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PROFESSOR: All right,
let's get started.

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Could everyone take
10 more seconds

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00:00:25,880 --> 00:00:27,360
on the clicker question.

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00:00:27,360 --> 00:00:35,250
And as a reminder, hopefully I
don't need to remind any of

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you, but exam 1 is on Wednesday,
so rather than our

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clicker question being on
something from last class,

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which is exam 2 material, let's
just make sure everyone

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00:00:44,120 --> 00:00:48,250
remembers some small topic from
exam 1 material, which is

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00:00:48,250 --> 00:00:50,170
the idea of angular nodes.

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00:00:50,170 --> 00:00:54,160
So I was hoping to see more like
99% on this, but you do

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still have two more days
before the exam.

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00:00:56,590 --> 00:00:59,410
Remember, we're talking about
angular nodes here, so you

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need to read the question
carefully.

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For an angular node, we're just
talking about what the l

22
00:01:05,380 --> 00:01:08,290
value is, so whatever l is
equal to is equal to the

23
00:01:08,290 --> 00:01:10,410
number of angular
nodes you have.

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00:01:10,410 --> 00:01:14,800
For an f orbital, what is the
quantum number l equal to?

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00:01:14,800 --> 00:01:15,340
Three.

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00:01:15,340 --> 00:01:18,370
Good, so everyone that
recognized that probably got

27
00:01:18,370 --> 00:01:20,880
the right answer of three
angular nodes here.

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00:01:20,880 --> 00:01:23,370
So let's switch to
today's notes.

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00:01:23,370 --> 00:01:26,580
Two more quick things about the
exam, the first is that

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00:01:26,580 --> 00:01:29,020
just remember on Wednesday,
don't come here, the exam is

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00:01:29,020 --> 00:01:30,980
not here, don't come here.

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00:01:30,980 --> 00:01:33,580
It's in Walker, so make
sure you go to Walker.

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00:01:33,580 --> 00:01:36,480
And also, keep in mind that
I have office hours

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00:01:36,480 --> 00:01:38,220
today from 3 to 5.

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00:01:38,220 --> 00:01:40,460
All your TA's have either
already have their extra

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office hours, or there are some
that will be going on

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00:01:43,090 --> 00:01:45,650
tonight or tomorrow, so keep
those in mind as your

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00:01:45,650 --> 00:01:48,250
finishing up your studying
for the exam.

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00:01:48,250 --> 00:01:51,340
So, today we're moving on,
we're talking about Lewis

40
00:01:51,340 --> 00:01:52,340
structures.

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00:01:52,340 --> 00:01:55,550
This is a really good topic to
do the class before an exam

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00:01:55,550 --> 00:01:57,700
because it's a little bit
of a lighter topic.

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00:01:57,700 --> 00:02:00,250
We remember that Lewis
structures are an idea that

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00:02:00,250 --> 00:02:02,190
are pre-quantum mechanics.

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00:02:02,190 --> 00:02:05,130
So that means that we don't
have to worry about things

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00:02:05,130 --> 00:02:07,170
like wave functions when we're
talking about Lewis

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00:02:07,170 --> 00:02:09,980
structures, but because they're
so simple to use and

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00:02:09,980 --> 00:02:13,930
because they so often predict
the electron configuration of

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00:02:13,930 --> 00:02:17,100
molecules accurately, we end up
using them all the time in

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00:02:17,100 --> 00:02:19,610
chemistry, so it's very valuable
to know how to draw

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00:02:19,610 --> 00:02:22,830
them correctly and to know
how to work with them.

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00:02:22,830 --> 00:02:25,760
So we'll talk specifically about
drawing Lewis structures

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00:02:25,760 --> 00:02:28,520
and then about formal charge
and resonance, which are

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00:02:28,520 --> 00:02:31,060
within Lewis structures.

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00:02:31,060 --> 00:02:33,460
So remember, that when we talked
about Lewis structure,

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00:02:33,460 --> 00:02:36,640
the organizing principle behind
Lewis structures is the

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00:02:36,640 --> 00:02:40,420
idea that within the molecule
the atoms are going to arrange

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00:02:40,420 --> 00:02:43,930
their valence electrons, such
that each atom within the

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00:02:43,930 --> 00:02:48,320
molecule has a complete octet
or full outer shell.

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So this is the idea of the octet
rule that Lewis came up

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with way back in 1902.

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00:02:54,360 --> 00:02:58,480
So, at the end of the class on
Friday, we saw that list of

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eight steps that you always need
to go through when you

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draw a Lewis structure.

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00:03:02,300 --> 00:03:06,140
Once you're doing this on your
own, especially, for example,

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00:03:06,140 --> 00:03:09,010
on exam 2, which is a ways down
the road, you won't be

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00:03:09,010 --> 00:03:11,510
able to look at those steps, so
you need to make sure that

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you can go through them without
looking at them, but

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00:03:13,800 --> 00:03:16,820
for now we can look at them as
we are actually learning how

70
00:03:16,820 --> 00:03:19,050
to draw the Lewis structures,
and rather just go through

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00:03:19,050 --> 00:03:21,250
them step-by-step, it's
more interesting to

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00:03:21,250 --> 00:03:22,700
do it with an example.

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00:03:22,700 --> 00:03:25,770
So let's hydrogen cyanide
as our first example.

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00:03:25,770 --> 00:03:27,780
So we have h c n.

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00:03:27,780 --> 00:03:31,360
So, in this example here, we
start with the first step --

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the first step in any Lewis
structure is drawing the

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00:03:34,310 --> 00:03:35,940
skeletal structure.

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00:03:35,940 --> 00:03:39,520
So, essentially drawing how the
atoms are arranged within

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our molecule.

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And in this case we have three
choices here in terms of

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what's going to be in the
middle, so we need to decide

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00:03:46,730 --> 00:03:48,040
that first.

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00:03:48,040 --> 00:03:51,640
In terms of where different
atoms are in a molecule, if

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you have a hydrogen atom or a
fluorine atom, you can pretty

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00:03:55,270 --> 00:03:58,120
much guarantee they're always
going to be terminal atoms.

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00:03:58,120 --> 00:04:00,620
By terminal I mean they're
only bonded to one thing.

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00:04:00,620 --> 00:04:03,080
So, for example, hydrogen or
fluorine they'll never be in

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00:04:03,080 --> 00:04:05,500
the middle, they'll always be
on the end of a molecule.

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00:04:05,500 --> 00:04:07,750
So that takes care of the
hydrogen, what about between

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00:04:07,750 --> 00:04:09,250
the carbon and the nitrogen?

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00:04:09,250 --> 00:04:12,100
In terms of picking a Lewis
structure that's going to be

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00:04:12,100 --> 00:04:15,340
the lowest energy, what you
want to do is put the atom

93
00:04:15,340 --> 00:04:17,730
with the lowest ionization
energy in the

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00:04:17,730 --> 00:04:19,300
center of your atom.

95
00:04:19,300 --> 00:04:21,840
This should make sense because
if something has a low

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00:04:21,840 --> 00:04:25,160
ionization energy, that means
it's not very electronegative,

97
00:04:25,160 --> 00:04:27,500
which means it's going to be a
lot happier giving up electron

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00:04:27,500 --> 00:04:30,020
density, which is essentially
what you're doing when you're

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00:04:30,020 --> 00:04:32,450
forming covalent bonds is you're
sharing some of your

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00:04:32,450 --> 00:04:33,740
electron density.

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00:04:33,740 --> 00:04:37,000
So, we keep the atoms with the
lowest ionization energy in

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00:04:37,000 --> 00:04:37,890
the center.

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00:04:37,890 --> 00:04:41,300
So, why don't you go ahead and
tell me, keeping that in mind,

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00:04:41,300 --> 00:04:45,910
which atom in terms of h c or
n would you expect to be in

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00:04:45,910 --> 00:04:49,550
the center of hydrogen
cyanide?

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00:04:49,550 --> 00:04:52,670
And I put a periodic table up
there, just the part you might

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00:04:52,670 --> 00:04:53,450
need to look at.

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00:04:53,450 --> 00:05:07,950
So this should be fast, so
let's take 10 seconds.

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00:05:07,950 --> 00:05:09,280
All right, good job everyone.

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00:05:09,280 --> 00:05:13,190
So most of you saw that carbon
should be in the center.

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00:05:13,190 --> 00:05:15,680
Carbon should be in the center
because it has the lowest

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00:05:15,680 --> 00:05:17,130
ionization energy.

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We know that ionization energy
is going to increase as we go

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across the periodic table, so
that means carbon has a lower

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00:05:24,770 --> 00:05:26,810
ionization energy than
nitrogen, which is

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00:05:26,810 --> 00:05:28,170
right next to us.

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So as I just said, we want to
put that one in the middle.

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00:05:30,920 --> 00:05:34,230
You got an extra hint here in
terms of the order, so even if

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00:05:34,230 --> 00:05:36,720
you had just forgotten what I
said, sometimes it's not a

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00:05:36,720 --> 00:05:38,970
terrible idea just to put it
in the order it's written,

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00:05:38,970 --> 00:05:41,240
that can give you a lot
of clues as well.

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00:05:41,240 --> 00:05:43,820
So, either of those ways of
figuring this out is the first

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00:05:43,820 --> 00:05:46,490
guess of what goes in the middle
will work pretty well.

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00:05:46,490 --> 00:05:49,570
So, let's go ahead and draw our
Lewis structure based on

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00:05:49,570 --> 00:05:53,800
the rest of the rules now
that we have a skeleton.

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00:05:53,800 --> 00:05:57,210
So our skeleton tells us that
carbon is in the middle, so

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00:05:57,210 --> 00:06:00,310
we'll put the h on one
side, and the n on

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00:06:00,310 --> 00:06:02,650
the other side there.

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00:06:02,650 --> 00:06:06,070
So, our second step, as we go
through our Lewis structure

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00:06:06,070 --> 00:06:09,420
rules, is to figure out how many
valence electrons we have

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00:06:09,420 --> 00:06:11,510
in our entire molecule.

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00:06:11,510 --> 00:06:14,340
So if we talk about hydrogen,
how many valence electrons are

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00:06:14,340 --> 00:06:16,390
we talking about?

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00:06:16,390 --> 00:06:17,380
1.

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00:06:17,380 --> 00:06:20,550
What about carbon?

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00:06:20,550 --> 00:06:21,760
I heard 4 and 6.

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00:06:21,760 --> 00:06:23,770
And remember, we're only
talking about valence

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00:06:23,770 --> 00:06:26,360
electrons, so the outer-most
shells.

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00:06:26,360 --> 00:06:30,360
So we're talking about four
valence electrons for carbon.

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00:06:30,360 --> 00:06:34,010
And then for nitrogen?

141
00:06:34,010 --> 00:06:37,260
Lots of options I have to
choose from from these

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00:06:37,260 --> 00:06:38,870
answers, but it's 5.

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00:06:38,870 --> 00:06:42,000
So, if you can't immediately
know, and you don't all have

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00:06:42,000 --> 00:06:44,210
periodic tables in front of you,
so that's fine, but if

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00:06:44,210 --> 00:06:46,940
you have a periodic table in
front of you, you need to be

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00:06:46,940 --> 00:06:49,210
able to count valence electrons,
so work on that if

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00:06:49,210 --> 00:06:51,450
it doesn't come naturally
to you in terms of

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00:06:51,450 --> 00:06:52,300
figuring that out.

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00:06:52,300 --> 00:06:56,250
So then in order to figure out
the complete number of valence

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00:06:56,250 --> 00:07:00,300
electrons in our molecule, we
just add 5 plus 4 plus 1.

