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PROFESSOR: All right.

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It's 12:05, so why don't you
go ahead and take 10 more

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seconds on the clicker
question today.

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This is about the periodic
trends that we

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discussed on Wednesday.

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So specifically, what we're
asking here is as we go across

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the periodic table, we want to
consider which has the smaller

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ionization energy.

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All right.

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So, let's focus our attention
up here now, whether it's

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between aluminum or whether
it's between phosphorous.

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And I also wanted you to
identify why it's also

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important to understand why we
have these trends, not just to

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memorize the trend itself.

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So, it turns out that the
majority of you got the

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correct answer, which is
that it's aluminum.

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The reason it's aluminum is
because aluminum has a lower z

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effective, so it's not being
pulled in as tightly by the

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nucleus, and if it's not being
pulled in as tightly, you're

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00:01:16,610 --> 00:01:19,330
going to have to put in less
energy in order to ionize it,

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so that's why it's actually
going to have the smaller

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ionization energy.

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So it looks like not too many
more than half of you got this

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correct, so make sure you can
look at your periodic table

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and figure out how to think
about ionization energy in

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terms of z effective, not just
in terms of memorizing what

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that trend is.

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All right, so we can go to
today's notes, and in terms of

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the notes, what we're going
to start with is finishing

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00:01:46,550 --> 00:01:49,700
material that's going to be
relevant for exam 1, and I

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00:01:49,700 --> 00:01:51,810
told you on Wednesday that
actually I'd give you some

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information today in terms of
what you need to do to prepare

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for exam 1.

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00:01:56,270 --> 00:01:59,240
So you should have gotten two
handouts as you came in, and

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if you didn't, please raise your
hand and a TA will come

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to you and get you that
second handout.

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But the second one says "exam
instructions and logistics."

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So, if everyone can
pull that out.

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So this is going to tell you
pretty much everything you

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need to know in terms of getting
ready for the exam,

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00:02:15,270 --> 00:02:17,210
which is next Wednesday.

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So when you go home today or
some time this weekend, make

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sure you read this
page in detail.

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I'm just going to go over a few
of the main points here.

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So, the first is that what we're
going over notes today

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and also on Monday, the exam
material ends at the end

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lecture notes from lecture 9.

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So that was Wednesday's class
-- not at the end of

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Wednesday's class, but at the
end of the lecture notes.

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So we're going to finish
up with those today.

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I'll be really, really clear
when we get through them, and

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that's where you can stop in
terms of studying for this.

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Also, on everything that was on
problem-sets 1 through 3.

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So, you turned in p-set 3 today,
but we'll have the

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answers posted for you this
afternoon, so you can start

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studying from p-set 3, even as
early as tonight, if you want

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to, because those answers
will be there for you.

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So, in terms of what it is that
you need to prepare and

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bring with you for the exam, you
need to bring your MIT ID,

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especially if you haven't been
showing up regularly to your

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recitations and you aren't 100%
sure if your TA knows

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exactly who you are, you need to
make sure you have your MIT

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ID with you.

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You can't take the exam unless
you're registered for the

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class, and we need to make
sure we can verify that.

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You also need to bring a
calculator, of course, because

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we'll be solving problems that
involve calculations.

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You can bring any calculator
you want, we don't actually

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have restrictions for calculator
types here, but

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what you can't do, is you can't
program any relevant

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chemical or information about
constants in there.

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It's OK to use certain
fundamental constants that

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come in a lot of calculators,
so there's nothing we can do

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about that.

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That's OK.

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If you're wondering what's OK
or what's not OK, it's very

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clearly written out in this
handout, so make sure you read

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through it, because it is your
responsibility to make sure

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that your calculator does
not have anything extra

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programmed in it.

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And if you have your calculator
all set up as you

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love and you don't want to
change it, then maybe you

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00:04:08,120 --> 00:04:11,010
should just go and get an $8.00
scientific calculator

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that doesn't have any of the
graphing functions, because

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you don't actually need them, so
that's a better option for

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00:04:15,580 --> 00:04:18,330
you, you can do that as well.

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And I had mentioned several
times that you do not need to

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memorize the majority of the
equations and you don't need

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to memorize any physical
constants.

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So if you flip the info page
over on the back here, what

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you'll see is the periodic
table, this is the same one

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that I've handed out in the
last two lectures -- the

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00:04:35,925 --> 00:04:38,490
periodic table without any
electron configurations.

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00:04:38,490 --> 00:04:40,740
This is exactly the sheet here,
it's exactly what you'll

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get on exam day.

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00:04:42,000 --> 00:04:44,870
You'll also see that they have
all the physical constants

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that you're going to need, and
also a bunch of the actual

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equations that we've been using
in the first couple

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weeks here.

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So you don't need to memorize
any of this, you're actually

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going to be handed this.

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There are a few equations that
you need to memorize -- those

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are the very simple -- very,
very simple equations, such as

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e equals h times nu -- hopefully
you don't have to

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sit down and try to memorize
that, hopefully we all know

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that already.

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But just to be really clear,
I've written out exactly which

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equations you do have to
memorize on the front here, so

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00:05:13,190 --> 00:05:17,730
long as you know those, the
rest you can just look up.

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And in terms of using these
equations in solving problems

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on the exam, and also using
these constants, make sure if

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you think there might be any
chance you're going to get any

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little part of a problem wrong
or do a calculation

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00:05:30,210 --> 00:05:33,280
inaccurately, you need to write
out every single step of

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00:05:33,280 --> 00:05:36,340
your thinking as you write out
these problems. We can't give

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00:05:36,340 --> 00:05:38,700
you any partial credit
whatsoever if we can't see

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your thought process.

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So it's important to write out
the equation you use, you need

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to write out the constants
that you use to

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fill in that equation.

