
00:13:51,180 --> 00:13:54,630
But you'll realize, if you
calculate the formal charge on

309
00:13:54,630 --> 00:13:56,470
this molecule, that
it's not the worst

310
00:13:56,470 --> 00:13:58,190
situation ever for carbon.

311
00:13:58,190 --> 00:14:01,680
At least it's formal charge is
zero, even if it doesn't have

312
00:14:01,680 --> 00:14:03,830
-- it would rather have
an extra bond

313
00:14:03,830 --> 00:14:05,420
and have a full octet.

314
00:14:05,420 --> 00:14:08,120
But it's not the worst scenario
that we can imagine.

315
00:14:08,120 --> 00:14:11,170
But still, radicals tend to be
incredibly reactive because

316
00:14:11,170 --> 00:14:13,030
they do want to fill
that octet.

317
00:14:13,030 --> 00:14:16,000
So, what happens when you have a
radical is it tends to react

318
00:14:16,000 --> 00:14:18,640
with the first thing that it
runs into, especially highly

319
00:14:18,640 --> 00:14:21,450
reactive radicals that are not
stabilized in some other way,

320
00:14:21,450 --> 00:14:24,030
which you'll tend to talk about
it organic chemistry --

321
00:14:24,030 --> 00:14:26,510
how you can stabilize
radicals.

322
00:14:26,510 --> 00:14:29,330
So the term free radical should
sound familiar to you,

323
00:14:29,330 --> 00:14:32,390
whether you've heard it in
chemistry before, or you

324
00:14:32,390 --> 00:14:35,330
haven't heard it in chemistry,
but maybe have heard it, I

325
00:14:35,330 --> 00:14:37,710
don't know, commercials
for facial

326
00:14:37,710 --> 00:14:40,150
products or other things.

327
00:14:40,150 --> 00:14:44,350
People like to talk about free
radicals, and they're sort of

328
00:14:44,350 --> 00:14:46,680
the hero that gets rid of
free radicals, which are

329
00:14:46,680 --> 00:14:47,320
antioxidants.

330
00:14:47,320 --> 00:14:51,450
So you hear in a lot of
different creams or products

331
00:14:51,450 --> 00:14:54,120
or vitamins that they have
antioxidants in them, which

332
00:14:54,120 --> 00:14:55,840
get rid of free radicals.

333
00:14:55,840 --> 00:14:58,010
The reason you would want to
get rid of free radicals is

334
00:14:58,010 --> 00:15:00,630
that free radicals can
damage DNA, so

335
00:15:00,630 --> 00:15:01,810
they're incredibly reactive.

336
00:15:01,810 --> 00:15:04,120
It makes sense that if they hit
a strand of DNA, they're

337
00:15:04,120 --> 00:15:06,250
going to react with the DNA,
you end up breaking the

338
00:15:06,250 --> 00:15:09,320
strands of DNA and causing
DNA damage.

339
00:15:09,320 --> 00:15:12,580
So, this is actually what
happens in aging because we

340
00:15:12,580 --> 00:15:14,950
have a lot of free radicals
in our body.

341
00:15:14,950 --> 00:15:17,940
We can introduce them
artificially, for example,

342
00:15:17,940 --> 00:15:20,760
cigarette smoke has a lot of
really dangerous free radicals

343
00:15:20,760 --> 00:15:24,000
that get into the cells in your
lungs, which damage your

344
00:15:24,000 --> 00:15:26,280
lung DNA, which can
cause lung cancer.

345
00:15:26,280 --> 00:15:29,290
But also, all of us are living
and breathing, which means

346
00:15:29,290 --> 00:15:33,250
we're having metabolism go on in
our body, which means that

347
00:15:33,250 --> 00:15:37,550
as we use oxygen and as we
metabolize our food, we are

348
00:15:37,550 --> 00:15:40,850
actually producing free
radicals as well.

349
00:15:40,850 --> 00:15:44,510
So it's kind of a paradox
because we need them because

350
00:15:44,510 --> 00:15:47,780
they are a natural by-product of
these important processes,

351
00:15:47,780 --> 00:15:50,070
but then they can go on and
damage cells, which is what

352
00:15:50,070 --> 00:15:54,160
kind of is causing aging
and can lead to cancer.

353
00:15:54,160 --> 00:15:58,090
We have enzymes in our body
that repair damage that is

354
00:15:58,090 --> 00:16:00,730
done by free radicals, that will
put the strands of DNA

355
00:16:00,730 --> 00:16:01,710
back together.

356
00:16:01,710 --> 00:16:04,370
And we also have antioxidants
in our body.

357
00:16:04,370 --> 00:16:08,920
So, you might know that, for
example, very brightly colored

358
00:16:08,920 --> 00:16:12,830
fruit is full of antioxidants,
they're full of chemicals that

359
00:16:12,830 --> 00:16:14,880
will neutralize free radicals.

360
00:16:14,880 --> 00:16:17,850
Lots of vitamins are also
antioxidants, so we have

361
00:16:17,850 --> 00:16:20,870
vitamin A on the top there
and vitamin E.

362
00:16:20,870 --> 00:16:22,980
So, the most common thing we
think of when we think of free

363
00:16:22,980 --> 00:16:25,110
radicals is very reactive,
bad for your

364
00:16:25,110 --> 00:16:28,250
body, causes DNA damage.

365
00:16:28,250 --> 00:16:30,530
But the reality is that
free radicals are also

366
00:16:30,530 --> 00:16:31,990
essential for life.

367
00:16:31,990 --> 00:16:34,530
So this is kind of interesting
to think about.

368
00:16:34,530 --> 00:16:38,080
And, for example, certain
enzymes or proteins actually

369
00:16:38,080 --> 00:16:41,950
use free radicals in order to
carry out the reactions that

370
00:16:41,950 --> 00:16:43,540
they carry out in your body.

371
00:16:43,540 --> 00:16:46,280
So, for example, this is a
picture or a snapshot of a

372
00:16:46,280 --> 00:16:49,350
protein, this is a crystal
structure of ribonucleotide

373
00:16:49,350 --> 00:16:51,830
reductase is what it's called.

374
00:16:51,830 --> 00:16:56,100
It's an enzyme that catalyzes
the reaction of an essential

375
00:16:56,100 --> 00:16:59,980
step in both DNA synthesis and
also DNA repair, and it

376
00:16:59,980 --> 00:17:04,090
requires having radicals within
its active site in

377
00:17:04,090 --> 00:17:05,830
order to carry out
the chemistry.

378
00:17:05,830 --> 00:17:08,910
So, this is kind of a neat
paradox, because radicals

379
00:17:08,910 --> 00:17:13,350
damage DNA, but in order to
repair your DNA, you need

380
00:17:13,350 --> 00:17:15,970
certain enzymes, and those
enzymes require different

381
00:17:15,970 --> 00:17:17,610
types of free radicals.

382
00:17:17,610 --> 00:17:20,090
So, free radicals are definitely
very interesting,

383
00:17:20,090 --> 00:17:22,720
and once we get -- or hopefully
you will get into

384
00:17:22,720 --> 00:17:24,680
organic chemistry at some point
and get to really think

385
00:17:24,680 --> 00:17:28,530
about what they do in terms
of a radical mechanism.

386
00:17:28,530 --> 00:17:31,120
We can think about radicals that
are also more stable, so

387
00:17:31,120 --> 00:17:35,690
let's do another example with
the molecule nitric acid.

388
00:17:35,690 --> 00:17:40,220
So we can again, draw the
skeleton here, and just by

389
00:17:40,220 --> 00:17:42,510
looking at it we might not know
it's a radical, but as we

390
00:17:42,510 --> 00:17:45,190
start to count valence
electrons, we should be able

391
00:17:45,190 --> 00:17:48,790
to figure it out very quickly,
because what we have is 11

392
00:17:48,790 --> 00:17:50,350
valence electrons.

393
00:17:50,350 --> 00:17:53,630
We need 16 electrons to
have full octets.

394
00:17:53,630 --> 00:17:56,270
So, we're left with 5
bonding electrons.

395
00:17:56,270 --> 00:17:59,760
We put a double bond in between
our nitrogen and our

396
00:17:59,760 --> 00:18:03,840
oxygen, so what we're left
over with is this single

397
00:18:03,840 --> 00:18:06,830
bonding electron, and we'll put
that on the nitrogen here.

398
00:18:06,830 --> 00:18:09,140
And I'll explain why we put it
on the nitrogen and not the

399
00:18:09,140 --> 00:18:11,080
oxygen in just a minute.