151
00:07:00,300 --> 00:07:02,610
So we end up having 10
valence electrons.

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00:07:02,610 --> 00:07:09,100
Step three in our Lewis
structure rules is to figure

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00:07:09,100 --> 00:07:12,560
out how many electronis we would
need in order for every

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00:07:12,560 --> 00:07:16,260
single atom in our molecule to
have a full valence shell.

155
00:07:16,260 --> 00:07:18,980
So, if we're talking about
hydrogen, that's our one

156
00:07:18,980 --> 00:07:21,520
exception so far to
the octet rule.

157
00:07:21,520 --> 00:07:24,670
So we actually only need two
electrons to fill up the

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00:07:24,670 --> 00:07:27,110
valence shell of hydrogen,
remember that's because all we

159
00:07:27,110 --> 00:07:29,640
need to fill up is the 1 s.

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00:07:29,640 --> 00:07:34,090
However, for carbon and nitrogen
we need 8 each.

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00:07:34,090 --> 00:07:37,050
So in terms of total numbers
that we would need to complete

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00:07:37,050 --> 00:07:40,510
our octets and fill our valence
shells, we would need

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00:07:40,510 --> 00:07:48,650
18 electrons.

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00:07:48,650 --> 00:07:48,980
All right.

165
00:07:48,980 --> 00:07:51,970
So let's bring down our
middle slide here.

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00:07:51,970 --> 00:07:54,890
So we have 18 electrons, and the
next thing that we need to

167
00:07:54,890 --> 00:07:57,780
figure out is how many bonding
electrons we have.

168
00:07:57,780 --> 00:08:00,500
So to figure out bonding
electrons, what we take is

169
00:08:00,500 --> 00:08:04,220
that number 18, which is our
total number of electrons we

170
00:08:04,220 --> 00:08:07,450
need to fill valence shells,
and we subtract it from our

171
00:08:07,450 --> 00:08:11,110
number of valence electrons,
which is 10.

172
00:08:11,110 --> 00:08:18,570
And what we find that we have
is 8 bonding electrons.

173
00:08:18,570 --> 00:08:23,810
And hopefully on your paper, you
can actually reach your h

174
00:08:23,810 --> 00:08:24,750
c n skeleton --

175
00:08:24,750 --> 00:08:27,940
I think I should probably
re-draw mine here.

176
00:08:27,940 --> 00:08:31,710
Because step five is that we
need to fill in our bonding

177
00:08:31,710 --> 00:08:33,920
electrons, and we start
it with filling in two

178
00:08:33,920 --> 00:08:35,850
electrons per bond.

179
00:08:35,850 --> 00:08:39,800
So I'm just going to re-draw
my skeleton.

180
00:08:39,800 --> 00:08:44,420
So, the first thing we do is
put two electrons between h

181
00:08:44,420 --> 00:08:47,970
and c, and then two electrons
between c and n.

182
00:08:47,970 --> 00:08:50,210
Remember, every time we have two
electrons that are being

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00:08:50,210 --> 00:08:54,400
shared, that's a single bond.

184
00:08:54,400 --> 00:08:56,930
The next thing that we want to
do is figure out do we have

185
00:08:56,930 --> 00:08:59,400
any bonding electrons left.

186
00:08:59,400 --> 00:09:03,090
So let's see, we started with
8 bonding electrons, and we

187
00:09:03,090 --> 00:09:08,990
used up only 4, so the answer
is yes, we have 4 bonding

188
00:09:08,990 --> 00:09:10,790
electrons left.

189
00:09:10,790 --> 00:09:13,350
So what we need to do is fill
in those extra bonding

190
00:09:13,350 --> 00:09:15,810
electrons into our bonds.

191
00:09:15,810 --> 00:09:17,850
Should I put an extra
pair of electrons

192
00:09:17,850 --> 00:09:19,890
here, does anyone think?

193
00:09:19,890 --> 00:09:20,590
No.

194
00:09:20,590 --> 00:09:22,850
The reason is because we already
have a full valence

195
00:09:22,850 --> 00:09:24,220
shell for our hydrogen,
it doesn't

196
00:09:24,220 --> 00:09:25,590
want anymore electrons.

197
00:09:25,590 --> 00:09:28,600
What about between the
carbon and nitrogen?

198
00:09:28,600 --> 00:09:29,080
Yes.

199
00:09:29,080 --> 00:09:31,840
Definitely, because both of
these are not anywhere near

200
00:09:31,840 --> 00:09:33,670
filling up their octets yet.

201
00:09:33,670 --> 00:09:37,820
So we can put actually all 4
of our extra electrons in

202
00:09:37,820 --> 00:09:39,780
between the carbon
and the nitrogen.

203
00:09:39,780 --> 00:09:44,960
Now we have 6 things around the
nitrogen, and we have 8

204
00:09:44,960 --> 00:09:47,700
around the carbon.

205
00:09:47,700 --> 00:09:51,970
So, what we do as our seventh
step is then figure out if we

206
00:09:51,970 --> 00:09:56,840
have any extra valence electrons
left at all.

207
00:09:56,840 --> 00:10:01,280
So we started with 10 valence
electrons, we used up 8 of

208
00:10:01,280 --> 00:10:04,600
those electrons in terms
of making bonds.

209
00:10:04,600 --> 00:10:08,380
So it turns out that we have
2 valence electrons left.

210
00:10:08,380 --> 00:10:11,920
So we need to add those 2
valence electrons left as lone

211
00:10:11,920 --> 00:10:14,010
pair electrons in
our structure.

212
00:10:14,010 --> 00:10:17,070
So, which atom is in need of
those lone pair electrons?

213
00:10:17,070 --> 00:10:19,290
The nitrogen.

214
00:10:19,290 --> 00:10:21,500
The reason being that's the only
one that didn't have a

215
00:10:21,500 --> 00:10:22,930
full octet yet.

216
00:10:22,930 --> 00:10:26,940
So now we're done, actually
there is one more step, which

217
00:10:26,940 --> 00:10:29,650
is to determine the
formal charge.

218
00:10:29,650 --> 00:10:32,760
This is a good way to actually
check if your Lewis structure

219
00:10:32,760 --> 00:10:34,170
is correct or not.

220
00:10:34,170 --> 00:10:37,490
We haven't actually learned how
to calculate the formal

221
00:10:37,490 --> 00:10:39,830
charge yet, we'll
learn it soon.

222
00:10:39,830 --> 00:10:42,650
So we won't do it for this
molecule, but we'll go back

223
00:10:42,650 --> 00:10:44,730
and do it for some of our other
examples, and you can go

224
00:10:44,730 --> 00:10:46,380
back and do it for this one.

225
00:10:46,380 --> 00:10:48,560
The other thing is that we
can re-write our h c

226
00:10:48,560 --> 00:10:50,860
n in terms of bonds.

227
00:10:50,860 --> 00:10:53,970
So we know every time we have
two electrons, that's a bond.

228
00:10:53,970 --> 00:10:57,130
So we have h, then we can
draw our bond as a line.

229
00:10:57,130 --> 00:10:59,870
And then we have a triple bond
there because we have 3 pairs

230
00:10:59,870 --> 00:11:01,750
of electrons.

231
00:11:01,750 --> 00:11:07,010
So it looks a lot less messy
if we just draw our Lewis

232
00:11:07,010 --> 00:11:11,540
structure like this for h c n,
where we have h bonded to c

233
00:11:11,540 --> 00:11:14,650
triple, bonded to n, and
then a lone pair on

234
00:11:14,650 --> 00:11:17,020
the nitrogen there.

235
00:11:17,020 --> 00:11:19,220
All right, so this is the same
procedure that we're going to

236
00:11:19,220 --> 00:11:21,940
go through, regardless of what
kind of Lewis structure we're

237
00:11:21,940 --> 00:11:22,850
going to draw.

238
00:11:22,850 --> 00:11:25,810
What you'll actually find in
terms of asking your TAs about

239
00:11:25,810 --> 00:11:28,590
the Lewis structure rules is
that sometimes they won't be

240
00:11:28,590 --> 00:11:31,350
as good at them as you are, and
the reason is once you've

241
00:11:31,350 --> 00:11:33,480
drawn enough of these
structures, you start to get a

242
00:11:33,480 --> 00:11:36,280
lot of chemical intuition about
what's right or what's

243
00:11:36,280 --> 00:11:39,090
not right -- it just looks wrong
to you if it's wrong.

244
00:11:39,090 --> 00:11:41,600
So your TA might take a minute,
so be patient with

245
00:11:41,600 --> 00:11:43,510
them if they see your structure
and they say oh, no,

246
00:11:43,510 --> 00:11:45,580
no, no, no that's wrong, that's
terrible, and they

247
00:11:45,580 --> 00:11:46,860
don't immediately know why.

248
00:11:46,860 --> 00:11:48,840
They might need to go through
the rules with you, you might

249
00:11:48,840 --> 00:11:49,540
need to remind them.

250
00:11:49,540 --> 00:11:51,130
Hopefully, they'll all
study them again, so

251
00:11:51,130 --> 00:11:52,470
this will be an issue.

252
00:11:52,470 --> 00:11:55,920
But what really happens is as
you go on in chemistry, you

253
00:11:55,920 --> 00:11:58,180
draw so many of these you can
just draw them without

254
00:11:58,180 --> 00:11:59,730
following the rules.

255
00:11:59,730 --> 00:12:01,920
Some of you might get almost to
that point, or you might be

256
00:12:01,920 --> 00:12:05,010
at that point now, but I
recommend this for you and for

257
00:12:05,010 --> 00:12:08,640
me and for the TAs, go through
the rules because there'll be

258
00:12:08,640 --> 00:12:11,790
cases where it's a little bit
tricky and it's always much

259
00:12:11,790 --> 00:12:15,310
faster to have gone through
step-by-step, than to try to

260
00:12:15,310 --> 00:12:18,170
just kind of hit or miss figure
out what's going to be

261
00:12:18,170 --> 00:12:19,680
right or wrong.

262
00:12:19,680 --> 00:12:24,480
So, let's try another example
here, and let's try a case now

263
00:12:24,480 --> 00:12:27,260
where instead of dealing with a
neutral molecule we have an

264
00:12:27,260 --> 00:12:30,700
ion, so we have c n minus.

265
00:12:30,700 --> 00:12:33,360
And what I'll mention to you
just in terms of the fact that

266
00:12:33,360 --> 00:12:37,390
we're finally dealing with real
molecules, which is -- or

267
00:12:37,390 --> 00:12:39,770
molecules that are made up of
more than one atom, which is

268
00:12:39,770 --> 00:12:42,680
kind of exciting for me and
maybe for some other of you

269
00:12:42,680 --> 00:12:46,390
that like to move into thinking
about what some of

270
00:12:46,390 --> 00:12:49,760
the consequences of these
molecules reacting might be.

271
00:12:49,760 --> 00:12:51,750
A lot of the examples that we're
going to give you in

272
00:12:51,750 --> 00:12:55,450
terms of trying out your Lewis
structures will be molecule

273
00:12:55,450 --> 00:12:58,010
that are used in organic
synthesis, or maybe they're

274
00:12:58,010 --> 00:13:00,360
molecules that react in
interesting ways with

275
00:13:00,360 --> 00:13:03,630
biomolecules in your body or
proteins in your body.