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And that we need to see your
work to get full credit, and

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then especially if you get
things wrong, we need to know

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where it went wrong, because
we do try to give as much

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partial credit as possible in
these exams, since there are a

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lot of places where small
mistakes can result in the

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wrong answer.

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So also, along those lines in
terms of test taking, make

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sure you also box your answers
and that you keep track of

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significant figures and that you
also remember to include

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your units.

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These are just little things
that can add up, so you just

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want to make sure you're
on top of those.

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And your TAs on Tuesday are
going to share a lot of other

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types of sort of exam strategies
in thinking about

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how you can approach an exam
when we're in a time situation

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like we are, so they'll share
some of their experience with

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you in terms of taking
these timed exams.

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So, in terms of practicing this
weekend, I mentioned that

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instead of getting a problem-set
today, what I am

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going to be posting is optional
extra problems. So

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they're optional, but they're
very, very, very highly

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00:06:42,700 --> 00:06:45,150
encouraged that you do these,
because this is going to give

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you practice for the types of
problems that are going to be

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on the exam.

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We're also posting a practice
exam for you to take, so after

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you're completely done your
studying, it's good to have

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everything done before you take
the practice exam, and

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then sit down just with this
sheet here and your

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calculator, and ideally a timer,
and make sure you can

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do the practice exam in the
allotted amount of time.

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So that way you can have an
idea if, oh, I do really

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understand this but I'm a little
bit slow, maybe I need

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to practice this one type of
problem a little bit longer so

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I can get up to speed so I'm
going to be able to get

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through all this in terms
of the exam time.

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All right.

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So let's move on to
today's topics.

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So, as I said, we're finishing
up with what we left off with

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yesterday -- or excuse
me, on Wednesday.

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This includes atomic radius and
the idea of isoelectronic

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00:07:31,340 --> 00:07:34,450
atoms. So that's going to
be the end of the exam 1

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00:07:34,450 --> 00:07:38,710
material, and then we'll move
on to exam 2 material, which

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is kind of exciting, because
we've been talking about just

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individual atoms and ions up to
this point, and now we can

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talk about molecules, so
we're going to start

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talking about bonding.

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So for some you that are less
interested in maybe the

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physical structure of an
individual atom, now some more

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exciting material for you might
be coming up if you like

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to think about how, instead,
molecules behave, either

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within bonding, within
themselves, or with other

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00:08:01,660 --> 00:08:03,650
molecules, that's what we're
going to be heading to

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00:08:03,650 --> 00:08:06,780
in this next unit.

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00:08:06,780 --> 00:08:09,920
So, we need to finish up
with periodic trends.

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And first, on your
lecture notes, I

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start with atomic radius.

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I was so proud of myself getting
the lecture notes

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finished early and handing them
in to CopyTech, and then

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I realized we didn't do

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electronegativity on Wednesday.

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So, if you can flip your lecture
notes over and just

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write on the blank space,
we're going to cover

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electronegativity first here,
and specifically, you can go

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back and fill this in to your
lecture 9 notes, if you want

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to stay organized, but I just
suggest just writing it on

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lecture 10 notes now
and going back.

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You can still keep organized,
which hopefully most of you

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like to do, and get it in the
right place in the notes.

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So, when we're talking about the
idea of electronegativity,

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essentially what we're talking
about is the ability for an

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atom to attract electron density
from another atom.

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00:09:05,300 --> 00:09:08,330
So it's just a measure of how
much does one given atom want

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to pull away electron
density from, let's

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say, an adjacent atom.

202
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So it's actually very related
to what we're talking about

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when we said electron affinity,
and it's also

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related to ionization energy,
and we can call

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electronegativity by symbol
here, and it turns out that

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it's going to be proportional
to 1/2 of the electron

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affinity of a given atom, plus
the ionization energy.

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So, in other words, we
can just think of

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00:09:36,340 --> 00:09:39,290
electronegativity as being the
average of that ionization

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00:09:39,290 --> 00:09:41,290
energy and the electron
affinity.

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This should make sense, because
if an atom has a very

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high electron affinity, that
means it's really happy taking

213
00:09:47,360 --> 00:09:50,440
an electron from another atom,
or taking a free electron --

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00:09:50,440 --> 00:09:51,970
that that's very favorable.

215
00:09:51,970 --> 00:09:54,720
If something has a high
ionization energy, it means

216
00:09:54,720 --> 00:09:56,730
that it really, really,
really does not want

217
00:09:56,730 --> 00:09:58,280
to give up an electron.

218
00:09:58,280 --> 00:10:00,830
So you can think about how these
2 things combined are

219
00:10:00,830 --> 00:10:03,890
going to be electronegativity,
which is a measure of how much

220
00:10:03,890 --> 00:10:06,430
an atom wants to pull electron
density away

221
00:10:06,430 --> 00:10:08,300
from another atom.

222
00:10:08,300 --> 00:10:12,100
So, if we think about
electronegativity as a

223
00:10:12,100 --> 00:10:16,450
periodic trend, we can just draw
our nice periodic table

224
00:10:16,450 --> 00:10:19,650
here, and let's separate
it into quadrants.

225
00:10:19,650 --> 00:10:22,320
So if we think about the upper
right hand part of the

226
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quadrant, well, this is where
we're going to have high

227
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electron affinity and high
ionization energy, so we're

228
00:10:28,960 --> 00:10:34,340
also going to see high
electronegativity here.

229
00:10:34,340 --> 00:10:37,980
And in contrast, in the lower
left hand part of the periodic

230
00:10:37,980 --> 00:10:41,200
table, these 2 quantities are
low, so also what we're going

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00:10:41,200 --> 00:10:46,740
to see is low electronegativity.