400
00:18:11,080 --> 00:18:16,350
But what we find is then once
we fill in the rest of the

401
00:18:16,350 --> 00:18:19,480
valence electrons in terms of
lone pairs, this is the

402
00:18:19,480 --> 00:18:21,460
structure that we get.

403
00:18:21,460 --> 00:18:24,340
And if you add up all of the
formal charges on the nitrogen

404
00:18:24,340 --> 00:18:27,450
and on the oxygen, what you'll
see is they're both 0.

405
00:18:27,450 --> 00:18:30,600
So if you happen to try drawing
this structure and you

406
00:18:30,600 --> 00:18:33,710
put the lone pair on oxygen and
then you figured out the

407
00:18:33,710 --> 00:18:36,900
formal charge and saw that you
had a split charge, a plus 1

408
00:18:36,900 --> 00:18:39,440
and a minus 1, the first thing
you might want to try is

409
00:18:39,440 --> 00:18:41,430
putting it on the other atom,
and once you did that you'd

410
00:18:41,430 --> 00:18:42,990
see that you had a
better structure

411
00:18:42,990 --> 00:18:46,500
with no formal charge.

412
00:18:46,500 --> 00:18:49,300
I have to mention what nitric
oxide does, because it's a

413
00:18:49,300 --> 00:18:50,850
very interesting molecule.

414
00:18:50,850 --> 00:18:54,670
Don't get it confused with
nitrous oxide, which is happy

415
00:18:54,670 --> 00:18:56,880
gas, that's n o 2.

416
00:18:56,880 --> 00:19:00,880
This is nitric oxide, and it's
actually much more interesting

417
00:19:00,880 --> 00:19:02,430
than nitrous oxide.

418
00:19:02,430 --> 00:19:04,690
It's a signaling molecule in
your body, it's one of the

419
00:19:04,690 --> 00:19:08,050
very few signaling molecules
that is a gas, and obviously,

420
00:19:08,050 --> 00:19:09,440
it's also a radical.

421
00:19:09,440 --> 00:19:12,350
What happens with n o is that
it's produced in the

422
00:19:12,350 --> 00:19:15,210
endothelium of your blood
vessels, so the inner lining

423
00:19:15,210 --> 00:19:20,280
of your blood vessels, and it
signals for smooth muscle that

424
00:19:20,280 --> 00:19:23,180
line your blood vessels
to relax, which causes

425
00:19:23,180 --> 00:19:25,900
vasodilation , and by
vasodilation, I just mean a

426
00:19:25,900 --> 00:19:27,850
widening of the blood vessels.

427
00:19:27,850 --> 00:19:31,320
So, n o signals for your blood
vessels to get wider and allow

428
00:19:31,320 --> 00:19:33,070
more blood to flow through.

429
00:19:33,070 --> 00:19:35,860
And if you think about what
consequences this could have,

430
00:19:35,860 --> 00:19:39,320
in terms of places where they
have high altitude, so they

431
00:19:39,320 --> 00:19:42,080
have lower oxygen levels, do
you think that they produce

432
00:19:42,080 --> 00:19:47,440
more or less and
n o their body?

433
00:19:47,440 --> 00:19:48,030
More?

434
00:19:48,030 --> 00:19:49,750
Yeah, it turns out they
do produce more.

435
00:19:49,750 --> 00:19:52,810
The reason they produce more is
that they want to have more

436
00:19:52,810 --> 00:19:55,340
blood flowing through their
veins so that they can get

437
00:19:55,340 --> 00:19:59,060
more oxygenated blood into
different parts of their body.

438
00:19:59,060 --> 00:20:02,770
N o is also a target in the
pharmaceutical industry.

439
00:20:02,770 --> 00:20:06,260
A very famous one that became
famous I guess over 10 years

440
00:20:06,260 --> 00:20:10,700
ago now, and this is from a drug
that actually targets one

441
00:20:10,700 --> 00:20:14,440
of n o's receptors, and this
drug has the net effect of

442
00:20:14,440 --> 00:20:18,190
vasodilation or widening of
blood vessels in a certain

443
00:20:18,190 --> 00:20:19,650
area in the body.

444
00:20:19,650 --> 00:20:22,880
So this is viagra, some of you
may be familiar, I think

445
00:20:22,880 --> 00:20:24,880
everyone's heard of viagra.

446
00:20:24,880 --> 00:20:27,370
Now you know how viagra works.

447
00:20:27,370 --> 00:20:31,770
Viagra breaks down, or it
inhibits the breakdown of n

448
00:20:31,770 --> 00:20:35,010
o's binding partner in just
certain areas, not everywhere

449
00:20:35,010 --> 00:20:36,910
in your body.

450
00:20:36,910 --> 00:20:40,060
So, in those areas, what happens
is you get more n o

451
00:20:40,060 --> 00:20:42,330
signaling, you get more
vasodilation, you get

452
00:20:42,330 --> 00:20:44,220
increased blood flow.

453
00:20:44,220 --> 00:20:45,770
So that's a little bit
of pharmacology

454
00:20:45,770 --> 00:20:47,750
for you here today.

455
00:20:47,750 --> 00:20:50,480
All right, so let's talk about
one more example in terms of

456
00:20:50,480 --> 00:20:52,990
the breakdown of the octet
rule with radicals.

457
00:20:52,990 --> 00:20:57,230
Let's think about molecular
oxygen.

458
00:20:57,230 --> 00:21:00,970
So let's go ahead and quickly
draw this Lewis structure.

459
00:21:00,970 --> 00:21:02,840
We have o 2.

460
00:21:02,840 --> 00:21:04,130
The second thing we
need to do is

461
00:21:04,130 --> 00:21:06,900
figure out valence electrons.

462
00:21:06,900 --> 00:21:11,060
6 plus 6, so we would
expect to see 12.

463
00:21:11,060 --> 00:21:17,850
For a complete octet we would
need 8 electrons each, so 16.

464
00:21:17,850 --> 00:21:25,250
So in terms of bonding
electrons, what we have is 4

465
00:21:25,250 --> 00:21:26,190
bonding electrons.

466
00:21:26,190 --> 00:21:29,890
So, we can go ahead and fill
those in as a double bond

467
00:21:29,890 --> 00:21:32,200
between the two oxygens.

468
00:21:32,200 --> 00:21:36,140
So, what we end up having left,
and this would be step

469
00:21:36,140 --> 00:21:40,820
six then because five was just
filling in that, is 12 minus

470
00:21:40,820 --> 00:21:45,890
4, so we have 8 lone pair
electrons left.

471
00:21:45,890 --> 00:21:51,290
So we can just fill it in to
our oxygens like this.

472
00:21:51,290 --> 00:21:53,970
All right, so using everything
we've learned about Lewis

473
00:21:53,970 --> 00:21:58,730
structures, we here have the
structure of molecular oxygen.

474
00:21:58,730 --> 00:22:01,100
And I just want to point out for
anyone that gets confused,

475
00:22:01,100 --> 00:22:05,500
when we talk about oxygen as
an atom, that's o, but

476
00:22:05,500 --> 00:22:08,240
molecular oxygen is actually
o 2, the same for molecular

477
00:22:08,240 --> 00:22:09,960
hydrogen, for example.

478
00:22:09,960 --> 00:22:12,430
All right, so let's look at
what the actual Lewis

479
00:22:12,430 --> 00:22:15,780
structure is for molecular
oxygen, and it turns out that

480
00:22:15,780 --> 00:22:19,030
actually we don't have a double
bond, we have a single

481
00:22:19,030 --> 00:22:21,030
bond, and we have
two radicals.

482
00:22:21,030 --> 00:22:23,820
And any time we have two
radicals, we talk about what's

483
00:22:23,820 --> 00:22:26,220
called a biradical.

484
00:22:26,220 --> 00:22:30,480
And while using this exception
to the Lewis structure rule,

485
00:22:30,480 --> 00:22:33,390
to the octet rule for odd
numbers of valence electrons

486
00:22:33,390 --> 00:22:37,020
can clue us into the fact that
we have a radical, there's

487
00:22:37,020 --> 00:22:40,220
really no way for us to use
Lewis structures to predict

488
00:22:40,220 --> 00:22:42,970
when we have a biradical, right,
because we would just

489
00:22:42,970 --> 00:22:45,660
predict that we would get this
Lewis structure here.

490
00:22:45,660 --> 00:22:48,270
So, when I first introduced
Lewis structures, I said these

491
00:22:48,270 --> 00:22:51,790
are great, they're really easy
to use and they work about 90%

492
00:22:51,790 --> 00:22:53,080
of the time.