276
00:13:03,630 --> 00:13:06,660
So, you already will have a head
start when you get on to

277
00:13:06,660 --> 00:13:09,120
later classes, like organic
chemistry or if you're

278
00:13:09,120 --> 00:13:12,480
thinking about biochemistry
where being able to draw the

279
00:13:12,480 --> 00:13:15,490
Lewis structure allows you to
think about, eventually, the

280
00:13:15,490 --> 00:13:18,290
reactivity of the molecule,
which becomes very interesting

281
00:13:18,290 --> 00:13:20,760
in thinking about how you're
going to synthesize a more

282
00:13:20,760 --> 00:13:23,790
complex molecule, or how that
molecule is going to interact

283
00:13:23,790 --> 00:13:26,580
with an active site in a
protein in the body.

284
00:13:26,580 --> 00:13:30,300
So, for example, just talking
about hydrogen cyanide or the

285
00:13:30,300 --> 00:13:34,750
cyanide anion, these are both
molecules which are used in

286
00:13:34,750 --> 00:13:37,780
organic synthesis, so
particularly the cyanide,

287
00:13:37,780 --> 00:13:41,320
anion and salts of the
cyanide anion.

288
00:13:41,320 --> 00:13:45,920
So either a potassium cyanide
or sodium cyanide, these are

289
00:13:45,920 --> 00:13:49,980
used in synthesis in terms of
making carbon-carbon bonds.

290
00:13:49,980 --> 00:13:52,120
So if you're trying to make
a more complicated organic

291
00:13:52,120 --> 00:13:55,160
molecule, carbon-carbon bonds
are one of the most difficult

292
00:13:55,160 --> 00:13:57,880
things to make an organic
chemistry, and it turns out

293
00:13:57,880 --> 00:14:00,900
that c n minus is a very
reactive molecule, so it's a

294
00:14:00,900 --> 00:14:04,460
good way, even though we'll go
over some drawbacks in a

295
00:14:04,460 --> 00:14:07,450
second, it is a good way to
make carbon-carbon bonds.

296
00:14:07,450 --> 00:14:08,550
It's very reactive.

297
00:14:08,550 --> 00:14:12,100
And because, of course, we have
this carbon here what you

298
00:14:12,100 --> 00:14:15,580
end up doing is adding a carbon
to your molecule.

299
00:14:15,580 --> 00:14:19,460
So, when you think about
cyanide, you might not think

300
00:14:19,460 --> 00:14:21,560
about organic reagents.

301
00:14:21,560 --> 00:14:23,370
Does anyone have something else
they think about when

302
00:14:23,370 --> 00:14:24,230
they think about cyanide?

303
00:14:24,230 --> 00:14:25,600
STUDENT: Death.

304
00:14:25,600 --> 00:14:28,370
PROFESSOR: Death -- that's
a good thought.

305
00:14:28,370 --> 00:14:31,980
Yes, cyanide and death, very
closely related as well.

306
00:14:31,980 --> 00:14:34,490
Cyanide is incredibly toxic,
it's a poison.

307
00:14:34,490 --> 00:14:37,280
That might be how you're more
familiar with cyanide.

308
00:14:37,280 --> 00:14:40,110
So if you're working with
cyanide in the lab as

309
00:14:40,110 --> 00:14:43,320
potassium cyanide or sodium
cyanide, those are what are

310
00:14:43,320 --> 00:14:46,660
called p h s's, or particularly
hazardous

311
00:14:46,660 --> 00:14:49,100
substances -- it's a rating for

312
00:14:49,100 --> 00:14:50,780
different kinds of chemicals.

313
00:14:50,780 --> 00:14:52,950
And what that means it's
there's all sorts of

314
00:14:52,950 --> 00:14:55,930
precautions and procedures you
take that are special when you

315
00:14:55,930 --> 00:14:56,980
deal with these.

316
00:14:56,980 --> 00:14:59,120
They're kept away from
other chemicals.

317
00:14:59,120 --> 00:15:02,880
You handle them very special
in terms of being extra

318
00:15:02,880 --> 00:15:07,330
careful in a very high
ventilation area, in hoods is

319
00:15:07,330 --> 00:15:08,490
how you handle them.

320
00:15:08,490 --> 00:15:13,050
So, yes, they're very poisonous,
and in fact, there

321
00:15:13,050 --> 00:15:16,880
are areas where you find this
toxic compound, cyanide.

322
00:15:16,880 --> 00:15:22,330
Other than just in poisons and
in organic synthesis shells,

323
00:15:22,330 --> 00:15:24,870
you might also find them in
some things we're more

324
00:15:24,870 --> 00:15:26,360
familiar with, such
as almonds.

325
00:15:26,360 --> 00:15:29,260
I don't know how many know that
there are trace , trace

326
00:15:29,260 --> 00:15:31,900
amounts, of cyanide
in almonds.

327
00:15:31,900 --> 00:15:34,530
I don't know if there any big
almond eaters out there.

328
00:15:34,530 --> 00:15:37,070
You don't have to worry, we're
definitely talking about

329
00:15:37,070 --> 00:15:38,160
trace, trace amounts.

330
00:15:38,160 --> 00:15:39,790
It's not going to hurt you.

331
00:15:39,790 --> 00:15:42,720
And actually, what we usually
eat are what are called sweet

332
00:15:42,720 --> 00:15:47,090
almonds, and there aren't actual
cyanide in the sweet

333
00:15:47,090 --> 00:15:49,350
almonds we eat, there's
precursors to cyanide, which

334
00:15:49,350 --> 00:15:50,640
might not make you
more comfortable.

335
00:15:50,640 --> 00:15:52,840
But the fact is there are
trace, trace amounts.

336
00:15:52,840 --> 00:15:56,150
This is nothing that we need to
worry in our food supply.

337
00:15:56,150 --> 00:15:59,130
However, some people
aren't so lucky.

338
00:15:59,130 --> 00:16:01,100
I don't know how many of you are
familiar with the Cassava

339
00:16:01,100 --> 00:16:05,070
plant, which is a kind of woody
shrub that's first been

340
00:16:05,070 --> 00:16:08,050
cultivated in South America,
but it's grown throughout

341
00:16:08,050 --> 00:16:12,020
Africa, the Caribbean, South
America still, many places

342
00:16:12,020 --> 00:16:15,450
around the world, and this
is a major source of

343
00:16:15,450 --> 00:16:18,590
carbohydrates for much of the
world, because Cassava root is

344
00:16:18,590 --> 00:16:21,140
very, very rich in
carbohydrates.

345
00:16:21,140 --> 00:16:24,940
It's not the best form of food
in that they're actually very

346
00:16:24,940 --> 00:16:28,820
poor in protein, and
unfortunately very, very rich

347
00:16:28,820 --> 00:16:30,180
in cyanide.

348
00:16:30,180 --> 00:16:32,080
So, these roots can
be very dangerous.

349
00:16:32,080 --> 00:16:35,010
There's different types of the
root that you can get called

350
00:16:35,010 --> 00:16:37,120
the bitter and the sweet.

351
00:16:37,120 --> 00:16:39,710
Hopefully you would all choose
the sweet if two are put in

352
00:16:39,710 --> 00:16:40,580
front of you.

353
00:16:40,580 --> 00:16:42,200
The bitter, of course,
are the ones that are

354
00:16:42,200 --> 00:16:43,800
very high in cyanide.

355
00:16:43,800 --> 00:16:46,450
If you eat these raw, which they
do in many places around

356
00:16:46,450 --> 00:16:49,740
the world, if you eat a bitter
one, you could, in fact, get

357
00:16:49,740 --> 00:16:51,670
enough cyanide to kill you.

358
00:16:51,670 --> 00:16:54,390
And there are ways to prepare
these, so it's important --

359
00:16:54,390 --> 00:16:57,560
this kind of thinking more along
the food science idea.

360
00:16:57,560 --> 00:16:59,930
There's a way to actually grind
down and prepare the

361
00:16:59,930 --> 00:17:03,180
flower, so that you promote the
enzymes within the plant

362
00:17:03,180 --> 00:17:05,950
to breakdown the cyanide
precursors.

363
00:17:05,950 --> 00:17:08,080
And if you put this in the
well-ventilated area, if you

364
00:17:08,080 --> 00:17:11,980
prepare this outside, the h
c n gas will actually be

365
00:17:11,980 --> 00:17:14,270
released into the air,
so you're safe,

366
00:17:14,270 --> 00:17:15,730
you can eat it later.

367
00:17:15,730 --> 00:17:18,970
About 80% of the cyanide at that
point is gone, so it does

368
00:17:18,970 --> 00:17:20,720
render the root much
more safe.

369
00:17:20,720 --> 00:17:23,540
But you do, in fact, have
to worry about long-term

370
00:17:23,540 --> 00:17:26,950
exposure, cyanide poisoning in
terms of long-term effects in

371
00:17:26,950 --> 00:17:29,070
certain populations that do
get the bulk of their

372
00:17:29,070 --> 00:17:31,460
carbohydrates from this
root, from the root

373
00:17:31,460 --> 00:17:33,080
of the Cassava plant.

374
00:17:33,080 --> 00:17:35,680
But in terms of us going to the
grocery store and thinking

375
00:17:35,680 --> 00:17:39,010
about things, probably we're all
breathing sighs of relief.

376
00:17:39,010 --> 00:17:40,470
I just told you almonds
are not a

377
00:17:40,470 --> 00:17:42,280
problem, no worries there.

378
00:17:42,280 --> 00:17:45,830
We probably don't find any forms
of the Cassava plant

379
00:17:45,830 --> 00:17:48,540
ever in the U.S. that are
going to have that high

380
00:17:48,540 --> 00:17:52,630
cyanide content, so we should
all be relieved.

381
00:17:52,630 --> 00:17:55,910
Unless, of course, you're a
smoker, or you're thinking of

382
00:17:55,910 --> 00:17:58,670
becoming a smoker, and then
maybe you should worry,

383
00:17:58,670 --> 00:18:01,850
because this is one of the
advertisements that was airing

384
00:18:01,850 --> 00:18:04,260
in terms of anti-smoking
campaign.

385
00:18:04,260 --> 00:18:08,730
Hydrogen cyanide is found
in cigarettes.

386
00:18:08,730 --> 00:18:11,740
So, if you're looking for
another reason to quit, if

387
00:18:11,740 --> 00:18:14,490
you're looking for a reason not
to start smoking, here's

388
00:18:14,490 --> 00:18:15,330
another good one.

389
00:18:15,330 --> 00:18:18,970
As I said, it's a particularly
hazardous substance, this is

390
00:18:18,970 --> 00:18:20,040
worked with in fume hoods.

391
00:18:20,040 --> 00:18:22,230
You don't want to inhale
it, it's definitely not

392
00:18:22,230 --> 00:18:23,750
recommended.

393
00:18:23,750 --> 00:18:27,260
The way, in the simplest terms
that cyanide can kill you, is

394
00:18:27,260 --> 00:18:30,110
it basically out-competes
your oxygen for the

395
00:18:30,110 --> 00:18:31,500
heme in your blood.

396
00:18:31,500 --> 00:18:34,580
So instead of carrying oxygen to
your cells, you're carrying

397
00:18:34,580 --> 00:18:36,700
cyanide to your cells.

398
00:18:36,700 --> 00:18:39,170
Obviously, the amounts that
are in cigarettes are not

399
00:18:39,170 --> 00:18:41,210
enough that people are dropping
dead of cyanide

400
00:18:41,210 --> 00:18:44,800
poisoning, but still it's not
a good idea if you can avoid

401
00:18:44,800 --> 00:18:47,730
eating or inhaling cyanide
-- you definitely want to

402
00:18:47,730 --> 00:18:49,610
minimize your exposure.