232
00:10:46,740 --> 00:10:49,680
And if we talk about what's
going on in areas, or with

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00:10:49,680 --> 00:10:52,210
atoms that have high
electronegativity, and we

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00:10:52,210 --> 00:10:56,130
think about whether they're
electron donors or electron

235
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acceptors, what would you expect
for an atom that has

236
00:10:59,500 --> 00:11:00,690
high electronegativity?

237
00:11:00,690 --> 00:11:04,280
Is it going to be an electron
donor or acceptor?

238
00:11:04,280 --> 00:11:04,650
Great.

239
00:11:04,650 --> 00:11:07,030
Yup, it's going to be an
electron acceptor, it wants to

240
00:11:07,030 --> 00:11:09,730
accept electrons, it wants to
accept electron density.

241
00:11:09,730 --> 00:11:14,310
So, in contrast, if it has a
low electronegativity, this

242
00:11:14,310 --> 00:11:21,540
then is going to be
an electron donor.

243
00:11:21,540 --> 00:11:24,020
All right, so it's very
common to talk about

244
00:11:24,020 --> 00:11:26,470
electronegativity of different
atoms, and you can look up

245
00:11:26,470 --> 00:11:27,580
tables of these.

246
00:11:27,580 --> 00:11:29,760
Often what you'll see is not
a table based on this

247
00:11:29,760 --> 00:11:32,330
definition, but something that's
called the Pauling

248
00:11:32,330 --> 00:11:34,000
definition of electronegativity,
but it's

249
00:11:34,000 --> 00:11:37,350
exactly the same idea and the
same trend as this more

250
00:11:37,350 --> 00:11:39,320
numerical way to think about
what the meaning of

251
00:11:39,320 --> 00:11:40,730
electronegativity is.

252
00:11:40,730 --> 00:11:44,620
All right, so now we can move
on to the start of today's

253
00:11:44,620 --> 00:11:46,740
notes, which is atomic radius.

254
00:11:46,740 --> 00:11:50,870
So this, in fact, is going to
be our last principle that

255
00:11:50,870 --> 00:11:54,690
we're going to talk about in
terms of periodic trends.

256
00:11:54,690 --> 00:11:57,500
So this is actually the most
straightforward, so sometimes

257
00:11:57,500 --> 00:11:59,880
it's nice to end with the
easiest concept, and that's

258
00:11:59,880 --> 00:12:01,110
what we're doing here.

259
00:12:01,110 --> 00:12:03,520
And if we're talking about
atomic radius, essentially

260
00:12:03,520 --> 00:12:06,000
we're talking about
atomic size.

261
00:12:06,000 --> 00:12:08,930
And immediately it should
probably come into your head

262
00:12:08,930 --> 00:12:12,070
that we don't actually have an
atomic radius that we can talk

263
00:12:12,070 --> 00:12:12,530
about, right?

264
00:12:12,530 --> 00:12:15,130
What I just spent many lectures
discussing is the

265
00:12:15,130 --> 00:12:19,500
fact that we can not know how
far away an electron is from

266
00:12:19,500 --> 00:12:22,350
the nucleus, so we can't
actually know the radius of a

267
00:12:22,350 --> 00:12:23,520
certain atom.

268
00:12:23,520 --> 00:12:25,010
And that's true.

269
00:12:25,010 --> 00:12:28,030
An atom's not a defined
sphere, for example.

270
00:12:28,030 --> 00:12:31,320
We can't define it as an exact
radius in terms of the

271
00:12:31,320 --> 00:12:33,840
definition we might think
of classically.

272
00:12:33,840 --> 00:12:36,470
So, keep that in mind when we're
talking about atomic

273
00:12:36,470 --> 00:12:39,400
radius, I'm not suddenly
changing my story and saying,

274
00:12:39,400 --> 00:12:41,270
yes, we do have a
distinct radius.

275
00:12:41,270 --> 00:12:43,890
Instead, what people have done
is come up with different ways

276
00:12:43,890 --> 00:12:46,160
to think about how they
can define a radius.

277
00:12:46,160 --> 00:12:48,680
And one common way to think
about it, is to think about

278
00:12:48,680 --> 00:12:52,780
the value of r, or the radius,
below which 90% of that

279
00:12:52,780 --> 00:12:55,060
electron density is going
to be contained.

280
00:12:55,060 --> 00:12:57,060
So we're not saying it's
all the electron

281
00:12:57,060 --> 00:12:58,830
density, it's just 90%.

282
00:12:58,830 --> 00:13:01,700
Because we know as we go to
infinity, even though the

283
00:13:01,700 --> 00:13:04,890
density gets smaller and smaller
and smaller, we still

284
00:13:04,890 --> 00:13:07,980
have electron density very far
away from the nucleus.

285
00:13:07,980 --> 00:13:11,130
So, what we're going to define
is just let's just capture 90%

286
00:13:11,130 --> 00:13:12,950
of that electron density.

287
00:13:12,950 --> 00:13:15,280
So, that's one way to think
about it, and there's also

288
00:13:15,280 --> 00:13:16,620
another way, and this
is the way that your

289
00:13:16,620 --> 00:13:17,790
book presents it.

290
00:13:17,790 --> 00:13:21,510
If you, in fact, have two of
the same atom right next to

291
00:13:21,510 --> 00:13:23,650
each other, let's say you have a
crystal, or let's say you're

292
00:13:23,650 --> 00:13:27,350
talking about a metal, what you
can do is just look at the

293
00:13:27,350 --> 00:13:31,010
distance between the two nuclei,
and split that in 1/2,

294
00:13:31,010 --> 00:13:33,410
and take the atomic
radius that way.