493
00:22:53,080 --> 00:22:55,750
This falls into that 10%
that Lewis structures

494
00:22:55,750 --> 00:22:56,930
don't work for us.

495
00:22:56,930 --> 00:23:00,260
It turns out, in order to
understand that this is the

496
00:23:00,260 --> 00:23:03,950
electron configuration for o
2, we need to use something

497
00:23:03,950 --> 00:23:07,870
called molecular orbital theory,
and just wait till

498
00:23:07,870 --> 00:23:10,190
next Wednesday and we will tell
you what that is, and we

499
00:23:10,190 --> 00:23:12,970
will, in fact, use
it for oxygen.

500
00:23:12,970 --> 00:23:15,650
But until that point, I'll just
tell you that molecular

501
00:23:15,650 --> 00:23:19,150
orbital theory takes into
account quantum mechanics,

502
00:23:19,150 --> 00:23:21,060
which Lewis theory does not.

503
00:23:21,060 --> 00:23:23,950
So that's why, in fact, there
are those 10% of cases that

504
00:23:23,950 --> 00:23:26,960
Lewis structures
don't work for.

505
00:23:26,960 --> 00:23:29,850
All right, the second case of
exceptions to the octet rule

506
00:23:29,850 --> 00:23:32,530
are when we have octet
deficient molecules.

507
00:23:32,530 --> 00:23:34,570
So basically, this means we're
going to have a molecule

508
00:23:34,570 --> 00:23:38,530
that's stable, even though it
doesn't have a complete octet.

509
00:23:38,530 --> 00:23:41,450
And these tend to happen in
group 13 molecules, and

510
00:23:41,450 --> 00:23:44,790
actually happen almost
exclusively in group 13

511
00:23:44,790 --> 00:23:48,210
molecules, specifically with
boron and aluminum.

512
00:23:48,210 --> 00:23:51,010
So, any time you see a Lewis
structure with boron or

513
00:23:51,010 --> 00:23:53,760
aluminum, you want to just
remember that I should look

514
00:23:53,760 --> 00:23:57,080
out to make sure that these
might have an incomplete

515
00:23:57,080 --> 00:24:01,070
octet, so look out for that
when you see those atoms.

516
00:24:01,070 --> 00:24:05,910
So, let's look at b f 3
as our example here.

517
00:24:05,910 --> 00:24:09,290
And what we see for b f 3
is the number of valence

518
00:24:09,290 --> 00:24:13,420
electrons that we have are 24,
because the valence number of

519
00:24:13,420 --> 00:24:16,330
electrons for boron
is 3, and then 3

520
00:24:16,330 --> 00:24:19,910
times 7 for each fluorine.

521
00:24:19,910 --> 00:24:24,070
For total filled octets we need
32, so that means we need

522
00:24:24,070 --> 00:24:25,420
8 bonding electrons.

523
00:24:25,420 --> 00:24:28,920
So, let's assign two to each
bond here, and then we're

524
00:24:28,920 --> 00:24:31,160
going to have two extra bonding
electrons, so let's

525
00:24:31,160 --> 00:24:33,350
just arbitrarily pick
a fluorine to give

526
00:24:33,350 --> 00:24:35,850
a double bond to.

527
00:24:35,850 --> 00:24:38,340
And then we can fill in the lone
pair electrons, we have

528
00:24:38,340 --> 00:24:40,250
16 left over.

529
00:24:40,250 --> 00:24:42,660
So thinking about what the
formal charge is, if we want

530
00:24:42,660 --> 00:24:45,470
to figure out the formal charge
for the boron here,

531
00:24:45,470 --> 00:24:48,590
what we're talking about is the
valence number for boron,

532
00:24:48,590 --> 00:24:52,430
which is 3, minus 0 because
there are no lone pairs, minus

533
00:24:52,430 --> 00:24:55,450
1/2 of 8 because there are
eight shared electrons.

534
00:24:55,450 --> 00:24:58,520
We get a formal charge
of minus 1.

535
00:24:58,520 --> 00:25:01,380
What is our formal charge
since we learned this on

536
00:25:01,380 --> 00:25:05,250
Monday for thinking
about the double

537
00:25:05,250 --> 00:25:07,770
bonded fluorine in boron?

538
00:25:07,770 --> 00:25:10,280
So, look at your notes and look
at the fluorine that has

539
00:25:10,280 --> 00:25:12,940
a double bond with it, and I
want you to go ahead and tell

540
00:25:12,940 --> 00:25:32,030
me what that formal
charge should be.

541
00:25:32,030 --> 00:25:46,650
All right, let's take 10
more seconds on that.

542
00:25:46,650 --> 00:25:50,400
OK, so 49%.

543
00:25:50,400 --> 00:25:54,800
So, let's go look back at the
notes, we'll talk about why

544
00:25:54,800 --> 00:25:58,610
about 50% of you are right, and
50% need to review, which

545
00:25:58,610 --> 00:26:00,880
I totally understand you haven't
had time to do yet,

546
00:26:00,880 --> 00:26:04,190
your formal charge rules from
Monday's class, there were

547
00:26:04,190 --> 00:26:05,520
other things going on.

548
00:26:05,520 --> 00:26:08,490
But let's talk about how we
figure out formal charge.

549
00:26:08,490 --> 00:26:12,390
Formal charge is just the number
of valence electrons

550
00:26:12,390 --> 00:26:14,720
you have. So fluorine has 7.

551
00:26:14,720 --> 00:26:17,140
You should be able to look at a
periodic table and see that

552
00:26:17,140 --> 00:26:18,550
fluorine has seven.

553
00:26:18,550 --> 00:26:21,760
What we subtract from that is
the number of lone pair

554
00:26:21,760 --> 00:26:25,280
electrons, and there are four
lone pair electrons on this

555
00:26:25,280 --> 00:26:28,770
double bonded fluorine,
so it's minus 4.

556
00:26:28,770 --> 00:26:32,150
Then we subtract 1/2 of
the shared electrons.

557
00:26:32,150 --> 00:26:34,870
Well we have a double bond with
boron here, so we have a

558
00:26:34,870 --> 00:26:37,050
total of 4 shared electrons.

559
00:26:37,050 --> 00:26:41,350
And when we do the subtraction
here, what we end up with is a

560
00:26:41,350 --> 00:26:46,180
formal charge plus 1 on the
double bonded fluorine.

561
00:26:46,180 --> 00:26:48,580
Without even doing a
calculation, what do you think

562
00:26:48,580 --> 00:26:50,540
that the formal charge
should be on you

563
00:26:50,540 --> 00:26:52,940
single bonded fluorines?

564
00:26:52,940 --> 00:26:53,400
Good.

565
00:26:53,400 --> 00:26:56,700
OK, it should be
0 and it is 0.

566
00:26:56,700 --> 00:27:01,420
The reason it's zero in terms of
calculating it is 7 minus 6

567
00:27:01,420 --> 00:27:04,150
lone pair electrons minus
1/2 half of 2 shared

568
00:27:04,150 --> 00:27:05,840
electrons is 0.

569
00:27:05,840 --> 00:27:09,140
The reason that you all told
me, I think, and I hope, is

570
00:27:09,140 --> 00:27:12,890
that you know that the formal
charge on individual atoms has

571
00:27:12,890 --> 00:27:15,120
to equal the total charge
on the molecule.

572
00:27:15,120 --> 00:27:18,030
So if we already have a minus 1
and a plus 1, and we know we

573
00:27:18,030 --> 00:27:20,740
have no charge in the molecule,
and we only have one

574
00:27:20,740 --> 00:27:23,640
type of atom left to talk about,
that formal charge had

575
00:27:23,640 --> 00:27:25,240
better be 0.

576
00:27:25,240 --> 00:27:25,690
OK.

577
00:27:25,690 --> 00:27:27,940
So this looks pretty good in
terms of a Lewis structure, we

578
00:27:27,940 --> 00:27:29,760
figured out our formal
charges.

579
00:27:29,760 --> 00:27:32,080
These also look pretty good,
too, we don't have too much

580
00:27:32,080 --> 00:27:33,800
charge separation.

581
00:27:33,800 --> 00:27:37,640
But what actually it turns out
is that if you experimentally

582
00:27:37,640 --> 00:27:41,060
look at what type of bonds you
have, it turns out that all

583
00:27:41,060 --> 00:27:44,840
three of the b f bonds are equal
in length, and they all

584
00:27:44,840 --> 00:27:48,160
have a length that would
correspond to a single bond.