403
00:18:49,610 --> 00:18:53,920
And in terms of thinking about
it for organic chemistry or if

404
00:18:53,920 --> 00:18:56,880
you're interested in thinking
about the mechanism maybe by

405
00:18:56,880 --> 00:19:00,230
which it is toxic, a first
step would be to draw its

406
00:19:00,230 --> 00:19:01,770
Lewis structure.

407
00:19:01,770 --> 00:19:05,470
So, let's go ahead and make sure
we can draw that, if we

408
00:19:05,470 --> 00:19:08,200
have interest either in the area
of organic chemistry or

409
00:19:08,200 --> 00:19:10,670
biochemistry or biology here.

410
00:19:10,670 --> 00:19:14,300
So in terms of the first step of
skeletal structure, this is

411
00:19:14,300 --> 00:19:17,370
actually going to be easier
because we don't have a

412
00:19:17,370 --> 00:19:23,440
central atom, we just have
carbon and nitrogen here.

413
00:19:23,440 --> 00:19:27,160
Our next step is thinking
about valence electrons.

414
00:19:27,160 --> 00:19:32,440
So we have 4 plus 5, but we're
actually not done yet, because

415
00:19:32,440 --> 00:19:36,190
it's c n minus, so if we have
minus, we actually have an

416
00:19:36,190 --> 00:19:38,780
extra electron in
our molecule.

417
00:19:38,780 --> 00:19:40,650
So we need to add 1 more.

418
00:19:40,650 --> 00:19:45,660
If instead we had a positive
ion, a cation, what we would

419
00:19:45,660 --> 00:19:47,890
have to do is subtract 1.

420
00:19:47,890 --> 00:19:50,340
But here we're going to
add 1, so again, we

421
00:19:50,340 --> 00:19:54,100
have 10 valence electrons.

422
00:19:54,100 --> 00:19:57,150
And if we go on to step three
where we figure out how many

423
00:19:57,150 --> 00:20:00,810
we would need for full octets,
it's just going to be 2 times

424
00:20:00,810 --> 00:20:04,780
8, so we have 16.

425
00:20:04,780 --> 00:20:09,710
And step four is going to have
us figure out how many bonding

426
00:20:09,710 --> 00:20:16,230
electrons we have, so we have 16
minus 10, is going to be 6

427
00:20:16,230 --> 00:20:21,150
bonding electrons.

428
00:20:21,150 --> 00:20:29,210
So, step five tells us to add 2
electrons between each atom,

429
00:20:29,210 --> 00:20:32,260
so we add two there.

430
00:20:32,260 --> 00:20:35,290
And step six asks us,
well, do we have any

431
00:20:35,290 --> 00:20:36,630
bonding electrons left?

432
00:20:36,630 --> 00:20:40,120
So how many bonding electrons
do we have left?

433
00:20:40,120 --> 00:20:42,770
Yup, so we do, we have 4 left.

434
00:20:42,770 --> 00:20:45,600
We started with 6,
we only used 2.

435
00:20:45,600 --> 00:20:48,330
This is very easy molecule
because we know exactly where

436
00:20:48,330 --> 00:20:50,600
to put them without even having
to think, we only have

437
00:20:50,600 --> 00:20:54,790
one option, and we'll make a
triple bond between the carbon

438
00:20:54,790 --> 00:20:56,850
and the nitrogen.

439
00:20:56,850 --> 00:21:00,710
So, seven asks us if we have
any valence electrons left,

440
00:21:00,710 --> 00:21:03,970
and how many valence electrons
do we have left?

441
00:21:03,970 --> 00:21:06,070
Yeah, so also 4.

442
00:21:06,070 --> 00:21:09,540
We started with 10 valence
electrons, we used up 6 of

443
00:21:09,540 --> 00:21:13,090
those as bonding electrons, so
we have 4 left, which will be

444
00:21:13,090 --> 00:21:14,980
lone pair electrons.

445
00:21:14,980 --> 00:21:19,150
So, in order to fill our octet,
what we do is put two

446
00:21:19,150 --> 00:21:23,900
on the nitrogen and
two on the carbon.

447
00:21:23,900 --> 00:21:27,030
So, in terms of finishing our
Lewis structure, we're

448
00:21:27,030 --> 00:21:29,720
actually not done yet here,
even though we have full

449
00:21:29,720 --> 00:21:33,610
octets, and we've used up all of
our valence electrons, and

450
00:21:33,610 --> 00:21:36,400
the reason is because it's c n
minus, so we need to make sure

451
00:21:36,400 --> 00:21:39,290
that that's reflected in our
Lewis structure, so let's put

452
00:21:39,290 --> 00:21:43,600
it in brackets here,
and put a minus 1.

453
00:21:43,600 --> 00:21:45,920
And also I wanted to mention
in terms of checking your

454
00:21:45,920 --> 00:21:49,230
Lewis structures, regardless of
what they are, you should

455
00:21:49,230 --> 00:21:51,080
always go back and say
how many valence

456
00:21:51,080 --> 00:21:52,370
electrons did I have --

457
00:21:52,370 --> 00:21:57,290
I had 10, and then count 2, 4,
6, 8, 10, because you always

458
00:21:57,290 --> 00:21:59,770
need to make sure you have the
same number of valence

459
00:21:59,770 --> 00:22:02,590
electrons that you calculated
in your actual structure.

460
00:22:02,590 --> 00:22:05,270
That'll catch a lot of just
silly mistakes for you if you

461
00:22:05,270 --> 00:22:08,190
go back and see it and you
don't have all of that.

462
00:22:08,190 --> 00:22:11,260
Let's re-draw this, so it looks
a little bit neater,

463
00:22:11,260 --> 00:22:16,350
where we have a triple bond in
the middle instead, and again,

464
00:22:16,350 --> 00:22:19,240
we need our negative
1 charge there.

465
00:22:19,240 --> 00:22:23,010
And our eigth step in the
process, again, is formal

466
00:22:23,010 --> 00:22:32,880
charge, which we will talk
about very soon.

467
00:22:32,880 --> 00:22:33,200
All right.

468
00:22:33,200 --> 00:22:35,850
So let's try one more example
of drawing Lewis structures

469
00:22:35,850 --> 00:22:38,100
before we talk about
formal charge.

470
00:22:38,100 --> 00:22:40,800
And the last example that we're
going to talk about is

471
00:22:40,800 --> 00:22:45,250
thionyl chloride, so
it's s o c l 2.

472
00:22:45,250 --> 00:22:47,830
This is another good step
forward, because now we

473
00:22:47,830 --> 00:22:51,890
actually have four different
atoms in our molecule.

474
00:22:51,890 --> 00:22:55,220
I'll tell you a little about
thionyl chloride as well.

475
00:22:55,220 --> 00:22:58,100
This is another organic
chemistry reagent, it's also

476
00:22:58,100 --> 00:23:01,460
used extensively in the
pharmaceutical industry.

477
00:23:01,460 --> 00:23:03,790
And what it's used is to convert
one type of group,

478
00:23:03,790 --> 00:23:07,170
what's called a carboxylic acid
into another type of very

479
00:23:07,170 --> 00:23:10,400
reactive intermediate, which
is called an acid chloride.

480
00:23:10,400 --> 00:23:13,590
So I show that here, so in
green, you have what's called

481
00:23:13,590 --> 00:23:18,170
a carboxcylic acid group, a c o
o h, which gets converted by

482
00:23:18,170 --> 00:23:23,280
s o c l 2 to a c double bond
o c l or an acid chloride.

483
00:23:23,280 --> 00:23:25,020
This is the very reactive
intermediate.

484
00:23:25,020 --> 00:23:27,250
You'll learn a lot more about
this if you take organic

485
00:23:27,250 --> 00:23:30,470
chemistry, but, In fact, you
can then go on and make a

486
00:23:30,470 --> 00:23:33,730
bunch of other different kinds
of very interesting molecules.

487
00:23:33,730 --> 00:23:36,920
So, for example, this is the
synthesis of novacaine.

488
00:23:36,920 --> 00:23:38,020
This is what's used
in industry to

489
00:23:38,020 --> 00:23:39,650
actually make novacaine.

490
00:23:39,650 --> 00:23:41,430
Has anyone had a novacaine
procedure?

491
00:23:41,430 --> 00:23:42,060
Yes.

492
00:23:42,060 --> 00:23:47,910
I've had it also many times,
you usually get

493
00:23:47,910 --> 00:23:49,530
novacaine for cavities.

494
00:23:49,530 --> 00:23:51,480
There's some alternatives that
are used now as well.

495
00:23:51,480 --> 00:23:54,260
It's also used as a local
anesthetic for other types of

496
00:23:54,260 --> 00:23:55,580
small procedures.

497
00:23:55,580 --> 00:23:58,250
So, this is, in fact,
what's used to make

498
00:23:58,250 --> 00:23:59,780
novacaine in industry.

499
00:23:59,780 --> 00:24:02,540
You'll notice that a lot of
different kinds medications do

500
00:24:02,540 --> 00:24:05,810
you have chlorine in them,
you'll see that c l group.

501
00:24:05,810 --> 00:24:09,160
So, for example, you might be
familiar with Wellbutrin here,

502
00:24:09,160 --> 00:24:12,170
this is a type of
anti-depressant that a lot of

503
00:24:12,170 --> 00:24:14,070
people use right now that are
taking anti-depressants.

504
00:24:14,070 --> 00:24:19,200
It's on the market, very popular
in terms of your

505
00:24:19,200 --> 00:24:22,230
choices right now as an option
as an anti-depressant.

506
00:24:22,230 --> 00:24:24,070
Also, Lunesta.

507
00:24:24,070 --> 00:24:27,390
This was very big in an ad
campaign at least last year,

508
00:24:27,390 --> 00:24:30,010
I'm not sure if it still is,
with the little butterfly.

509
00:24:30,010 --> 00:24:32,430
This is the structure of
Lunesta, and you see the c l

510
00:24:32,430 --> 00:24:33,620
in it as well.

511
00:24:33,620 --> 00:24:36,700
I just wanted to point out that
although you see these

512
00:24:36,700 --> 00:24:41,100
chlorine atoms in these drugs,
what you almost never see is

513
00:24:41,100 --> 00:24:44,310
an acid chloride -- in fact, I
don't think I've ever seen an

514
00:24:44,310 --> 00:24:47,360
acid chloride in a final
pharmaceutical product or drug

515
00:24:47,360 --> 00:24:50,650
that we take, and the reason is
because they're so reactive

516
00:24:50,650 --> 00:24:53,340
that you wouldn't want to have
that in something you digest.

517
00:24:53,340 --> 00:24:56,420
So just keep in mind when you
do see the chlorine in these

518
00:24:56,420 --> 00:24:58,550
drugs, it's very different
from the acid chloride.

519
00:24:58,550 --> 00:25:02,290
So, for example, Wellbutrin,
it is very unlikely that it

520
00:25:02,290 --> 00:25:06,140
would have thionyl chloride in
order to make it, and if

521
00:25:06,140 --> 00:25:08,990
thionyl chloride was used at
some point in the synthesis,

522
00:25:08,990 --> 00:25:11,150
it was not to put that chlorine
atom on, it was to

523
00:25:11,150 --> 00:25:12,680
put something else on.

524
00:25:12,680 --> 00:25:16,190
But in terms of drugs that don't
look like maybe this

525
00:25:16,190 --> 00:25:19,230
compound was used in the
synthesis, many of them might

526
00:25:19,230 --> 00:25:23,170
have used thionyl chloride,
because it generates such a

527
00:25:23,170 --> 00:25:25,560
nice reactive intermediate that
you can go on and make a

528
00:25:25,560 --> 00:25:28,200
bunch of different compounds
from that intermediate.