295
00:13:33,410 --> 00:13:35,760
So, these are two different
definitions of how to think

296
00:13:35,760 --> 00:13:38,500
about atomic radius, but really
what you find when

297
00:13:38,500 --> 00:13:40,500
these are measured is they
come up with almost the

298
00:13:40,500 --> 00:13:43,570
identical values, so there are
tables you can look up of

299
00:13:43,570 --> 00:13:47,430
atomic radii and see these
values, and you can trust them

300
00:13:47,430 --> 00:13:50,670
that, yes, they work for both
this definition and for this

301
00:13:50,670 --> 00:13:54,770
definition here,
in most cases.

302
00:13:54,770 --> 00:13:56,710
And what we've been talking
about with all of these

303
00:13:56,710 --> 00:14:00,390
properties are, of course, how
can we figure out what that is

304
00:14:00,390 --> 00:14:02,850
for a certain atom by looking
at the periodic table, so we

305
00:14:02,850 --> 00:14:04,130
want to think about
the periodic

306
00:14:04,130 --> 00:14:06,190
trend for atomic radius.

307
00:14:06,190 --> 00:14:09,460
And we know as we go across a
row in the periodic table,

308
00:14:09,460 --> 00:14:12,340
what's happening is that z
effective or the effective

309
00:14:12,340 --> 00:14:14,920
pull on the nucleus
is increasing.

310
00:14:14,920 --> 00:14:18,100
So would you expect, therefore,
as we go across a

311
00:14:18,100 --> 00:14:23,040
row for the atomic radius, to
increase or to decrease?

312
00:14:23,040 --> 00:14:23,350
Good.

313
00:14:23,350 --> 00:14:24,020
OK, yes.

314
00:14:24,020 --> 00:14:26,880
We are expecting to see that
it decreases because it's

315
00:14:26,880 --> 00:14:30,430
feeling a stronger pull, all the
electrons are being pulled

316
00:14:30,430 --> 00:14:33,860
in closer to the nucleus, so
that atomic size is going to

317
00:14:33,860 --> 00:14:35,530
get smaller.

318
00:14:35,530 --> 00:14:38,530
This is in contrast to what's
happening as we go down a

319
00:14:38,530 --> 00:14:39,800
periodic table.

320
00:14:39,800 --> 00:14:42,660
So as we go down we're now
adding electrons to further

321
00:14:42,660 --> 00:14:45,240
and further away shells, so
what we're going to see is

322
00:14:45,240 --> 00:14:47,580
that the atomic radius is going
to increase as we're

323
00:14:47,580 --> 00:14:52,290
going down the periodic table.

324
00:14:52,290 --> 00:14:54,610
And we can look at
an example here.

325
00:14:54,610 --> 00:14:57,410
If we start in the upper left
hand corner of the periodic

326
00:14:57,410 --> 00:15:00,470
table with lithium, you can
see that as we go down the

327
00:15:00,470 --> 00:15:03,270
table, what you're seeing is
that that atomic radius is

328
00:15:03,270 --> 00:15:05,860
actually increasing,
as we would expect.

329
00:15:05,860 --> 00:15:09,060
Whereas, if we go across a row,
what we see is that the

330
00:15:09,060 --> 00:15:11,330
atomic radius is decreasing.

331
00:15:11,330 --> 00:15:14,210
So, again, this is one of the
more straightforward trends.

332
00:15:14,210 --> 00:15:15,960
You just need to remember
what's happening to z

333
00:15:15,960 --> 00:15:18,450
effective, which really tells
us what's happening with all

334
00:15:18,450 --> 00:15:21,170
the trends, and once you know
z effective, you can figure

335
00:15:21,170 --> 00:15:24,140
out, for example, what direction
the atomic radius

336
00:15:24,140 --> 00:15:26,130
should be going into.

337
00:15:26,130 --> 00:15:28,190
So, that's it for
periodic trends.

338
00:15:28,190 --> 00:15:29,950
We have talked about four
different ones.

339
00:15:29,950 --> 00:15:33,780
We talked about ionization
energy, electron affinity, we

340
00:15:33,780 --> 00:15:35,530
talked about electronegativity,
which is

341
00:15:35,530 --> 00:15:38,610
just kind of a combination of
the first two, and then ended

342
00:15:38,610 --> 00:15:41,190
with atomic radius here.

343
00:15:41,190 --> 00:15:44,190
And what you might have noted
is although we described how

344
00:15:44,190 --> 00:15:46,950
to make predictions about these
properties, I didn't

345
00:15:46,950 --> 00:15:49,090
talk too much about what it
actually means, what the

346
00:15:49,090 --> 00:15:51,680
ramifications of these different
properties are.

347
00:15:51,680 --> 00:15:53,830
And the reason we didn't do that
is because we're actually

348
00:15:53,830 --> 00:15:56,940
going to spend much of the rest
of the course relating

349
00:15:56,940 --> 00:16:00,360
these different properties to
the properties of molecules in

350
00:16:00,360 --> 00:16:03,950
terms of bonding, and also in
terms of chemical reactions.

351
00:16:03,950 --> 00:16:07,890
So, for example, if we have a
very electronegative atom

352
00:16:07,890 --> 00:16:10,820
within a certain molecule, what
you'll actually find is

353
00:16:10,820 --> 00:16:15,740
that it does affect how the
molecule is going to take part

354
00:16:15,740 --> 00:16:17,820
in different chemical or
biological reactions.

355
00:16:17,820 --> 00:16:20,420
And this will become more and
more clear as we actually talk

356
00:16:20,420 --> 00:16:22,740
about these reactions and
talk about bonding.