585
00:27:48,160 --> 00:27:51,790
So, experimentally, we know we
have to throw out this Lewis

586
00:27:51,790 --> 00:27:54,430
structure here, we have some
more information, let's think

587
00:27:54,430 --> 00:27:57,430
about how this could happen.

588
00:27:57,430 --> 00:28:00,330
So this could happen, for
example, is if we take this

589
00:28:00,330 --> 00:28:03,490
two of the electrons that are in
the b f double bond and we

590
00:28:03,490 --> 00:28:06,810
put it right on to the fluorine
here, so now we have

591
00:28:06,810 --> 00:28:08,280
all single bonds.

592
00:28:08,280 --> 00:28:11,630
And let's think about what the
formal charge situation would

593
00:28:11,630 --> 00:28:13,970
be in this case here.

594
00:28:13,970 --> 00:28:16,990
What happens here is now we
would have a formal charge of

595
00:28:16,990 --> 00:28:20,770
0 on the boron, we'd have a
formal charge of 0 on all of

596
00:28:20,770 --> 00:28:22,760
the fluorine molecules
as well.

597
00:28:22,760 --> 00:28:25,480
So, it turns out that actually
looking at formal charge, even

598
00:28:25,480 --> 00:28:28,220
though the first case didn't
look too bad, this case

599
00:28:28,220 --> 00:28:28,970
actually looks a lot better.

600
00:28:28,970 --> 00:28:32,320
We have absolutely no formal
charge separation whatsoever.

601
00:28:32,320 --> 00:28:35,390
It turns out again, boron and
aluminum, those are the two

602
00:28:35,390 --> 00:28:36,890
that you want to look out for.

603
00:28:36,890 --> 00:28:39,970
They can be perfectly happy
without a full octet, they're

604
00:28:39,970 --> 00:28:44,510
perfectly happy with 6 instead
of 8 in terms of electrons in

605
00:28:44,510 --> 00:28:45,410
their valence shell.

606
00:28:45,410 --> 00:28:48,650
So that is our exception
the number two.

607
00:28:48,650 --> 00:28:51,230
We have one more exception and
this is a valence shell

608
00:28:51,230 --> 00:28:53,950
expansion, and this can be the
hardest to look out for,

609
00:28:53,950 --> 00:28:57,770
students tend to forget to look
for this one, but it's

610
00:28:57,770 --> 00:28:59,860
very important as well, because
there are a lot of

611
00:28:59,860 --> 00:29:01,610
structures that are affected
for this .

612
00:29:01,610 --> 00:29:04,660
And this is only applicable if
we're talking about a central

613
00:29:04,660 --> 00:29:08,520
atom that has an n value or
a principle quantum number

614
00:29:08,520 --> 00:29:11,420
that's equal to or greater
than three.

615
00:29:11,420 --> 00:29:16,110
What happens when we have n
that's equal to or greater to

616
00:29:16,110 --> 00:29:19,790
three, is that now, in addition
to s orbitals and p

617
00:29:19,790 --> 00:29:23,090
orbitals, what else do we
have available to us?

618
00:29:23,090 --> 00:29:24,350
D orbitals, great.

619
00:29:24,350 --> 00:29:28,470
So what we see is we have some
empty d orbitals, which means

620
00:29:28,470 --> 00:29:31,310
that we can have more than
eight electrons that fit

621
00:29:31,310 --> 00:29:33,400
around that central atom.

622
00:29:33,400 --> 00:29:35,380
If you're looking to see if this
is going to happen, do

623
00:29:35,380 --> 00:29:37,020
you think this would happen
with a large or

624
00:29:37,020 --> 00:29:41,350
small central atom?

625
00:29:41,350 --> 00:29:43,430
So think of it in terms
of just fitting.

626
00:29:43,430 --> 00:29:47,320
We've got to fit more than
8 electrons around here.

627
00:29:47,320 --> 00:29:50,860
Yeah, so it's going to be, we
need to have a large central

628
00:29:50,860 --> 00:29:53,050
atom in order for this
to take place.

629
00:29:53,050 --> 00:29:55,470
Literally, we just need to fit
everything around is probably

630
00:29:55,470 --> 00:29:58,050
the easiest way to
think about it.

631
00:29:58,050 --> 00:30:02,160
And what happens is it also
tends to have small atoms that

632
00:30:02,160 --> 00:30:02,860
it's bonded to.

633
00:30:02,860 --> 00:30:06,900
Again, just think of it in terms
of all fitting in there.

634
00:30:06,900 --> 00:30:10,160
So, let's take an
example p c l 5.

635
00:30:10,160 --> 00:30:12,660
The first example is the more
straightforward example,

636
00:30:12,660 --> 00:30:15,240
because let's start to draw the
Lewis structure, and what

637
00:30:15,240 --> 00:30:19,250
we see is that phosphorous has
five chlorines around it.

638
00:30:19,250 --> 00:30:21,980
So we already know if we want
to form five bonds we've

639
00:30:21,980 --> 00:30:23,120
broken our octet rule.

640
00:30:23,120 --> 00:30:25,570
But let's go through and figure
this out and see how

641
00:30:25,570 --> 00:30:26,660
that happens.

642
00:30:26,660 --> 00:30:29,900
What we know is we need
40 valence electrons,

643
00:30:29,900 --> 00:30:31,090
we have those --

644
00:30:31,090 --> 00:30:35,410
5 from the phosphorous, and
we have 7 from each of the

645
00:30:35,410 --> 00:30:39,190
chlorine atoms. If we were to
fill out all of those octets,

646
00:30:39,190 --> 00:30:42,850
that would be 48 electrons.

647
00:30:42,850 --> 00:30:45,710
So what we end up with when
we do our Lewis structure

648
00:30:45,710 --> 00:30:48,270
calculation is that we only
have 8 bonding electrons

649
00:30:48,270 --> 00:30:49,560
available to us.

650
00:30:49,560 --> 00:30:52,270
So we can fill those in between
the phosphorous and

651
00:30:52,270 --> 00:30:55,240
the chlorine, those 8
bonding electrons.

652
00:30:55,240 --> 00:30:59,000
So, this is obviously
a problem.

653
00:30:59,000 --> 00:31:04,080
To make 5 p c l bonds we need
10 shared electrons, and we

654
00:31:04,080 --> 00:31:06,990
know that that's the situation
because it's called p c l 5

655
00:31:06,990 --> 00:31:10,530
and not p c l 4, so we can go
right ahead and add in that

656
00:31:10,530 --> 00:31:13,220
extra electron pair.

657
00:31:13,220 --> 00:31:16,350
So we've used up 10 for bonding,
so that means what we

658
00:31:16,350 --> 00:31:19,740
have left is 30 lone pair
electrons, and I would not

659
00:31:19,740 --> 00:31:22,120
recommend filling all of these
in your notes right now, you

660
00:31:22,120 --> 00:31:24,500
can go back and do that, but
just know the rest end up

661
00:31:24,500 --> 00:31:27,300
filling up the octets for
all of the chlorines.

662
00:31:27,300 --> 00:31:30,450
So, in this first case where you
actually need to make more

663
00:31:30,450 --> 00:31:33,170
than for bonds, you will
immediately know you need to

664
00:31:33,170 --> 00:31:37,240
use this exception to the Lewis
structure octet rule,

665
00:31:37,240 --> 00:31:39,130
but sometimes it won't
be as obvious.

666
00:31:39,130 --> 00:31:43,090
So, let's look at c r o 4, the
2 minus version here, so a

667
00:31:43,090 --> 00:31:47,530
chromate ion, and if we draw
the skeletal structure, we

668
00:31:47,530 --> 00:31:51,940
have four things that the
chromate needs to bond to.

669
00:31:51,940 --> 00:31:54,200
So, let's do the Lewis
structure again.

670
00:31:54,200 --> 00:31:57,320
When we figure out the valence
electrons, we have total, we

671
00:31:57,320 --> 00:32:01,760
have 6 from the chromium, we
have 6 from each of the

672
00:32:01,760 --> 00:32:06,450
different oxygens, and where
did this 2 come from?

673
00:32:06,450 --> 00:32:07,780
Yup, the negative charge.

674
00:32:07,780 --> 00:32:10,600
So, remember, we have 2 extra
electrons hanging out in our

675
00:32:10,600 --> 00:32:12,520
molecule, so we need
to include those.

676
00:32:12,520 --> 00:32:13,670
We have a total of 32.

677
00:32:13,670 --> 00:32:16,870
40 are needed to
fill up octets.