529
00:25:28,200 --> 00:25:28,490
All right.

530
00:25:28,490 --> 00:25:30,760
So let's think about how to draw
the Lewis structure for

531
00:25:30,760 --> 00:25:34,620
thionyl chloride -- oh,
actually, let me let you tell

532
00:25:34,620 --> 00:25:37,050
me how we should start
this Lewis structure.

533
00:25:37,050 --> 00:25:39,890
So, which atom would you expect
to be in the center of

534
00:25:39,890 --> 00:25:50,170
a Lewis structure for
thionyl chloride?

535
00:25:50,170 --> 00:25:50,410
All right.

536
00:25:50,410 --> 00:25:59,080
Let's take 10 seconds on that.

537
00:25:59,080 --> 00:26:01,780
Looks like we have some fast
thinking here, a lot of last

538
00:26:01,780 --> 00:26:03,650
minute answers coming in.

539
00:26:03,650 --> 00:26:04,880
OK.

540
00:26:04,880 --> 00:26:08,850
We have a split decision, so
-- you know what, actually,

541
00:26:08,850 --> 00:26:10,510
let's think about this
for a second.

542
00:26:10,510 --> 00:26:14,940
So hopefully, it was a time
issue in terms of looking at

543
00:26:14,940 --> 00:26:18,670
the periodic table, because
let's have you tell me what

544
00:26:18,670 --> 00:26:22,480
are we looking for here?

545
00:26:22,480 --> 00:26:22,770
Yeah.

546
00:26:22,770 --> 00:26:22,980
OK.

547
00:26:22,980 --> 00:26:25,440
We're looking for the lowest
ionization energy.

548
00:26:25,440 --> 00:26:28,230
So, this one can be tricky
because oxygen looks like it's

549
00:26:28,230 --> 00:26:30,540
in the middle because of the way
it's written, but we need

550
00:26:30,540 --> 00:26:33,250
to start by looking at the
lowest ionization energy.

551
00:26:33,250 --> 00:26:37,170
So, if we look on the periodic
table, comparing, for example,

552
00:26:37,170 --> 00:26:41,720
s to o, if we have s it's
below o, what happens to

553
00:26:41,720 --> 00:26:44,760
ionization energy as
we go down a table?

554
00:26:44,760 --> 00:26:46,070
It decreases.

555
00:26:46,070 --> 00:26:48,550
If you're still not completely
up on the periodic trends,

556
00:26:48,550 --> 00:26:51,130
that is stuff that's going to be
on the first exam, so make

557
00:26:51,130 --> 00:26:54,280
sure that you're able to do this
without taking too much

558
00:26:54,280 --> 00:26:55,340
time to think about it.

559
00:26:55,340 --> 00:26:58,420
We would expect the ionization
energy to decrease, that means

560
00:26:58,420 --> 00:27:00,880
that sulfur has our lowest
ionization energy.

561
00:27:00,880 --> 00:27:01,480
All right.

562
00:27:01,480 --> 00:27:05,950
So, let's go ahead and draw our
Lewis structure here with

563
00:27:05,950 --> 00:27:08,390
sulfur in the middle.

564
00:27:08,390 --> 00:27:13,030
So, we can put our sulfur in
the middle, and then it

565
00:27:13,030 --> 00:27:16,120
doesn't really matter how we
draw the rest of it, where we

566
00:27:16,120 --> 00:27:18,930
put our c l's versus where
we put our oxygens.

567
00:27:18,930 --> 00:27:23,680
We'll just put them in some way
around our sulfur atom.

568
00:27:23,680 --> 00:27:26,050
So that's our step one.

569
00:27:26,050 --> 00:27:28,600
For our step two,
what we need is

570
00:27:28,600 --> 00:27:31,130
number of valence electrons.

571
00:27:31,130 --> 00:27:33,600
So we have 2 for each
of the chlorine.

572
00:27:33,600 --> 00:27:35,570
How many valence electrons
are in chlorine?

573
00:27:35,570 --> 00:27:39,230
All right.

574
00:27:39,230 --> 00:27:41,860
So it's 7 that are in chlorine,
it's the same as

575
00:27:41,860 --> 00:27:45,500
fluorine or any of the
others in that row or

576
00:27:45,500 --> 00:27:46,440
in that group rather.

577
00:27:46,440 --> 00:27:51,990
2 times 7, plus we have 6 in
the sulfur, and oxygen is

578
00:27:51,990 --> 00:27:55,000
right above sulfur, so
that also has 6.

579
00:27:55,000 --> 00:27:59,240
So we end up having 26 valence
electrons that we're

580
00:27:59,240 --> 00:28:01,220
dealing with here.

581
00:28:01,220 --> 00:28:04,870
Our step three is to figure out
how many bonding electrons

582
00:28:04,870 --> 00:28:08,070
that we need, or excuse me, how
many total electrons that

583
00:28:08,070 --> 00:28:11,630
we need to fill up our octets,
so that's just going to be 4

584
00:28:11,630 --> 00:28:15,730
times 8, which is 32.

585
00:28:15,730 --> 00:28:20,810
And then we take 32 minus 26.

586
00:28:20,810 --> 00:28:25,360
So what we end up with in terms
of our bonding electrons

587
00:28:25,360 --> 00:28:27,540
is going to be 6 bonding
electrons.

588
00:28:27,540 --> 00:28:30,550
So we can go right ahead
and fill these in.

589
00:28:30,550 --> 00:28:36,060
1 2, 3 4, 5 and 6.

590
00:28:36,060 --> 00:28:37,910
And that was step five.

591
00:28:37,910 --> 00:28:40,080
Step six is thinking about
do we have any

592
00:28:40,080 --> 00:28:42,970
bonding electrons left?

593
00:28:42,970 --> 00:28:44,240
Nope, we used them all up.

594
00:28:44,240 --> 00:28:46,920
So we don't need to put any
more bonds in there.

595
00:28:46,920 --> 00:28:52,280
And step seven is how many
electrons do we have left over

596
00:28:52,280 --> 00:28:54,130
that are going to go
into lone pairs?

597
00:28:54,130 --> 00:28:55,800
How many?

598
00:28:55,800 --> 00:28:56,290
20.

599
00:28:56,290 --> 00:29:01,580
26 minus 6, so that tells us 20,
and these are all going to

600
00:29:01,580 --> 00:29:02,770
be lone pairs.

601
00:29:02,770 --> 00:29:04,780
Well, we're talking about a
pretty high number here, so to

602
00:29:04,780 --> 00:29:08,330
make counting easier, we'll
just say 10 lone pairs,

603
00:29:08,330 --> 00:29:10,480
because 20 lone pair electrons
is the same

604
00:29:10,480 --> 00:29:12,130
thing as 10 lone pairs.

605
00:29:12,130 --> 00:29:14,310
And all we need to do
is go over here now

606
00:29:14,310 --> 00:29:16,270
and fill up our octets.

607
00:29:16,270 --> 00:29:23,670
So oxygen gets 3 pairs, and each
chlorine gets 3 pairs, so

608
00:29:23,670 --> 00:29:26,900
now we're up to 9 pairs.

609
00:29:26,900 --> 00:29:31,840
And what we have left here is
the sulfur, which will also

610
00:29:31,840 --> 00:29:33,800
get a pair.

611
00:29:33,800 --> 00:29:36,320
So, if you look at all of these,
we have full octets for

612
00:29:36,320 --> 00:29:39,100
all of them, and if we count
up all of the valence

613
00:29:39,100 --> 00:29:44,520
electrons, it's going to be
equal to our number 26 here.

614
00:29:44,520 --> 00:29:47,400
And the last thing we do for any
of our structures to check

615
00:29:47,400 --> 00:29:50,080
them and figure out are these
valid or not valid, are these

616
00:29:50,080 --> 00:29:54,380
good Lewis structures is to
check the formal charge.

617
00:29:54,380 --> 00:29:56,520
So now that we have enough
practice drawing Lewis

618
00:29:56,520 --> 00:29:59,450
structures, let's talk about
actually figuring out this

619
00:29:59,450 --> 00:30:02,670
formal charge.

620
00:30:02,670 --> 00:30:05,500
So when we talk about formal
charge, basically formal

621
00:30:05,500 --> 00:30:09,990
charge is the measure of the
extent to which an individual

622
00:30:09,990 --> 00:30:12,060
atom within your molecule
has either

623
00:30:12,060 --> 00:30:14,400
gained or lost an electron.

624
00:30:14,400 --> 00:30:17,880
So as we said when we first
introduced covalent bonds,

625
00:30:17,880 --> 00:30:20,510
it's a sharing of electrons,
but it's not

626
00:30:20,510 --> 00:30:22,120
always an equal sharing.

627
00:30:22,120 --> 00:30:24,700
Sometimes we have a very
electronegative atom that's

628
00:30:24,700 --> 00:30:27,330
going to take more of its equal

629
00:30:27,330 --> 00:30:29,270
share of electron density.

630
00:30:29,270 --> 00:30:31,500
So for example, that might
have a formal charge of

631
00:30:31,500 --> 00:30:35,980
negative 1, because to some
extent it has gained that much

632
00:30:35,980 --> 00:30:38,210
electron density that
it now has a formal

633
00:30:38,210 --> 00:30:40,000
charge that's negative.

634
00:30:40,000 --> 00:30:44,450
So, when we think about any
type of formal charges, we

635
00:30:44,450 --> 00:30:48,520
have to assign these based on a
formula here, which is very

636
00:30:48,520 --> 00:30:50,260
easy to follow.

637
00:30:50,260 --> 00:30:54,310
Formal charge equals v
minus l minus 1/2 s.

638
00:30:54,310 --> 00:30:57,200
It's even easier to follow
if we know what all

639
00:30:57,200 --> 00:30:59,820
of those stand for.

640
00:30:59,820 --> 00:31:04,960
So, f c, I think you all
know is formal charge.

641
00:31:04,960 --> 00:31:10,940
Does anyone have
a guess for v?

642
00:31:10,940 --> 00:31:12,530
Everyone has a guess, great.

643
00:31:12,530 --> 00:31:14,930
Valence electrons.

644
00:31:14,930 --> 00:31:17,610
What about l?

645
00:31:17,610 --> 00:31:20,120
Lone pairs.

646
00:31:20,120 --> 00:31:23,530
So, lone pair electrons,
actually, not lone pairs

647
00:31:23,530 --> 00:31:25,780
themselves.

648
00:31:25,780 --> 00:31:28,440
And then s?

649
00:31:28,440 --> 00:31:28,710
Good.

650
00:31:28,710 --> 00:31:30,760
That's a tricky one,
shared electrons.

651
00:31:30,760 --> 00:31:35,930
All right.

652
00:31:35,930 --> 00:31:38,900
So this means we can actually
calculate this for any

653
00:31:38,900 --> 00:31:41,680
molecule that we've drawn the
Lewis structure for, because

654
00:31:41,680 --> 00:31:44,560
we actually do need to draw the
Lewis structure before we

655
00:31:44,560 --> 00:31:47,290
know, for example, how many of
each of these we have, or at

656
00:31:47,290 --> 00:31:50,080
least go through the rules.

657
00:31:50,080 --> 00:31:53,090
And what's important to keep in
mind about formal charge is

658
00:31:53,090 --> 00:31:56,570
if we have a neutral atom,
such as we did in thionyl

659
00:31:56,570 --> 00:32:00,650
chloride here, the sum of the
individual formal charges on

660
00:32:00,650 --> 00:32:04,840
individual atoms within the
molecule have to equal 0.