357
00:16:22,740 --> 00:16:24,930
But you need to be able to
predict what kind of

358
00:16:24,930 --> 00:16:27,690
properties a certain atom's
going to have within a

359
00:16:27,690 --> 00:16:29,500
molecule, whether you're talking
about something, for

360
00:16:29,500 --> 00:16:31,730
example, that's very
electronegative, or something

361
00:16:31,730 --> 00:16:33,960
that is not electronegative at
all, it is going to make a

362
00:16:33,960 --> 00:16:36,560
difference in terms of thinking
about how molecules

363
00:16:36,560 --> 00:16:37,790
are structured and
also how they

364
00:16:37,790 --> 00:16:41,310
interact with other molecules.

365
00:16:41,310 --> 00:16:43,780
However, I can give you at least
one example while we're

366
00:16:43,780 --> 00:16:47,100
still on just talking about
atoms. So we haven't gotten to

367
00:16:47,100 --> 00:16:50,150
molecules yet, we're just
talking about single atoms or

368
00:16:50,150 --> 00:16:53,350
single ions, but what's nice
is just talking about this

369
00:16:53,350 --> 00:16:56,130
very straightforward principle
of atomic radius.

370
00:16:56,130 --> 00:17:00,110
We can already use that in terms
of single ions to think

371
00:17:00,110 --> 00:17:03,240
about a really complex
biological issue, which is to

372
00:17:03,240 --> 00:17:05,030
talk about ion channels.

373
00:17:05,030 --> 00:17:07,310
So, that is just a quick example
for some of you, you

374
00:17:07,310 --> 00:17:10,070
might be very familiar with ion
channels, others might not

375
00:17:10,070 --> 00:17:12,870
know what these are, so I'll
just tell you quite briefly

376
00:17:12,870 --> 00:17:15,570
that ion channels are
these very massive

377
00:17:15,570 --> 00:17:17,540
transmembrane proteins.

378
00:17:17,540 --> 00:17:21,600
Essentially, what they are is
it's a protein that spans the

379
00:17:21,600 --> 00:17:23,760
membrane of a cell.

380
00:17:23,760 --> 00:17:26,560
And what they do is they
regulate the influx of ions

381
00:17:26,560 --> 00:17:27,920
across that cell.

382
00:17:27,920 --> 00:17:30,420
So the influx of ions from the
outside of the cell to the

383
00:17:30,420 --> 00:17:32,440
inside of the cell,
for example.

384
00:17:32,440 --> 00:17:35,980
And you can think of ion
channels as being gated, by

385
00:17:35,980 --> 00:17:39,340
gated it means the gate can be
closed and no ions are going

386
00:17:39,340 --> 00:17:41,500
through, as in this case here.

387
00:17:41,500 --> 00:17:45,660
Or you can talk about the gate
being open, and in this case,

388
00:17:45,660 --> 00:17:48,670
you can see that you will
have an influx of ions.

389
00:17:48,670 --> 00:17:52,060
So, ion channels are important
for maintaining a voltage

390
00:17:52,060 --> 00:17:54,820
difference between the inside of
the cell and outside of the

391
00:17:54,820 --> 00:17:57,560
cell, and they're found in all
sorts of cell types in your

392
00:17:57,560 --> 00:18:00,520
body, but if you think about
where they're most prevalent,

393
00:18:00,520 --> 00:18:04,380
it turns out that they're most
prevalent in muscle cells and

394
00:18:04,380 --> 00:18:07,360
also in nerve cells,
so in your neurons.

395
00:18:07,360 --> 00:18:10,810
And essentially, what they do
in neurons is they underlie

396
00:18:10,810 --> 00:18:14,250
those nerve impulses, or those,
essentially what we

397
00:18:14,250 --> 00:18:17,450
call electrical signaling
between neurons -- you might

398
00:18:17,450 --> 00:18:21,020
also call that the action
potential of the neurons.

399
00:18:21,020 --> 00:18:24,510
So essentially, they regulate
this action potential, and

400
00:18:24,510 --> 00:18:28,160
they do so by helping to
establish and then control the

401
00:18:28,160 --> 00:18:30,810
voltage gradient within the
cells. so, essentially,

402
00:18:30,810 --> 00:18:34,070
they're establishing or
controlling or changing the

403
00:18:34,070 --> 00:18:37,640
difference between the charge
inside the cell and the charge

404
00:18:37,640 --> 00:18:39,480
outside cell.

405
00:18:39,480 --> 00:18:42,320
And when we talk about any type
of ion channel, there are

406
00:18:42,320 --> 00:18:44,590
just tons of different kinds of
ion channels, and you can

407
00:18:44,590 --> 00:18:46,620
characterize them in a
few different ways.

408
00:18:46,620 --> 00:18:48,700
So, for example, you could
characterize them in terms of

409
00:18:48,700 --> 00:18:51,370
how they're gated, and basically
how they open or

410
00:18:51,370 --> 00:18:54,030
close -- that's one way to talk
about different types.

411
00:18:54,030 --> 00:18:56,420
Another way to talk about
different types is to think

412
00:18:56,420 --> 00:18:58,920
about which ion they're
selected for.

413
00:18:58,920 --> 00:19:02,090
And all ion channels are
selective for a single type of

414
00:19:02,090 --> 00:19:06,030
ion, and we can think about
how that selectivity takes

415
00:19:06,030 --> 00:19:08,640
place, and that's where this
idea of atomic radius is going

416
00:19:08,640 --> 00:19:10,280
to become very important.

417
00:19:10,280 --> 00:19:13,780
So, for example, if we look at
sodium channels, and sodium

418
00:19:13,780 --> 00:19:16,340
channels are some of the
particularly prevalent ones

419
00:19:16,340 --> 00:19:19,010
when we're talking about
neurons, if you think about

420
00:19:19,010 --> 00:19:22,880
the cell membrane, and this
little green cartoon is me

421
00:19:22,880 --> 00:19:26,090
trying to show a sodium channel
here, and in this

422
00:19:26,090 --> 00:19:28,910
case, you can see that it's
closed, such that no ions are

423
00:19:28,910 --> 00:19:30,080
getting through.