678
00:32:16,870 --> 00:32:20,820
So again, we have 8 bonding
electrons available, so we can

679
00:32:20,820 --> 00:32:24,560
go ahead and fill these in
between each of the bonds.

680
00:32:24,560 --> 00:32:27,850
What happens is that we then
have 24 lone pair electrons

681
00:32:27,850 --> 00:32:31,810
left, and we can fill
those in like this.

682
00:32:31,810 --> 00:32:34,350
And the problem comes
now when we figure

683
00:32:34,350 --> 00:32:35,810
out the formal charge.

684
00:32:35,810 --> 00:32:38,310
So, when we do that what we find
is that the chromium has

685
00:32:38,310 --> 00:32:42,510
a formal charge of plus 1, and
that each of the oxygens has a

686
00:32:42,510 --> 00:32:44,040
total charge of minus 1.

687
00:32:44,040 --> 00:32:47,240
So we actually have a bit of
charge separation here.

688
00:32:47,240 --> 00:32:49,560
Without even doing a
calculation, what is the total

689
00:32:49,560 --> 00:32:53,420
charge of these that
are added up?

690
00:32:53,420 --> 00:32:55,140
OK, it's minus 2,
that's right.

691
00:32:55,140 --> 00:32:57,850
We know that the total charge of
each of the formal charges

692
00:32:57,850 --> 00:33:00,290
has to add up to minus 2,
because that's the charge in

693
00:33:00,290 --> 00:33:01,210
our molecule.

694
00:33:01,210 --> 00:33:04,130
We can also just calculate it
-- the chromate gives us a

695
00:33:04,130 --> 00:33:07,030
plus 2, then we have 4 times
minus 1 for each of the

696
00:33:07,030 --> 00:33:09,600
oxygens, so we have a minus 2.

697
00:33:09,600 --> 00:33:11,950
So, we have some charge
separation here, and in some

698
00:33:11,950 --> 00:33:15,380
cases, if we're not at n equals
3 or higher, there's

699
00:33:15,380 --> 00:33:17,470
really nothing we can do about
it, this would be the best

700
00:33:17,470 --> 00:33:18,580
structure we can do.

701
00:33:18,580 --> 00:33:22,530
But since we have these d
orbitals available, we can use

702
00:33:22,530 --> 00:33:25,100
them, and it turns out that
experimentally this is what's

703
00:33:25,100 --> 00:33:29,180
found, that the length and the
strength are not single bonds,

704
00:33:29,180 --> 00:33:32,855
but they're actually something
between a single bond and a

705
00:33:32,855 --> 00:33:33,970
double bond.

706
00:33:33,970 --> 00:33:37,220
So how do we get a 1 and 1/2
bond, for example, what's the

707
00:33:37,220 --> 00:33:39,710
term that let's us do that?

708
00:33:39,710 --> 00:33:40,360
Resonance.

709
00:33:40,360 --> 00:33:40,920
That's right.

710
00:33:40,920 --> 00:33:43,130
So that's exactly what's
happening here.

711
00:33:43,130 --> 00:33:46,160
So, if we went ahead and drew
this structure here where we

712
00:33:46,160 --> 00:33:51,230
have now two double bonds and
two single bonds, that would

713
00:33:51,230 --> 00:33:54,920
be in resonance with another
structure where we have two

714
00:33:54,920 --> 00:33:58,220
double bonds instead to these
two oxygens, and now, single

715
00:33:58,220 --> 00:34:00,020
bonds to these two oxygens.

716
00:34:00,020 --> 00:34:02,810
We can actually also have
several other resonance

717
00:34:02,810 --> 00:34:04,140
structures as well.

718
00:34:04,140 --> 00:34:06,500
Remember, the definition of a
resonance structure is where

719
00:34:06,500 --> 00:34:09,470
all the atoms stay the same,
but what we can do is move

720
00:34:09,470 --> 00:34:11,830
around the electrons -- we're
moving around those extra two

721
00:34:11,830 --> 00:34:14,280
electrons that can be
in double bonds.

722
00:34:14,280 --> 00:34:17,140
So, why don't you tell me
how many other resonance

723
00:34:17,140 --> 00:34:27,170
structures you would expect to
see for this chromate ion?

724
00:34:27,170 --> 00:34:50,960
All right, let's take 10
more seconds on this.

725
00:34:50,960 --> 00:34:51,640
All right.

726
00:34:51,640 --> 00:34:52,260
This is good.

727
00:34:52,260 --> 00:34:55,980
I know this is a real split
response, but the right answer

728
00:34:55,980 --> 00:34:58,360
is the one that is indicated
in the graph

729
00:34:58,360 --> 00:34:59,930
here that it's four.

730
00:34:59,930 --> 00:35:02,380
This takes a little bit of time
to get used to thinking

731
00:35:02,380 --> 00:35:04,890
about all the different Lewis
structures you can have. So,

732
00:35:04,890 --> 00:35:06,990
you guys should all go back
home if you can't see it

733
00:35:06,990 --> 00:35:10,330
immediately right now and try
drawing out those four other

734
00:35:10,330 --> 00:35:12,360
Lewis structures, for chromate,

735
00:35:12,360 --> 00:35:13,700
there are four others.

736
00:35:13,700 --> 00:35:16,240
You'll probably get a chance to
literally do this example

737
00:35:16,240 --> 00:35:19,420
in recitation where you draw
out all four, but it's even

738
00:35:19,420 --> 00:35:21,500
better to make sure you
understand it before you get

739
00:35:21,500 --> 00:35:22,450
to that point.

740
00:35:22,450 --> 00:35:25,800
So, we can go back to
the class notes.

741
00:35:25,800 --> 00:35:28,280
So it turns out there's four
other Lewis structures, so

742
00:35:28,280 --> 00:35:30,410
basically just think about
all the other different

743
00:35:30,410 --> 00:35:32,910
combinations where you can have
single and double bonds,

744
00:35:32,910 --> 00:35:35,350
and when you draw those out,
you end up with four.

745
00:35:35,350 --> 00:35:37,730
So, for every single one of
these Lewis structures, we

746
00:35:37,730 --> 00:35:40,680
could figure out what the formal
charges are, and what

747
00:35:40,680 --> 00:35:43,600
we would find is that it's 0
on the chromium, it's 0 for

748
00:35:43,600 --> 00:35:46,630
the double bonded oxygens, and
it's going to be negative 1

749
00:35:46,630 --> 00:35:48,320
for the single bonded oxygens.

750
00:35:48,320 --> 00:35:52,050
So, what you can see is that in
this situation, we end up

751
00:35:52,050 --> 00:35:55,310
having less formal charge
separation, and that's what

752
00:35:55,310 --> 00:35:58,180
we're looking for, that's the
more stable structure.

753
00:35:58,180 --> 00:36:02,570
So any time you can have an
expanded octet -- an expanded

754
00:36:02,570 --> 00:36:05,940
valence shell, where you have n
is equal to or greater than

755
00:36:05,940 --> 00:36:09,510
3, and by expanding and adding
more electrons into that

756
00:36:09,510 --> 00:36:12,380
valence shell, you lower
the charge separation,

757
00:36:12,380 --> 00:36:13,560
you want to do that.

758
00:36:13,560 --> 00:36:17,000
I also want to point out, I
basically said there's 6

759
00:36:17,000 --> 00:36:19,600
different ways we can draw this
in terms of drawing all

760
00:36:19,600 --> 00:36:20,880
the resonance structures.

761
00:36:20,880 --> 00:36:23,340
You might be wondering if you
have to figure out the formal

762
00:36:23,340 --> 00:36:25,810
charge for each structure
individually, and the answer

763
00:36:25,810 --> 00:36:28,560
is no, you can pick any single
structure and the formal

764
00:36:28,560 --> 00:36:30,360
charges will work
out the same.

765
00:36:30,360 --> 00:36:32,900
So, for example, if you pick
this structure and your friend

766
00:36:32,900 --> 00:36:35,590
picks this structure, you'll
both get the right answer that

767
00:36:35,590 --> 00:36:38,930
there's just the negative 1 on
the oxygens and no other

768
00:36:38,930 --> 00:36:42,130
formal charges in the molecule.

769
00:36:42,130 --> 00:36:42,480
All right.

770
00:36:42,480 --> 00:36:45,880
So those are the end of our
exceptions to the octet rule

771
00:36:45,880 --> 00:36:48,080
for Lewis structures, that's
everything we're going to say

772
00:36:48,080 --> 00:36:49,670
about Lewis structures.