661
00:32:04,840 --> 00:32:07,620
So if we add them all up, there
should be no net charge

662
00:32:07,620 --> 00:32:11,240
on the molecule, if the
molecule is neutral.

663
00:32:11,240 --> 00:32:15,460
So, if we think about the second
case here where we have

664
00:32:15,460 --> 00:32:18,700
c n minus, now we're talking
about a molecule with a net

665
00:32:18,700 --> 00:32:20,370
charge of negative 1.

666
00:32:20,370 --> 00:32:23,350
So that means if we add up all
of the formal charges within

667
00:32:23,350 --> 00:32:26,500
the molecule, what we would
expect to see is that they sum

668
00:32:26,500 --> 00:32:29,940
up to give a net charge
of negative 1.

669
00:32:29,940 --> 00:32:33,840
So we can do this for any final
charge we have, if we a

670
00:32:33,840 --> 00:32:37,110
molecule that has a charge of
plus 2, then all of the formal

671
00:32:37,110 --> 00:32:41,470
charges should add up
to plus 2 and so on.

672
00:32:41,470 --> 00:32:44,470
So, let's just figure this out
for some of the examples we

673
00:32:44,470 --> 00:32:45,820
did, so for the cyanide anion.

674
00:32:45,820 --> 00:32:49,250
So, if we want to figure out
the formal charge on the

675
00:32:49,250 --> 00:32:52,810
carbon, we need to take the
number of valence electrons,

676
00:32:52,810 --> 00:32:54,110
so that's 4.

677
00:32:54,110 --> 00:32:58,530
We need to subtract the lone
pair, what number is that?

678
00:32:58,530 --> 00:32:59,600
It's 2.

679
00:32:59,600 --> 00:33:03,140
And then 1/2 of the number
of shared electrons.

680
00:33:03,140 --> 00:33:06,050
So, shared electrons are the
ones that are shared between

681
00:33:06,050 --> 00:33:10,770
the carbon and the nitrogen, so
we have 6 shared electrons,

682
00:33:10,770 --> 00:33:12,120
and we want to take
1/2 of that.

683
00:33:12,120 --> 00:33:13,770
So we end up with a
formal charge on

684
00:33:13,770 --> 00:33:16,370
carbon of negative 1.

685
00:33:16,370 --> 00:33:18,330
We can do the same thing
for nitrogen.

686
00:33:18,330 --> 00:33:21,410
So in terms of nitrogen that
starts off with a valence

687
00:33:21,410 --> 00:33:25,630
number of 5, again we have 2
lone pair electrons in the

688
00:33:25,630 --> 00:33:29,900
nitrogen, and again, we have 6
electrons that are shared.

689
00:33:29,900 --> 00:33:31,750
So what we see is that
the formal charge on

690
00:33:31,750 --> 00:33:34,000
the nitrogen is 0.

691
00:33:34,000 --> 00:33:37,270
Also, formal charges can be
checked, as I just said.

692
00:33:37,270 --> 00:33:41,110
Negative 1 plus 0 should add up
to negative 1, if in fact,

693
00:33:41,110 --> 00:33:43,270
we're correct for
the c n anion.

694
00:33:43,270 --> 00:33:48,040
And it does, so we know that
we're probably on target in

695
00:33:48,040 --> 00:33:50,990
terms of calculating
our formal charge.

696
00:33:50,990 --> 00:33:53,440
So, let's think about our second
example -- actually our

697
00:33:53,440 --> 00:33:55,290
third, but the second one we're
going to talk about in

698
00:33:55,290 --> 00:33:58,490
terms of formal charge, which
is thionyl chloride.

699
00:33:58,490 --> 00:34:01,220
So why don't you tell me what
the formal charge should be on

700
00:34:01,220 --> 00:34:03,910
the sulfur atom of
thionyl chloride?

701
00:34:03,910 --> 00:34:38,050
All right.

702
00:34:38,050 --> 00:34:53,610
Let's take 10 more seconds.

703
00:34:53,610 --> 00:34:56,970
OK, so the majority got it.

704
00:34:56,970 --> 00:35:00,460
So hopefully next time we do a
formal charge question, we'll

705
00:35:00,460 --> 00:35:01,740
get everyone back up to speed.

706
00:35:01,740 --> 00:35:04,030
But we've just introduced it, so
let's go back to the class

707
00:35:04,030 --> 00:35:06,890
notes and explain why this
is the correct answer.

708
00:35:06,890 --> 00:35:10,400
So if we look at sulfur, what
we need to do is take the

709
00:35:10,400 --> 00:35:13,850
valence electrons in sulfur,
and there are 6.

710
00:35:13,850 --> 00:35:16,190
By looking at the periodic table
it's right underneath

711
00:35:16,190 --> 00:35:19,700
oxygen, so those both have
6 valence electrons.

712
00:35:19,700 --> 00:35:23,890
There are 2 lone pair electrons
on sulfur -- we only

713
00:35:23,890 --> 00:35:29,140
have 2 lone pairs -- or 1 lone
pair, 2 lone pair electrons.

714
00:35:29,140 --> 00:35:32,870
And then we end up having 6
shared electrons, 2 from each

715
00:35:32,870 --> 00:35:35,710
of the bonds, so we end up
with a formal charge on

716
00:35:35,710 --> 00:35:37,980
sulfur of plus 1.

717
00:35:37,980 --> 00:35:42,040
If we go to the oxygen atom,
now we're talking about

718
00:35:42,040 --> 00:35:45,810
starting with 6 in terms of
valence electrons again, but

719
00:35:45,810 --> 00:35:50,040
instead of 2, you can see we
have 6 lone pair electrons

720
00:35:50,040 --> 00:35:54,180
around the oxygen minus 1/2 of
2, so we have minus 1 is our

721
00:35:54,180 --> 00:35:55,360
formal charge.

722
00:35:55,360 --> 00:35:58,380
And if we talk about chlorine,
and both of the chlorines are

723
00:35:58,380 --> 00:36:01,870
the same in this case, we start
with a valence number of

724
00:36:01,870 --> 00:36:06,140
7 for chlorine, and then we
subtract 6, because it had 6

725
00:36:06,140 --> 00:36:09,340
lone pair electrons around each
of the chlorine atoms.

726
00:36:09,340 --> 00:36:14,700
Then minus 1/2 of 2, because
we only have one bond or 2

727
00:36:14,700 --> 00:36:16,460
electrons in a bond.

728
00:36:16,460 --> 00:36:19,140
So again, we should be able
to check all of our formal

729
00:36:19,140 --> 00:36:23,010
charges and make sure they add
up to 0, which they do, and

730
00:36:23,010 --> 00:36:25,390
that makes sense, because we
have a neutral atom in terms

731
00:36:25,390 --> 00:36:28,140
of thionyl chloride.

732
00:36:28,140 --> 00:36:30,700
Another thing to mention in
terms of thinking about if you

733
00:36:30,700 --> 00:36:34,090
have a good or a bad Lewis
structure, is that when you

734
00:36:34,090 --> 00:36:37,550
figure out the formal charge on
each of the atoms, it's the

735
00:36:37,550 --> 00:36:40,290
more electronegative atoms that
you would expect to have

736
00:36:40,290 --> 00:36:42,850
that negative charge, and that
should make sense to you

737
00:36:42,850 --> 00:36:46,310
because electronegative atoms
want to have electron density,

738
00:36:46,310 --> 00:36:49,350
they want to pull in electron
density to them, so it would

739
00:36:49,350 --> 00:36:51,730
make sense that they have more
of it, which would give them a

740
00:36:51,730 --> 00:36:53,630
negative charge.

741
00:36:53,630 --> 00:36:58,030
So, if we compare the sulfur to
the oxygen, the oxygen it

742
00:36:58,030 --> 00:37:00,120
turns out is more
electronegative and that is

743
00:37:00,120 --> 00:37:04,690
what holds the negative charge
in this molecule.

744
00:37:04,690 --> 00:37:07,570
Another thing I want to point
out, and for some of you just

745
00:37:07,570 --> 00:37:09,930
ignore what I'm saying, if
you haven't thought about

746
00:37:09,930 --> 00:37:12,450
oxidation number, if you haven't
heard of that before,

747
00:37:12,450 --> 00:37:14,230
don't worry we'll get to
it in the second half

748
00:37:14,230 --> 00:37:15,440
with Professor Drennan.

749
00:37:15,440 --> 00:37:17,520
But for those of you from high
school that have learned about

750
00:37:17,520 --> 00:37:20,570
oxidation number and maybe are
starting to think about it

751
00:37:20,570 --> 00:37:23,230
when you look at these
molecules, formal charge is --

752
00:37:23,230 --> 00:37:25,890
it's not the same thing as
oxidation number, so separate

753
00:37:25,890 --> 00:37:27,300
those two things in your head.

754
00:37:27,300 --> 00:37:29,760
We'll get to oxidation number
in the second half of this

755
00:37:29,760 --> 00:37:31,690
course, but it's not
in any way the same

756
00:37:31,690 --> 00:37:35,190
idea as formal charge.

757
00:37:35,190 --> 00:37:35,450
All right.

758
00:37:35,450 --> 00:37:38,000
So formal charge can actually
help us out when we're trying

759
00:37:38,000 --> 00:37:41,200
to decide between several Lewis
structures that look

760
00:37:41,200 --> 00:37:43,870
like they might be comparable in
terms of which might be the

761
00:37:43,870 --> 00:37:46,170
lower energy or the more
stable structure.

762
00:37:46,170 --> 00:37:48,540
The examples we've done so
far have been pretty

763
00:37:48,540 --> 00:37:51,480
straightforward, so we haven't
needed to use formal charge to

764
00:37:51,480 --> 00:37:53,120
make this kind of decision.

765
00:37:53,120 --> 00:37:56,620
But we could, for example, look
at a case where we have

766
00:37:56,620 --> 00:37:59,620
several different structures
that look pretty good, and the

767
00:37:59,620 --> 00:38:02,290
one we want to determine as
being the lowest energy

768
00:38:02,290 --> 00:38:05,740
structure is the one in which
the absolute values of the

769
00:38:05,740 --> 00:38:08,940
formal charges are going to be
lower, so essentially that

770
00:38:08,940 --> 00:38:11,120
they have less charge
separation.

771
00:38:11,120 --> 00:38:14,140
Those are going to be the more
stable or the lower energy

772
00:38:14,140 --> 00:38:16,530
structures.

773
00:38:16,530 --> 00:38:19,780
So, for example, let's look
at thiocyanate ion, we

774
00:38:19,780 --> 00:38:22,380
have c s and n.

775
00:38:22,380 --> 00:38:25,220
What we've learned so far is as
a first approximation, what

776
00:38:25,220 --> 00:38:28,540
we want to do is put the atom
with the lowest ionization

777
00:38:28,540 --> 00:38:30,270
energy in the middle here.

778
00:38:30,270 --> 00:38:33,670
So let's compare those
ionization energies.

779
00:38:33,670 --> 00:38:38,800
We have 10 90 for carbon, 1,000
for sulfur, and 1,400

780
00:38:38,800 --> 00:38:39,950
for nitrogen.

781
00:38:39,950 --> 00:38:42,650
So, thinking about ionization
energy, which atom would you

782
00:38:42,650 --> 00:38:46,590
put in the middle here?

783
00:38:46,590 --> 00:38:48,530
So, a lot of people
I hear are saying

784
00:38:48,530 --> 00:38:50,070
sulfur, and that's right.