424
00:19:30,080 --> 00:19:33,240
However, when that gate is
opened, the sodium channel is

425
00:19:33,240 --> 00:19:36,570
now going to be incredibly
selective and only let through

426
00:19:36,570 --> 00:19:39,320
sodium ions and no other
type of ion.

427
00:19:39,320 --> 00:19:41,480
And this is really interesting
to think about because you can

428
00:19:41,480 --> 00:19:44,300
imagine in our body we have
concentrations of all types of

429
00:19:44,300 --> 00:19:47,780
ions, and specifically, some
seem very, very similar to

430
00:19:47,780 --> 00:19:48,460
each other.

431
00:19:48,460 --> 00:19:51,520
So we could think about
comparing the potassium ion to

432
00:19:51,520 --> 00:19:52,850
a sodium ion.

433
00:19:52,850 --> 00:19:54,740
They have the same charge
of plus one.

434
00:19:54,740 --> 00:19:57,130
The only thing that's different
is that they're one

435
00:19:57,130 --> 00:20:02,000
down on the periodic table,
potassium is down one row, so

436
00:20:02,000 --> 00:20:03,890
it's going to be a little
bigger, but when we're

437
00:20:03,890 --> 00:20:06,900
thinking about size, it maybe
does not seem that significant

438
00:20:06,900 --> 00:20:08,370
to talk about the size.

439
00:20:08,370 --> 00:20:10,500
But what we find out
is that it is.

440
00:20:10,500 --> 00:20:13,690
So, what happens, this is
another view of a sodium

441
00:20:13,690 --> 00:20:15,930
channel, so this is actually
looking a little bit more at

442
00:20:15,930 --> 00:20:17,330
the protein structure.

443
00:20:17,330 --> 00:20:19,980
What all of these channels
have is what's called a

444
00:20:19,980 --> 00:20:23,100
selectivity filter, so this
filter filters out the type of

445
00:20:23,100 --> 00:20:25,990
ion that's going to be
allowed through.

446
00:20:25,990 --> 00:20:27,560
And there's two parts
of the filter.

447
00:20:27,560 --> 00:20:30,310
First we need to select
for actual charge.

448
00:20:30,310 --> 00:20:32,780
So the way that it does this
is the filter is actually

449
00:20:32,780 --> 00:20:35,490
lined with all of this negative
charge, and for those

450
00:20:35,490 --> 00:20:38,330
of you that are more into
biology or biochemistry,

451
00:20:38,330 --> 00:20:41,020
that's because of negatively
charged amino acid residues,

452
00:20:41,020 --> 00:20:44,680
but all you need to think about
is that it has negative

453
00:20:44,680 --> 00:20:48,360
charge in the inside of this
pore, and what happens then is

454
00:20:48,360 --> 00:20:50,650
that if something has a positive
charge, it's going to

455
00:20:50,650 --> 00:20:54,000
be stabilized to enter this
pore, whereas any negative

456
00:20:54,000 --> 00:20:55,470
ions are going to be repelled.

457
00:20:55,470 --> 00:20:58,510
So that's the first step in
being selective, but now how

458
00:20:58,510 --> 00:20:59,920
do we differentiate between
these sodium

459
00:20:59,920 --> 00:21:00,920
and potassium ions.

460
00:21:00,920 --> 00:21:04,640
And the answer is just really
beautifully simple, and it's

461
00:21:04,640 --> 00:21:07,340
just that the pore gets really,
really tiny, to the

462
00:21:07,340 --> 00:21:10,000
point that it gets so small that
all that can fit through

463
00:21:10,000 --> 00:21:14,880
this pore is one a single ion,
one single sodium ion,

464
00:21:14,880 --> 00:21:18,760
solvated by one single
water molecule.

465
00:21:18,760 --> 00:21:21,360
And that's all that's big enough
to pass through or

466
00:21:21,360 --> 00:21:23,120
small enough to pass through.

467
00:21:23,120 --> 00:21:26,820
And if we go up even just one
row on the periodic table to

468
00:21:26,820 --> 00:21:30,200
potassium, what we actually see
is now that it's going to

469
00:21:30,200 --> 00:21:33,070
be too large, and, in fact, a
potassium solvated with one

470
00:21:33,070 --> 00:21:36,620
water molecule won't go
through our channel.

471
00:21:36,620 --> 00:21:39,480
So, this is just one example
of how these properties can

472
00:21:39,480 --> 00:21:41,760
already, even our understanding
just talking

473
00:21:41,760 --> 00:21:44,760
about single atoms, can already
make an impact in

474
00:21:44,760 --> 00:21:47,600
these biological systems. And
actually, a question that

475
00:21:47,600 --> 00:21:50,400
might come up, I just explained,
the sodium channel,

476
00:21:50,400 --> 00:21:53,260
you might say, well, how do
potassium channels work then,

477
00:21:53,260 --> 00:21:55,830
because I can understand how you
can filter something big

478
00:21:55,830 --> 00:21:57,820
out, but how do you filter
out something small.