773
00:36:49,670 --> 00:36:52,040
And remember, that when we talk
about Lewis structures,

774
00:36:52,040 --> 00:36:55,140
what they tell us is the
electron configuration in

775
00:36:55,140 --> 00:36:57,520
covalent bonds, so that
valence shell electron

776
00:36:57,520 --> 00:36:58,740
configuration.

777
00:36:58,740 --> 00:37:02,100
So we talked a lot about
covalent bonds before we got

778
00:37:02,100 --> 00:37:05,290
into Lewis structures, and then
how to represent covalent

779
00:37:05,290 --> 00:37:07,490
bonds by Lewis structures.

780
00:37:07,490 --> 00:37:10,730
So now I'll say a little bit
about ionic bonds, which are

781
00:37:10,730 --> 00:37:14,080
the other extreme, and when you
have an ionic bond, what

782
00:37:14,080 --> 00:37:18,420
you have now is a complete
transfer of either one or many

783
00:37:18,420 --> 00:37:22,370
electrons between two atoms. So
the key word for covalent

784
00:37:22,370 --> 00:37:25,440
bond was electron sharing, the
key word for ionic bonds is

785
00:37:25,440 --> 00:37:27,660
electron transfer.

786
00:37:27,660 --> 00:37:30,700
And the bonding between the two
atoms ends up resulting

787
00:37:30,700 --> 00:37:33,700
from an attraction that we're
very familiar with, which is

788
00:37:33,700 --> 00:37:36,130
the Coulomb or the electrostatic
attraction

789
00:37:36,130 --> 00:37:37,930
between the negatively
charged and the

790
00:37:37,930 --> 00:37:41,610
positively charged ions.

791
00:37:41,610 --> 00:37:42,690
So let's take an example.

792
00:37:42,690 --> 00:37:45,940
The easiest one to think about
is where we have a negative 1

793
00:37:45,940 --> 00:37:47,360
and a positive 1 ion.

794
00:37:47,360 --> 00:37:51,460
So this is salt, n a c l --
actually lots of things are

795
00:37:51,460 --> 00:37:54,610
call salt, but this is what
we think of a table salt.

796
00:37:54,610 --> 00:37:57,970
So, let's think about what we
have to do if we want the form

797
00:37:57,970 --> 00:38:01,690
sodium chloride from the neutral
sodium and chlorine

798
00:38:01,690 --> 00:38:04,570
atoms. So, the first thing that
we're going to need to do

799
00:38:04,570 --> 00:38:08,540
is we need to convert sodium
into sodium plus.

800
00:38:08,540 --> 00:38:10,610
What does this process
look like to you?

801
00:38:10,610 --> 00:38:13,470
Is this one of those periodic
trends, perhaps?

802
00:38:13,470 --> 00:38:17,150
Can anyone name what we're
looking at here?

803
00:38:17,150 --> 00:38:18,930
Exactly, ionization energy.

804
00:38:18,930 --> 00:38:21,050
So, if we're going to talk about
the energy difference

805
00:38:21,050 --> 00:38:23,420
here, what we're going to
be talking about is the

806
00:38:23,420 --> 00:38:27,150
ionization energy, or the energy
it takes to rip off an

807
00:38:27,150 --> 00:38:32,370
electron from sodium in order
to form the sodium plus ion.

808
00:38:32,370 --> 00:38:34,080
So, we can just put right
here, that's 494

809
00:38:34,080 --> 00:38:37,200
kilojoules per mole.

810
00:38:37,200 --> 00:38:39,880
The next thing that we want to
look at is chlorine, so in

811
00:38:39,880 --> 00:38:42,680
terms of chlorine we need to
go to chlorine minus, so we

812
00:38:42,680 --> 00:38:44,860
actually need to add
an electron.

813
00:38:44,860 --> 00:38:48,370
This is actually the reverse of
one of the periodic trends

814
00:38:48,370 --> 00:38:49,120
we talked about.

815
00:38:49,120 --> 00:38:53,160
Which trend is that this
is the reverse of?

816
00:38:53,160 --> 00:38:54,500
Electron affinity, right.

817
00:38:54,500 --> 00:38:57,330
Because if we go backwards we're
saying how badly does

818
00:38:57,330 --> 00:38:59,310
chlorine want to grab
an electron?

819
00:38:59,310 --> 00:39:02,360
Chlorine wants to do this very
badly, and it turns out the

820
00:39:02,360 --> 00:39:05,060
electron affinity for chlorine
is huge, it's 349 kilojoules

821
00:39:05,060 --> 00:39:08,790
per mole, but remember, we're
going in reverse, so we need

822
00:39:08,790 --> 00:39:13,820
to talk about it as negative
349 kilojoules per mole.

823
00:39:13,820 --> 00:39:17,060
So if we talk about the sum of
what's happening here, what we

824
00:39:17,060 --> 00:39:20,480
need to do is think about going
from the neutrals to the

825
00:39:20,480 --> 00:39:24,420
ions, so we can just add those
two energies together, and

826
00:39:24,420 --> 00:39:28,690
what we end up with is plus 145
kilojoules per mole, in

827
00:39:28,690 --> 00:39:30,590
order to go from neutral
sodium in

828
00:39:30,590 --> 00:39:33,350
chlorine to the ions.

829
00:39:33,350 --> 00:39:36,440
So, the problem here is that we
have to actually put energy

830
00:39:36,440 --> 00:39:39,730
into our system, so this doesn't
seem favorable, right.

831
00:39:39,730 --> 00:39:42,200
What's favorable is when we
actually get energy out and

832
00:39:42,200 --> 00:39:45,420
our energy gets lower, but what
we're saying here is that

833
00:39:45,420 --> 00:39:47,850
we actually need to
put in energy.

834
00:39:47,850 --> 00:39:49,720
So another way to say
this is this process

835
00:39:49,720 --> 00:39:51,820
actually requires energy.

836
00:39:51,820 --> 00:39:55,290
It does not emit energy, it
does not give off excess

837
00:39:55,290 --> 00:39:57,340
energy, it requires energy.

838
00:39:57,340 --> 00:40:00,230
So, we need to think about how
can we solve this problem in

839
00:40:00,230 --> 00:40:03,240
terms of thinking about ionic
bonds, and the answer is

840
00:40:03,240 --> 00:40:04,590
Coulomb attraction.

841
00:40:04,590 --> 00:40:07,460
So there's one more force that
we need to talk about, and

842
00:40:07,460 --> 00:40:10,160
that is when we talk about the
attraction between the

843
00:40:10,160 --> 00:40:13,520
negatively and the positively
charged ions, such that we

844
00:40:13,520 --> 00:40:15,150
form sodium chloride.

845
00:40:15,150 --> 00:40:18,900
So this process here has a
delta energy, a change in

846
00:40:18,900 --> 00:40:22,570
energy of negative 589
kilojoules per mole.

847
00:40:22,570 --> 00:40:25,230
So that's huge, we're giving
off a lot of energy by this

848
00:40:25,230 --> 00:40:26,140
attraction.

849
00:40:26,140 --> 00:40:30,820
So if we add up the net energy
for all of this process, all

850
00:40:30,820 --> 00:40:35,220
we need to do is add negative
589 to plus 145.

851
00:40:35,220 --> 00:40:38,480
So what we end up getting is the
net energy change is going

852
00:40:38,480 --> 00:40:42,700
to be negative 444 kilojoules
per mole, so you can see that,

853
00:40:42,700 --> 00:40:46,490
in fact, it is very favorable
for neutral sodium and neutral

854
00:40:46,490 --> 00:40:53,090
chloride to form sodium chloride
in an ionic bond.

855
00:40:53,090 --> 00:40:56,680
And the net increase then,
is a decrease in energy.

856
00:40:56,680 --> 00:40:59,880
So, I just gave you the number
in terms of what that Coulomb

857
00:40:59,880 --> 00:41:02,710
potential would be in
attraction, but we can I

858
00:41:02,710 --> 00:41:06,480
easily calculate it as well
using this equation here where

859
00:41:06,480 --> 00:41:10,440
the energy is equal to the
charge on each of the ions,

860
00:41:10,440 --> 00:41:13,070
and this is just multiplied by
the value of charge for an

861
00:41:13,070 --> 00:41:17,880
electron divided by 4 pi epsilon
nought times r, are r

862
00:41:17,880 --> 00:41:21,420
is just the distance in terms
of the bond length we could

863
00:41:21,420 --> 00:41:22,330
talk about.