785
00:38:50,070 --> 00:38:52,530
So, in terms of ionization
energy, we would expect to see

786
00:38:52,530 --> 00:38:53,450
sulfur in the middle.

787
00:38:53,450 --> 00:38:56,780
So if we went through and drew
out our Lewis structure

788
00:38:56,780 --> 00:38:59,690
following each of our steps,
what we would get is this as

789
00:38:59,690 --> 00:39:02,230
our Lewis structure here, and we
could figure out all of the

790
00:39:02,230 --> 00:39:03,380
formal charges.

791
00:39:03,380 --> 00:39:05,830
But what I'm going to tell you
already is this is a case

792
00:39:05,830 --> 00:39:08,710
where, in fact, it's an
exception to the idea that the

793
00:39:08,710 --> 00:39:11,600
lowest energy structure has the
lowest ionization energy

794
00:39:11,600 --> 00:39:14,180
in the middle, and we can figure
this out when we look

795
00:39:14,180 --> 00:39:15,380
at formal charge.

796
00:39:15,380 --> 00:39:17,880
It's always a good first
approximation, because you

797
00:39:17,880 --> 00:39:19,940
need to start somewhere in
terms of drawing Lewis

798
00:39:19,940 --> 00:39:22,500
structures, but then if you go
and figure out the formal

799
00:39:22,500 --> 00:39:25,750
charge and you just have lots
of charge separation or very

800
00:39:25,750 --> 00:39:29,360
high charges, like a plus 2 and
a minus 2 and a minus 1

801
00:39:29,360 --> 00:39:31,850
all different places in the
atom, what it should tell you

802
00:39:31,850 --> 00:39:33,690
is maybe there's a
better structure.

803
00:39:33,690 --> 00:39:36,070
So, let's think of all of the
combinations that we could

804
00:39:36,070 --> 00:39:38,120
have in terms of
this molecule.

805
00:39:38,120 --> 00:39:40,620
So in one case, we could
actually put carbon in the

806
00:39:40,620 --> 00:39:43,270
middle, in one place, we could
put sulfur in the middle, and

807
00:39:43,270 --> 00:39:44,960
in one case we could
put nitrogen.

808
00:39:44,960 --> 00:39:47,280
And then we can go ahead and
let's quickly write out what

809
00:39:47,280 --> 00:39:49,720
the formal charges for
all of these will be.

810
00:39:49,720 --> 00:39:53,410
So in our first structure, we
would find for the nitrogen we

811
00:39:53,410 --> 00:39:56,980
have a formal charge 5 minus
4 minus 2, because we're

812
00:39:56,980 --> 00:40:00,840
starting with 5 valence
electrons, so that is a formal

813
00:40:00,840 --> 00:40:03,940
charge of minus 1.

814
00:40:03,940 --> 00:40:08,070
For the carbon, we start with 4
valence electrons, we have 0

815
00:40:08,070 --> 00:40:12,370
lone pair electrons minus 4,
and we end up with a formal

816
00:40:12,370 --> 00:40:14,790
charge of 0.

817
00:40:14,790 --> 00:40:18,220
For the sulfur, we start off
with 6 valence electrons,

818
00:40:18,220 --> 00:40:22,730
minus 4 lone pair electrons,
minus 2, taking in account our

819
00:40:22,730 --> 00:40:26,820
bonding electrons, so we end up
with a formal charge of 0.

820
00:40:26,820 --> 00:40:28,080
All right, so this looks
pretty good.

821
00:40:28,080 --> 00:40:30,180
We don't actually have much
charge separation

822
00:40:30,180 --> 00:40:31,530
in this case here.

823
00:40:31,530 --> 00:40:34,650
Let's take a look at the lowest
ionization energy in

824
00:40:34,650 --> 00:40:35,950
the center case.

825
00:40:35,950 --> 00:40:39,030
And what you find out if you do
these calculations, is that

826
00:40:39,030 --> 00:40:42,980
you have a negative 1 for your
formal charge on nitrogen, you

827
00:40:42,980 --> 00:40:47,210
have a negative 2 for your
formal charge on carbon, and

828
00:40:47,210 --> 00:40:51,330
you have a positive 2 for your
formal charge on sulfur.

829
00:40:51,330 --> 00:40:53,940
If we look at our last structure
here where we have

830
00:40:53,940 --> 00:40:56,230
nitrogen the middle, we can
also figure out all those

831
00:40:56,230 --> 00:40:59,640
formal charges, and in this
case we have plus 1 on the

832
00:40:59,640 --> 00:41:04,560
nitrogen, we have minus 2 on the
carbon, and then we end up

833
00:41:04,560 --> 00:41:08,450
with a 0 on the sulfur there.

834
00:41:08,450 --> 00:41:10,460
So let's go to a clicker
question.

835
00:41:10,460 --> 00:41:13,950
If we call this structure a,
b, and c -- you can look on

836
00:41:13,950 --> 00:41:16,260
your notes, it also says
structure a, b, and c on your

837
00:41:16,260 --> 00:41:17,480
notes, so you didn't
need to have just

838
00:41:17,480 --> 00:41:19,250
memorized that slide.

839
00:41:19,250 --> 00:41:23,080
But I want you to tell me in
terms of thinking about formal

840
00:41:23,080 --> 00:41:25,420
charge, which Lewis structure
would you predict to be the

841
00:41:25,420 --> 00:41:29,670
most stable?

842
00:41:29,670 --> 00:41:43,780
And this should be fast, so
let's take 10 seconds on this.

843
00:41:43,780 --> 00:41:44,600
Okay, great.

844
00:41:44,600 --> 00:41:47,050
So let's go back to
our notes here.

845
00:41:47,050 --> 00:41:48,920
And the reason that this
should be so fast is we

846
00:41:48,920 --> 00:41:50,210
already did all the
calculations

847
00:41:50,210 --> 00:41:51,650
for the formal charges.

848
00:41:51,650 --> 00:41:54,740
So what we see is that structure
a is the most stable

849
00:41:54,740 --> 00:41:57,510
because we have the least
separation of charge in the

850
00:41:57,510 --> 00:42:00,130
case of structure a.

851
00:42:00,130 --> 00:42:03,360
And you can do this any time if
you have Lewis structures

852
00:42:03,360 --> 00:42:04,980
that you're choosing between.

853
00:42:04,980 --> 00:42:08,750
You won't always draw out every
single possibility that

854
00:42:08,750 --> 00:42:09,890
you have to start with.

855
00:42:09,890 --> 00:42:12,070
Often a good thing to start
with is to put the lowest

856
00:42:12,070 --> 00:42:14,960
ionization energy atom in the
middle, and if you don't have

857
00:42:14,960 --> 00:42:18,290
charge separation then go with
that structure, but if you do

858
00:42:18,290 --> 00:42:20,450
find you have a lot of
separation, such as the case

859
00:42:20,450 --> 00:42:23,570
in negative 2, positive 2, and
minus 1, then you want to say

860
00:42:23,570 --> 00:42:26,320
wait a second, this is really
bad in terms of formal charge,

861
00:42:26,320 --> 00:42:30,880
let me go ahead and see what
other options I have here.

862
00:42:30,880 --> 00:42:36,260
So, we can also get into a case
where we have similar

863
00:42:36,260 --> 00:42:39,270
values in terms of absolute
values of formal charge

864
00:42:39,270 --> 00:42:41,280
between two different molecules
we're deciding

865
00:42:41,280 --> 00:42:43,320
between in their Lewis
structures.

866
00:42:43,320 --> 00:42:45,580
And in this case, the
tie-breaker goes to the

867
00:42:45,580 --> 00:42:48,660
molecule in which the negative
charge is on the most

868
00:42:48,660 --> 00:42:51,030
electronegative atom.

869
00:42:51,030 --> 00:42:53,440
So, let's look at an example
of this here.

870
00:42:53,440 --> 00:42:56,740
So we could say, for example,
this molecule here, this --

871
00:42:56,740 --> 00:42:59,130
now we're dealing with a lot
of different atoms in the

872
00:42:59,130 --> 00:43:01,340
molecule, much more complicated
than the initial

873
00:43:01,340 --> 00:43:04,080
case of the cyanide ion
where we only had two.

874
00:43:04,080 --> 00:43:06,660
So, how do we figure out first
how to draw the skeletal

875
00:43:06,660 --> 00:43:09,270
structure of this
molecule here?

876
00:43:09,270 --> 00:43:13,360
And one thing I want to tell
you to start out with is

877
00:43:13,360 --> 00:43:17,380
something about this
c h 3 group here.

878
00:43:17,380 --> 00:43:23,210
Any time you see a c h 3, this
means a methyl group.

879
00:43:23,210 --> 00:43:27,510
And if you draw out what a
methyl group is -- hopefully

880
00:43:27,510 --> 00:43:35,880
you won't have to spell it --
then what you have is a carbon

881
00:43:35,880 --> 00:43:40,910
in the middle with three
hydrogens around it, and then

882
00:43:40,910 --> 00:43:43,360
it can only be bonded
to one other thing.

883
00:43:43,360 --> 00:43:46,990
So any time you see c h 3 here,
remember that that's

884
00:43:46,990 --> 00:43:49,410
methyl and that's going to
be a terminal group.

885
00:43:49,410 --> 00:43:51,740
So, you already have a hint that
methyl groups are never

886
00:43:51,740 --> 00:43:54,150
in the middle, they always have
to be on the outside.

887
00:43:54,150 --> 00:43:57,190
So that's a good start for us
putting together a skeletal

888
00:43:57,190 --> 00:43:59,320
structure for this
compound here.

889
00:43:59,320 --> 00:44:01,460
The other tip I'm going to give
you is any time you see a

890
00:44:01,460 --> 00:44:05,080
chain molecule, by chain I just
mean many different atoms

891
00:44:05,080 --> 00:44:06,480
written out in a row.

892
00:44:06,480 --> 00:44:10,440
A convention is that typically
you will put a terminal atom.

893
00:44:10,440 --> 00:44:13,600
We know that h is always
terminal, it's always on the

894
00:44:13,600 --> 00:44:15,720
end, never in the center.

895
00:44:15,720 --> 00:44:19,190
Right after the molecule
that it's attached to.

896
00:44:19,190 --> 00:44:22,370
So, for instance, this would
suggest to us by the way it's

897
00:44:22,370 --> 00:44:25,810
written, that the hydrogen is
attached to the nitrogen and

898
00:44:25,810 --> 00:44:27,440
not the oxygen.

899
00:44:27,440 --> 00:44:30,390
So, if we use those two tips
to try to figure out a

900
00:44:30,390 --> 00:44:33,300
structure, a skeletal structure,
we would get this

901
00:44:33,300 --> 00:44:36,960
structure here if we write out
the full Lewis structure.

902
00:44:36,960 --> 00:44:41,640
We could I think well, maybe
this isn't written out in

903
00:44:41,640 --> 00:44:44,090
terms of that convention, which
sometimes it's not, so

904
00:44:44,090 --> 00:44:46,790
let's also try writing it, such
that we have the hydrogen

905
00:44:46,790 --> 00:44:48,600
and the oxygen atom there.

906
00:44:48,600 --> 00:44:50,370
So that would give us
this structure here.

907
00:44:50,370 --> 00:44:53,532
So notice a difference in these
structures, is this has

908
00:44:53,532 --> 00:44:57,550
an n h bond whereas this
has an o h bond.