479
00:21:57,820 --> 00:22:00,590
And it turns out that size
exclusion is also the

480
00:22:00,590 --> 00:22:04,420
principle that's in play with
potassium channels as well,

481
00:22:04,420 --> 00:22:06,800
but in this case it's a little
more complex, because what

482
00:22:06,800 --> 00:22:10,780
happens is these negative
residues the are in the pore

483
00:22:10,780 --> 00:22:13,120
need to stabilize the potassium
as it goes through,

484
00:22:13,120 --> 00:22:16,920
and the potassium is large
enough to make all the

485
00:22:16,920 --> 00:22:19,930
contacts it needs, but the
sodium, which you can picture

486
00:22:19,930 --> 00:22:23,410
being smaller, actually can't
reach all of the stabilizing

487
00:22:23,410 --> 00:22:25,770
charges that it needs to to
get through the pore.

488
00:22:25,770 --> 00:22:28,380
So, again, it is based on size,
it's a little bit less

489
00:22:28,380 --> 00:22:30,790
intuitive than the idea of just
straining out all of the

490
00:22:30,790 --> 00:22:31,340
potassium ions.

491
00:22:31,340 --> 00:22:34,950
But again, many of these ion
channels have this size

492
00:22:34,950 --> 00:22:37,700
exclusion pore, it's a very
important part of them.

493
00:22:37,700 --> 00:22:41,890
All right, so that was a quick
aside on thinking about how

494
00:22:41,890 --> 00:22:43,960
these properties can,
in fact, relate to

495
00:22:43,960 --> 00:22:45,800
something in our body.

496
00:22:45,800 --> 00:22:49,340
Let's move on to the last topic
in terms of this first

497
00:22:49,340 --> 00:22:51,650
exam, which is thinking about
the idea of isoelectronic

498
00:22:51,650 --> 00:22:56,000
atoms, or isoelectronic ions.

499
00:22:56,000 --> 00:22:58,950
And isoelectronic is very
straightforward, it just means

500
00:22:58,950 --> 00:23:01,520
having the same electron
configuration.

501
00:23:01,520 --> 00:23:04,890
The easiest way to look at this
is just to do an example.

502
00:23:04,890 --> 00:23:07,220
So let's take the
example of neon.

503
00:23:07,220 --> 00:23:10,680
This has the electron
configuration of 1 s 2, 2

504
00:23:10,680 --> 00:23:12,530
s 2, and 2 p 6.

505
00:23:12,530 --> 00:23:16,140
It looks like we're cut off the
screen a little bit here,

506
00:23:16,140 --> 00:23:18,600
but you can see I've just
circled it there.

507
00:23:18,600 --> 00:23:21,150
So we can go ahead and think
about, well, are there any

508
00:23:21,150 --> 00:23:23,130
other atoms that are going
to have the same electron

509
00:23:23,130 --> 00:23:23,790
configuration?

510
00:23:23,790 --> 00:23:26,110
The answer to that is definitely
no -- if they had

511
00:23:26,110 --> 00:23:27,732
the same electron configuration,
they would, in

512
00:23:27,732 --> 00:23:29,200
fact, be neon.

513
00:23:29,200 --> 00:23:31,590
But we can think about different
ions that have this

514
00:23:31,590 --> 00:23:33,070
electron configuration.

515
00:23:33,070 --> 00:23:36,510
So for example, if we think
about fluorine, that has an

516
00:23:36,510 --> 00:23:41,570
electron configuration of 1 s
2, 2 s 2, 2 p 5, so all we

517
00:23:41,570 --> 00:23:44,210
would need to do is add one more
electron to get the same

518
00:23:44,210 --> 00:23:46,270
configuration as for neon.

519
00:23:46,270 --> 00:23:49,300
So if we want to write out what
that would be, it would

520
00:23:49,300 --> 00:23:55,980
just be to say that f minus is
isoelectronic with neon.

521
00:23:55,980 --> 00:24:00,770
So, we can say that -- if we
have neon here and we want to

522
00:24:00,770 --> 00:24:03,860
think about what's
isoelectronic, f minus would

523
00:24:03,860 --> 00:24:04,170
be isoelectronic.

524
00:24:04,170 --> 00:24:10,230
We also have oxygen -- what
would the charge on oxygen be?

525
00:24:10,230 --> 00:24:10,690
Um-hmm, right.

526
00:24:10,690 --> 00:24:12,240
2 minus.

527
00:24:12,240 --> 00:24:16,110
Then also, nitrogen, 3 minus --
these are all going to be

528
00:24:16,110 --> 00:24:18,080
isoelectronic with neon.

529
00:24:18,080 --> 00:24:20,540
We can go in the other
direction, so let's go to

530
00:24:20,540 --> 00:24:24,020
sodium, but we would need to
take away an electron to make

531
00:24:24,020 --> 00:24:25,670
it isoelectronic.

532
00:24:25,670 --> 00:24:30,710
So we would say sodium plus,
or magnesium 2 plus, we can

533
00:24:30,710 --> 00:24:36,960
just keep going -- aluminum 3
plus, silicone 4 plus, and we

534
00:24:36,960 --> 00:24:40,080
can go on and on in either
direction all the way across

535
00:24:40,080 --> 00:24:42,670
and down the periodic table.

536
00:24:42,670 --> 00:24:45,380
So, that's the idea of
isoelectronic ions.

537
00:24:45,380 --> 00:24:48,260
These are all isoelectronic,
they all have the same

538
00:24:48,260 --> 00:24:50,980
electron configuration.

539
00:24:50,980 --> 00:24:53,110
And we can also think
about going back to

540
00:24:53,110 --> 00:24:54,830
atomic size for a second.

541
00:24:54,830 --> 00:24:58,130
What the relationship is between
these ions and their

542
00:24:58,130 --> 00:25:03,470
parent atoms. So, for example,
if we think of the fluorine

543
00:25:03,470 --> 00:25:08,190
minus case, would you expect
fluorine minus to be larger or

544
00:25:08,190 --> 00:25:11,940
smaller than neutral fluorine?

545
00:25:11,940 --> 00:25:12,350
Okay.