864
00:41:22,330 --> 00:41:24,790
So, let's calculate and make
sure that I didn't tell you a

865
00:41:24,790 --> 00:41:25,690
false number here.

866
00:41:25,690 --> 00:41:28,440
Let's say we do the calculation
with the bond

867
00:41:28,440 --> 00:41:30,460
length that we've looked
up, which is 2 .

868
00:41:30,460 --> 00:41:33,040
3 6 angstroms for
the bond length

869
00:41:33,040 --> 00:41:34,520
between sodium and chloride.

870
00:41:34,520 --> 00:41:37,230
So we should be able to figure
out the Coulombic

871
00:41:37,230 --> 00:41:37,710
attraction for this.

872
00:41:37,710 --> 00:41:47,540
So, if we talk about the energy
of attraction, we need

873
00:41:47,540 --> 00:41:52,140
to multiply plus 1, that's the
charge on the sodium, times

874
00:41:52,140 --> 00:41:56,370
minus 1, the charge on the
chlorine, times the charge in

875
00:41:56,370 --> 00:41:58,030
an electron, 1 .

876
00:41:58,030 --> 00:42:04,640
6 0 2 times 10 the negative 19
Coulombs, and that's all

877
00:42:04,640 --> 00:42:12,470
divided by 4 pi, and then I've
written out epsilon nought in

878
00:42:12,470 --> 00:42:14,610
your notes, so I won't write
it on the board.

879
00:42:14,610 --> 00:42:17,910
And then r, so r is
going to be 2 .

880
00:42:17,910 --> 00:42:23,170
3 6 and times -- what is
angstrom, everyone?

881
00:42:23,170 --> 00:42:26,080
Yup, 10 to the negative 10.

882
00:42:26,080 --> 00:42:30,190
So 10 to the negative
10 meters.

883
00:42:30,190 --> 00:42:34,780
So, if we do this calculation
here, what we end up with is

884
00:42:34,780 --> 00:42:42,770
negative 9.774 times 10 to
the negative 19 joules.

885
00:42:42,770 --> 00:42:46,310
So that's what we have in
terms of our energy.

886
00:42:46,310 --> 00:42:49,330
That does not look the same as
what we saw -- yup, do you

887
00:42:49,330 --> 00:42:49,910
have a question?

888
00:42:49,910 --> 00:42:54,720
STUDENT: [INAUDIBLE]

889
00:42:54,720 --> 00:42:56,690
PROFESSOR: OK.

890
00:42:56,690 --> 00:42:58,770
Luckily, although, I did not
write it in my own notes, I

891
00:42:58,770 --> 00:43:01,090
did it when I put in my
calculator, thank you.

892
00:43:01,090 --> 00:43:04,040
So you need to square this value
here and then you should

893
00:43:04,040 --> 00:43:07,600
get this value right here,
negative 9.77.

894
00:43:07,600 --> 00:43:11,320
All right, so what we need to
do though is convert from

895
00:43:11,320 --> 00:43:13,560
joules into kilojoules per mole,
because that's what we

896
00:43:13,560 --> 00:43:14,650
were using.

897
00:43:14,650 --> 00:43:20,580
So if we multiply that number
there by kilojoules per mole

898
00:43:20,580 --> 00:43:25,540
-- or excuse me, first
kilojoules per joule, so we

899
00:43:25,540 --> 00:43:30,430
have 1,000 joules in
every kilojoule.

900
00:43:30,430 --> 00:43:35,830
And then we multiply that by
Avagadro's number, 6.022 times

901
00:43:35,830 --> 00:43:40,490
10 to the 23 per mole.

902
00:43:40,490 --> 00:43:45,770
What we end up with
is negative 589

903
00:43:45,770 --> 00:43:48,090
kilojoules per mole.

904
00:43:48,090 --> 00:43:51,570
So this is that same Coulombic
attraction that we saw in the

905
00:43:51,570 --> 00:43:53,290
first place.

906
00:43:53,290 --> 00:43:57,100
So, notice that you will
naturally get out a negative

907
00:43:57,100 --> 00:43:59,760
charge here, remember negative
means an attractive force in

908
00:43:59,760 --> 00:44:02,450
this case, because you
have the plus and

909
00:44:02,450 --> 00:44:05,390
the minus 1 in here.

910
00:44:05,390 --> 00:44:09,630
So we should be able to easily
do that calculation, and what

911
00:44:09,630 --> 00:44:12,060
we end up getting matches up
with what I just told you,

912
00:44:12,060 --> 00:44:14,470
luckily, and thank you for
catching the square, that's an

913
00:44:14,470 --> 00:44:17,070
important part in getting
the right answer.

914
00:44:17,070 --> 00:44:21,120
So, experimentally then, what we
find is that the change in

915
00:44:21,120 --> 00:44:23,070
energy for this reaction
is negative 444

916
00:44:23,070 --> 00:44:25,850
kilojoules per mole.

917
00:44:25,850 --> 00:44:28,980
If we look experimentally what
we see, it's actually a little

918
00:44:28,980 --> 00:44:32,800
bit different, it's negative
411 kilojoules per mole.

919
00:44:32,800 --> 00:44:36,530
So, in terms of this class, this
is the method that we're

920
00:44:36,530 --> 00:44:38,470
going to use, and we're going
to say this gets us close

921
00:44:38,470 --> 00:44:41,070
enough such that we can make
comparisons and have a

922
00:44:41,070 --> 00:44:43,870
meaningful conversations about
different types of ionic bonds

923
00:44:43,870 --> 00:44:46,070
and the attraction
between them.

924
00:44:46,070 --> 00:44:49,500
But let's think about where this
discrepancy comes from,

925
00:44:49,500 --> 00:44:53,710
and before I do that I want to
point out, one term we use a

926
00:44:53,710 --> 00:44:57,190
lot is change in energy for a
reaction where, for example,

927
00:44:57,190 --> 00:44:58,640
you break a bond.

928
00:44:58,640 --> 00:45:02,520
Remember that the negative of
the change in energy is what's

929
00:45:02,520 --> 00:45:04,570
called delta e sub d.

930
00:45:04,570 --> 00:45:07,630
We first saw this when we first
introduced the idea of

931
00:45:07,630 --> 00:45:08,330
covalent bonds.

932
00:45:08,330 --> 00:45:14,600
Do you remember what this term
here means, delta e sub d?

933
00:45:14,600 --> 00:45:17,750
A little bit and some no's,
which this was pre-exam, I

934
00:45:17,750 --> 00:45:19,550
understand, you still need to
review those notes, it's

935
00:45:19,550 --> 00:45:21,320
dissociation energy.

936
00:45:21,320 --> 00:45:24,950
So you get a negative energy
out by breaking the bond.

937
00:45:24,950 --> 00:45:29,360
The dissociation energy means
how much energy that bond is

938
00:45:29,360 --> 00:45:31,870
worth in terms of strength, so
it's the opposite of the

939
00:45:31,870 --> 00:45:35,350
energy you get out of breaking
the bond -- or excuse me, the

940
00:45:35,350 --> 00:45:37,680
energy that you get out
of forming the bond.

941
00:45:37,680 --> 00:45:40,090
It's the amount of energy you
need to put in to break the

942
00:45:40,090 --> 00:45:42,620
bond is dissociation energy.

943
00:45:42,620 --> 00:45:44,470
It takes this much energy
to dissociate your

944
00:45:44,470 --> 00:45:44,860
bond, excuse me.

945
00:45:44,860 --> 00:45:45,230
All right.

946
00:45:45,230 --> 00:45:48,670
So, let's take a look here at
our predictions, so I just put

947
00:45:48,670 --> 00:45:51,070
them both ways so we
don't get confused.

948
00:45:51,070 --> 00:45:53,730
The dissociation
energy is 444.

949
00:45:53,730 --> 00:45:55,570
The change in energy
for forming the

950
00:45:55,570 --> 00:45:57,630
bond is negative 444.

951
00:45:57,630 --> 00:46:00,220
We made the following
approximations, which explain

952
00:46:00,220 --> 00:46:03,350
why, in fact, we got a different
experimental energy,

953
00:46:03,350 --> 00:46:04,540
if we look at that.

954
00:46:04,540 --> 00:46:07,060
The first thing is that we
ignored any repulsive

955
00:46:07,060 --> 00:46:07,730
interactions.

956
00:46:07,730 --> 00:46:12,220
If you think about salt, it's
not just two single atoms that

957
00:46:12,220 --> 00:46:13,390
you are talking about.