909
00:44:57,550 --> 00:45:01,120
And if we were to think about
which one of these is better,

910
00:45:01,120 --> 00:45:03,750
it turns out that it's the
same in terms of formal

911
00:45:03,750 --> 00:45:05,790
charges, so that doesn't
help us out.

912
00:45:05,790 --> 00:45:08,490
So we need to go to this
second case where we're

913
00:45:08,490 --> 00:45:11,650
instead going to think about
electronegativity, and we want

914
00:45:11,650 --> 00:45:13,660
to think about which atom is
the most electronegative.

915
00:45:13,660 --> 00:45:18,550
So, in this case, we see that
our formal charge is negative

916
00:45:18,550 --> 00:45:22,200
on the nitrogen, in this case
it's negative on oxygen.

917
00:45:22,200 --> 00:45:25,660
Which of those two is more
electronegative?

918
00:45:25,660 --> 00:45:26,390
The oxygen.

919
00:45:26,390 --> 00:45:28,700
So you should be able to look at
your periodic table and see

920
00:45:28,700 --> 00:45:31,250
this, or also I've written
the trend here.

921
00:45:31,250 --> 00:45:35,630
So that means that the more
stable molecule is going to be

922
00:45:35,630 --> 00:45:38,210
this molecule here, which
actually puts the negative

923
00:45:38,210 --> 00:45:41,520
charge on be more
electronegative atom.

924
00:45:41,520 --> 00:45:46,290
So this is our lower
energy structure.

925
00:45:46,290 --> 00:45:49,190
So, these are the different ways
that we can actually go

926
00:45:49,190 --> 00:45:51,790
ahead and use formal charge when
we're choosing between

927
00:45:51,790 --> 00:45:53,650
different types of
Lewis structures.

928
00:45:53,650 --> 00:45:56,520
So one last concept that I want
introduces is this idea

929
00:45:56,520 --> 00:45:58,080
of resonance.

930
00:45:58,080 --> 00:46:01,670
And resonance is the idea that
sometimes one single Lewis

931
00:46:01,670 --> 00:46:04,340
structure does not adequately
describe the electron

932
00:46:04,340 --> 00:46:09,320
configuration around a given
molecule, so instead you need

933
00:46:09,320 --> 00:46:12,320
to draw two different Lewis
structures to describe that

934
00:46:12,320 --> 00:46:13,740
more appropriately.

935
00:46:13,740 --> 00:46:17,810
So, let's quickly go through the
Lewis structure for ozone.

936
00:46:17,810 --> 00:46:20,280
I have the skeletal structure
written up there, I've written

937
00:46:20,280 --> 00:46:22,140
it twice and you'll see
why in a minute.

938
00:46:22,140 --> 00:46:23,910
It's easy to write the skeletal
structure, because

939
00:46:23,910 --> 00:46:26,030
it's all oxygen, we don't have
to worry about what's going to

940
00:46:26,030 --> 00:46:27,540
go in the middle.

941
00:46:27,540 --> 00:46:30,790
In this case, we're going to
have 3 times 6 for valence

942
00:46:30,790 --> 00:46:31,090
electrons --

943
00:46:31,090 --> 00:46:35,550
6 valence electrons for each
oxygen, so we have 18 total

944
00:46:35,550 --> 00:46:37,860
valence electrons.

945
00:46:37,860 --> 00:46:41,740
So, in order to fill up our
shell, what we need is 3 times

946
00:46:41,740 --> 00:46:44,090
8 or 24 electrons.

947
00:46:44,090 --> 00:46:48,820
This leaves us with
24 minus 18, or 6

948
00:46:48,820 --> 00:46:51,260
bonding electrons left.

949
00:46:51,260 --> 00:46:53,960
So what we can do is
fill that in here.

950
00:46:53,960 --> 00:46:59,360
Clearly, we put 2 for each bond,
and now we end up having

951
00:46:59,360 --> 00:47:01,770
2 remaining bonding
electrons left.

952
00:47:01,770 --> 00:47:04,100
So here is where our question
comes, because

953
00:47:04,100 --> 00:47:04,960
where do we put it?

954
00:47:04,960 --> 00:47:08,640
There's absolutely nothing that
tells us which atoms we

955
00:47:08,640 --> 00:47:11,680
should put it between, because
they're both oxygen-oxygen.

956
00:47:11,680 --> 00:47:15,320
So, let's just arbitrarily put
it between these two in this

957
00:47:15,320 --> 00:47:18,880
case here, but actually there's
no reason we couldn't

958
00:47:18,880 --> 00:47:21,730
also put it between oxygen b
and c, so I'm going to draw

959
00:47:21,730 --> 00:47:25,020
another structure where
we have it here.

960
00:47:25,020 --> 00:47:28,500
So in terms of remaining valence
electrons we have 12,

961
00:47:28,500 --> 00:47:31,330
so we can finish off each of
our Lewis structures, so

962
00:47:31,330 --> 00:47:34,120
that's our first structure
there, and our second

963
00:47:34,120 --> 00:47:35,080
structure there.

964
00:47:35,080 --> 00:47:37,690
We could also figure out the
formal charges, and obviously

965
00:47:37,690 --> 00:47:40,820
the formal charges between these
two atoms, they're going

966
00:47:40,820 --> 00:47:42,500
to be identical, we're
only dealing with

967
00:47:42,500 --> 00:47:44,810
oxygen atoms here.

968
00:47:44,810 --> 00:47:47,230
So, we need to think about what
this means -- which is

969
00:47:47,230 --> 00:47:50,060
the more stable structure,
because we have two different

970
00:47:50,060 --> 00:47:50,710
structures here.

971
00:47:50,710 --> 00:47:54,290
In this case we have a double
bond between a and b, and in

972
00:47:54,290 --> 00:47:56,870
this case we have it
between b and c.

973
00:47:56,870 --> 00:48:00,370
So, presumably, if we follow our
rules so far only one of

974
00:48:00,370 --> 00:48:02,420
these should be correct.

975
00:48:02,420 --> 00:48:05,620
And again, if we figure out the
formal charges, let's go

976
00:48:05,620 --> 00:48:10,230
through this quickly, we get
0 plus 1 and negative 1 for

977
00:48:10,230 --> 00:48:11,720
structure 1.

978
00:48:11,720 --> 00:48:16,270
We get negative 1 plus 1
and 0 for structure 2.

979
00:48:16,270 --> 00:48:18,780
So as I said, they're going to
be identical in terms of

980
00:48:18,780 --> 00:48:21,120
making the decision that way.

981
00:48:21,120 --> 00:48:24,040
And what it turns out is
experimental evidence tells us

982
00:48:24,040 --> 00:48:26,410
that these two structures
are equivalent.

983
00:48:26,410 --> 00:48:28,850
And by that what we mean is
that they're absolutely

984
00:48:28,850 --> 00:48:31,980
identical, and it turns out that
this here is not a double

985
00:48:31,980 --> 00:48:34,730
bond, it's not a single bond
either, it's actually

986
00:48:34,730 --> 00:48:35,670
something in between.

987
00:48:35,670 --> 00:48:38,980
So if we look at its length,
it's actually shorter than a

988
00:48:38,980 --> 00:48:41,190
single bond, but longer
than a double bond.

989
00:48:41,190 --> 00:48:44,170
Or if we look at how strong it
is, it's actually stronger

990
00:48:44,170 --> 00:48:47,010
than a single bond, but weaker
than a double bond.

991
00:48:47,010 --> 00:48:50,420
And we find the same thing for
these two atoms here, it's not

992
00:48:50,420 --> 00:48:54,490
actually a double bond, it's
somewhere between a single

993
00:48:54,490 --> 00:48:56,360
bond and a double bond.

994
00:48:56,360 --> 00:48:59,620
So this is a case where we have
resonance structures, or

995
00:48:59,620 --> 00:49:01,820
we call this a resonance
hybrid.

996
00:49:01,820 --> 00:49:03,950
So the reality of the situation
is that it's a

997
00:49:03,950 --> 00:49:06,780
combination between these
2 structures.

998
00:49:06,780 --> 00:49:08,800
And an important thing to
remember when we talk about

999
00:49:08,800 --> 00:49:12,530
resonance hybrids is that the
structure it's not 1/2 the

1000
00:49:12,530 --> 00:49:15,910
time this structure, and 1/2
of the time this structure,

1001
00:49:15,910 --> 00:49:19,390
it's actually some combination
or some average between the

1002
00:49:19,390 --> 00:49:20,440
two structures.

1003
00:49:20,440 --> 00:49:22,970
But since in drawing Lewis
structures we don't have a way

1004
00:49:22,970 --> 00:49:26,120
to represent that -- actually,
in some cases you do, you can

1005
00:49:26,120 --> 00:49:30,200
draw a dotted line that means a
1 and 1/2 bond, but most in

1006
00:49:30,200 --> 00:49:33,570
most cases, we just draw out
both resonance structures, and

1007
00:49:33,570 --> 00:49:35,730
the way that we say it's a
resonance structure is that we

1008
00:49:35,730 --> 00:49:38,750
put it in the brackets and we
put an arrow between it.

1009
00:49:38,750 --> 00:49:40,860
So, when you think about
resonance structures, some

1010
00:49:40,860 --> 00:49:43,540
students tend to just get
confused and be picturing this

1011
00:49:43,540 --> 00:49:45,470
flickering back and forth.

1012
00:49:45,470 --> 00:49:49,270
A good example to keep in mind
is the idea of a mule.

1013
00:49:49,270 --> 00:49:52,780
So most of you know, hopefully,
that a mule is a

1014
00:49:52,780 --> 00:49:57,670
combination of a donkey
and a horse.

1015
00:49:57,670 --> 00:50:00,590
A mule is not spending 1/2 of
its time as a donkey, and 1/2

1016
00:50:00,590 --> 00:50:04,620
of its time as a horse, we don't
see it flickering back

1017
00:50:04,620 --> 00:50:07,520
and forth between the two,
that's not what we see.

1018
00:50:07,520 --> 00:50:10,845
Instead what we see is it's an
average, it's part like a

1019
00:50:10,845 --> 00:50:12,680
horse, it's part
like a donkey.

1020
00:50:12,680 --> 00:50:16,940
So if we want to put that in
chemical terms, we want to

1021
00:50:16,940 --> 00:50:19,580
make sure we put these in
brackets here, and remember,

1022
00:50:19,580 --> 00:50:22,980
this is the resonance arrow,
it's not a reaction arrow,

1023
00:50:22,980 --> 00:50:25,140
it's a resonance arrow, so
make sure you mark it up

1024
00:50:25,140 --> 00:50:27,080
correctly like that.

1025
00:50:27,080 --> 00:50:30,050
When we talk about resonance
structures, the key word is

1026
00:50:30,050 --> 00:50:33,120
that the electrons
are de-localized.

1027
00:50:33,120 --> 00:50:36,060
So they're not just between
two atoms here.

1028
00:50:36,060 --> 00:50:38,380
Now, for example, in our
structure with ozone it's

1029
00:50:38,380 --> 00:50:41,340
between all three atoms.

1030
00:50:41,340 --> 00:50:44,240
And the final point I want to
make today, and this is very,

1031
00:50:44,240 --> 00:50:47,720
very important, so make sure
that you do understand this.

1032
00:50:47,720 --> 00:50:50,590
When we talk about resonance
structures, we're talking

1033
00:50:50,590 --> 00:50:54,560
about cases that have the same
arrangement of atoms, the key

1034
00:50:54,560 --> 00:50:57,365
is the atoms are the same, and
the thing that is different is

1035
00:50:57,365 --> 00:51:00,080
the arrangement of
electrons here.