958
00:46:13,390 --> 00:46:16,090
It's actually in a whole network
or whole lattice of

959
00:46:16,090 --> 00:46:18,920
other molecules, so you actually
have some other

960
00:46:18,920 --> 00:46:21,730
chlorines around that are going
to be having repulsive

961
00:46:21,730 --> 00:46:25,310
interactions with our chlorine
that we're talking about.

962
00:46:25,310 --> 00:46:27,630
We're going to ignore those,
make the approximation that

963
00:46:27,630 --> 00:46:30,840
those don't matter, at this
point, in these calculations.

964
00:46:30,840 --> 00:46:33,370
And the result for that is that
we end up with a larger

965
00:46:33,370 --> 00:46:36,670
dissociation energy than
the experimental value.

966
00:46:36,670 --> 00:46:38,750
That's because the bond is going
to be a little bit more

967
00:46:38,750 --> 00:46:41,780
broken than it was in our
calculation, because we do

968
00:46:41,780 --> 00:46:44,870
have these repulsive
interactions.

969
00:46:44,870 --> 00:46:48,040
The other thing that we did is
that we treated both sodium

970
00:46:48,040 --> 00:46:51,250
and the chlorine as
point charges.

971
00:46:51,250 --> 00:46:53,380
And this is what actually
allowed us to make this

972
00:46:53,380 --> 00:46:56,290
calculation and calculate the
Coulomb potential so easily,

973
00:46:56,290 --> 00:46:58,320
we just treated them as if
they're point charges.

974
00:46:58,320 --> 00:47:01,270
We're ignoring quantum mechanics
in this -- this is

975
00:47:01,270 --> 00:47:04,020
sort of the class where we
ignore quantum mechanics, we

976
00:47:04,020 --> 00:47:05,550
ignored it for Lewis
structures,

977
00:47:05,550 --> 00:47:06,730
we're ignoring it here.

978
00:47:06,730 --> 00:47:10,170
We will be back to paying a lot
of attention to quantum

979
00:47:10,170 --> 00:47:13,630
mechanics in lecture 14 when we
talk about MO theory, but

980
00:47:13,630 --> 00:47:15,560
for now, these are
approximations, these are

981
00:47:15,560 --> 00:47:18,220
models where we don't take
it into consideration.

982
00:47:18,220 --> 00:47:21,010
And I think you'll agree that we
come reasonably close such

983
00:47:21,010 --> 00:47:23,360
that we'll be able to make
comparisons between different

984
00:47:23,360 --> 00:47:24,580
kinds of ionic bonds.

985
00:47:24,580 --> 00:47:27,050
All right.

986
00:47:27,050 --> 00:47:29,690
So, the last thing I want to
introduce today is talking

987
00:47:29,690 --> 00:47:31,840
about polar covalent bonds.

988
00:47:31,840 --> 00:47:34,150
We've now covered the
two extremes.

989
00:47:34,150 --> 00:47:38,070
One extreme is complete total
electron sharing -- if we have

990
00:47:38,070 --> 00:47:41,560
a perfectly covalent bond,
we have perfect sharing.

991
00:47:41,560 --> 00:47:45,870
The other is electron transfer
in terms of ionic bonds.

992
00:47:45,870 --> 00:47:49,340
So when we talk about a polar
covalent bond, what we're now

993
00:47:49,340 --> 00:47:53,010
talking about is an unequal
sharing of electrons between

994
00:47:53,010 --> 00:47:54,100
two atoms.

995
00:47:54,100 --> 00:47:56,450
So, this is essentially
something we've seen before,

996
00:47:56,450 --> 00:47:59,350
we just never formally talked
about what we would call it.

997
00:47:59,350 --> 00:48:02,830
This is any time you have a
bond forming between two

998
00:48:02,830 --> 00:48:06,340
non-metals that have different
electronegativities, so, for

999
00:48:06,340 --> 00:48:10,200
example, hydrogen
choride, h c l.

1000
00:48:10,200 --> 00:48:13,880
The electronegativity for
hydrogen is 2.2, for

1001
00:48:13,880 --> 00:48:15,940
chlorine it's 3.2.

1002
00:48:15,940 --> 00:48:19,120
And in general, what we say is
we consider a difference in

1003
00:48:19,120 --> 00:48:22,330
terms of a first approximation
if the difference in

1004
00:48:22,330 --> 00:48:24,280
electronegativity
is more than 0.

1005
00:48:24,280 --> 00:48:27,560
5, so this is on the Pauling
electronegativity scale.

1006
00:48:27,560 --> 00:48:31,850
So what we end up having is we
sort of have a kind of, and

1007
00:48:31,850 --> 00:48:34,840
what we call it is a partial
negative charge on the

1008
00:48:34,840 --> 00:48:37,220
chlorine, and a partial
positive

1009
00:48:37,220 --> 00:48:38,720
charge in the hydrogen.

1010
00:48:38,720 --> 00:48:41,150
The reason we have that is
because the chlorine's more

1011
00:48:41,150 --> 00:48:43,970
electronegative, it wants to
pull more of that shared

1012
00:48:43,970 --> 00:48:45,770
electron density to itself.

1013
00:48:45,770 --> 00:48:47,900
If it has more electron density,
it's going to have a

1014
00:48:47,900 --> 00:48:50,350
little bit of a negative charge
and the hydrogen's

1015
00:48:50,350 --> 00:48:53,150
going to be left with a little
bit of a positive charge.

1016
00:48:53,150 --> 00:48:56,420
So, we can compare this, for
example to, molecular hydrogen

1017
00:48:56,420 --> 00:48:58,940
where they're going to have
that complete sharing, so

1018
00:48:58,940 --> 00:49:02,380
there's not going to be a delta
plus or a delta minus,

1019
00:49:02,380 --> 00:49:05,300
delta is going to be equal to
zero on each of the atoms.

1020
00:49:05,300 --> 00:49:09,670
They are completely sharing
their electrons.

1021
00:49:09,670 --> 00:49:13,000
And we can also explain this
in another way by talking

1022
00:49:13,000 --> 00:49:16,670
about a dipole moment where we
have a charged distribution

1023
00:49:16,670 --> 00:49:19,340
that results in this dipole,
this electric dipole.

1024
00:49:19,340 --> 00:49:22,540
And we talk about this using
the term mu, which is a

1025
00:49:22,540 --> 00:49:24,290
measurement of what
the dipole is.

1026
00:49:24,290 --> 00:49:28,610
A dipole is always written in
terms of writing an arrow from

1027
00:49:28,610 --> 00:49:31,040
the positive charge to
the negative charge.

1028
00:49:31,040 --> 00:49:33,780
In chemistry, we are always
incredibly interested in what

1029
00:49:33,780 --> 00:49:36,900
the electrons are doing, so we
tend to pay attention to them

1030
00:49:36,900 --> 00:49:37,900
in terms of arrows.

1031
00:49:37,900 --> 00:49:40,500
Oh, the electrons are going over
to the chlorine, so we're

1032
00:49:40,500 --> 00:49:43,290
going to draw our arrow toward
the chlorine atom.

1033
00:49:43,290 --> 00:49:47,530
So, we measure this here, so mu
is equal to q times r, the

1034
00:49:47,530 --> 00:49:49,040
distance between the two.

1035
00:49:49,040 --> 00:49:52,960
And q, that charge is just equal
to the partial negative

1036
00:49:52,960 --> 00:49:57,460
or the partial positive times
the charge on the electron.

1037
00:49:57,460 --> 00:50:01,540
So this is measured in Coulomb
meters, you won't ever see a

1038
00:50:01,540 --> 00:50:03,990
measurement of electronegativity
in Coulomb

1039
00:50:03,990 --> 00:50:06,700
meters -- we tend to talk about
it in terms of debye or

1040
00:50:06,700 --> 00:50:12,310
1 d, or sometimes there's no
units at all, so the d is just

1041
00:50:12,310 --> 00:50:15,970
assumed, and it's because 1
debye is just equal to this

1042
00:50:15,970 --> 00:50:18,770
very tiny number of Coulomb
meters and it's a lot easier

1043
00:50:18,770 --> 00:50:21,850
to work with debye's here.

1044
00:50:21,850 --> 00:50:25,470
So, when we talk about polar
molecules, we can actually

1045
00:50:25,470 --> 00:50:29,040
extend our idea of talking about
polar bonds to talking

1046
00:50:29,040 --> 00:50:30,170
about polar molecules.

1047
00:50:30,170 --> 00:50:32,860
So, actually let's start
with that on Monday.

1048
00:50:32,860 --> 00:50:35,380
So everyone have a
great weekend.