546
00:25:12,350 --> 00:25:14,840
I heard mostly larger, but a
little bit of a mix in there,

547
00:25:14,840 --> 00:25:17,780
and it turns out that
larger is correct.

548
00:25:17,780 --> 00:25:20,750
And we can think about why --
essentially we have fluorine

549
00:25:20,750 --> 00:25:22,980
and now we're adding
another electron.

550
00:25:22,980 --> 00:25:25,260
So you can picture that fluorine
is going to get

551
00:25:25,260 --> 00:25:27,060
larger in this case.

552
00:25:27,060 --> 00:25:28,330
And that would be true
for all of the

553
00:25:28,330 --> 00:25:30,320
negatively charged ions.

554
00:25:30,320 --> 00:25:32,970
So, by the same logic, that
means that all of our

555
00:25:32,970 --> 00:25:36,330
positively charged ions are, in
fact, going to be smaller

556
00:25:36,330 --> 00:25:39,370
in terms of radius, compared
to their neutral parents.

557
00:25:39,370 --> 00:25:42,640
Not only are we taking away an
electron here, but we're also

558
00:25:42,640 --> 00:25:45,810
going to decrease shielding,
so the electrons that are

559
00:25:45,810 --> 00:25:48,220
already in there are going to
feel a higher z effective and

560
00:25:48,220 --> 00:25:53,640
will be pulling and the atom
will be getting smaller.

561
00:25:53,640 --> 00:25:56,670
And this is just a picture
showing some of these sizes

562
00:25:56,670 --> 00:25:57,500
with their parent.

563
00:25:57,500 --> 00:26:01,620
So, for example, a lithium here,
you can see how lithium

564
00:26:01,620 --> 00:26:05,200
plus is smaller than the actual
lithium atom in its

565
00:26:05,200 --> 00:26:06,190
neutral state.

566
00:26:06,190 --> 00:26:10,610
Whereas for fluorine, fluorine
is smaller than f minus is the

567
00:26:10,610 --> 00:26:13,570
one that's the outer
shell shown here.

568
00:26:13,570 --> 00:26:15,960
So, let's do a clicker question
on isoelectronic

569
00:26:15,960 --> 00:26:22,450
atoms. And now we're asking you
to look at krypton, so the

570
00:26:22,450 --> 00:26:24,240
atomic mass is 36.

571
00:26:24,240 --> 00:26:26,760
You can actually just grab
that handout, the second

572
00:26:26,760 --> 00:26:29,830
handout on the exam and look at
the periodic table there.

573
00:26:29,830 --> 00:26:32,200
So, which of the following ions
listed is isoelectronic

574
00:26:32,200 --> 00:26:43,950
with krypton?

575
00:26:43,950 --> 00:26:58,050
OK, let's take 10
seconds on that.

576
00:26:58,050 --> 00:26:58,520
OK, good.

577
00:26:58,520 --> 00:27:03,170
This might be our all-time
high, 89% got this right.

578
00:27:03,170 --> 00:27:04,170
This is great.

579
00:27:04,170 --> 00:27:07,980
So, selenium 2 minus is what's
going to be isoelectronic,

580
00:27:07,980 --> 00:27:10,610
because if you add two electrons
to selenium, you'll

581
00:27:10,610 --> 00:27:13,490
get the same electron
configuration that you have

582
00:27:13,490 --> 00:27:14,470
for krypton here.

583
00:27:14,470 --> 00:27:18,070
OK, I think we can safely
go back to notes.

584
00:27:18,070 --> 00:27:20,345
So, I said I would announce
it, that's the

585
00:27:20,345 --> 00:27:22,000
end of exam 1 material.

586
00:27:22,000 --> 00:27:24,350
So, if you compartmentalize
things in your brain in

587
00:27:24,350 --> 00:27:28,250
certain ways, put that off into
the end of the exam 1

588
00:27:28,250 --> 00:27:29,470
part of your brain, and
now we're going to

589
00:27:29,470 --> 00:27:31,010
move on to exam 2.

590
00:27:31,010 --> 00:27:33,220
Remember, for exam 2, you
still need to know and

591
00:27:33,220 --> 00:27:36,290
understand everything you
learned in exam 1, but you can

592
00:27:36,290 --> 00:27:41,400
put off learning it completely
until we get through, at

593
00:27:41,400 --> 00:27:44,660
least, next Wednesday before we
start maybe spending time

594
00:27:44,660 --> 00:27:47,580
on these concepts outside
of class.

595
00:27:47,580 --> 00:27:51,035
So, we're going to start with
talking about bonding, and any

596
00:27:51,035 --> 00:27:54,090
time we have a chemical bond,
basically what we're talking

597
00:27:54,090 --> 00:27:56,860
about is having two atoms where
the arrangement of their

598
00:27:56,860 --> 00:28:00,640
nuclei and they're electrons are
such that the bonded atoms

599
00:28:00,640 --> 00:28:04,720
results in a lower energy than
for the separate atoms. So we

600
00:28:04,720 --> 00:28:07,480
know we always want to have
our systems in as low an

601
00:28:07,480 --> 00:28:10,450
energy as possible, so it makes
sense that a bond would

602
00:28:10,450 --> 00:28:13,740
happen any time we got a lower
energy when we combine two

603
00:28:13,740 --> 00:28:18,110
atoms, versus when we
keep them separate.

604
00:28:18,110 --> 00:28:20,290
So specifically, today
we're going to talk

605
00:28:20,290 --> 00:28:22,020
about covalent bonds.

606
00:28:22,020 --> 00:28:25,560
A covalent bond is any time we
have a pair of electrons that

607
00:28:25,560 --> 00:28:28,470
is shared between two different
atoms. And the key

608
00:28:28,470 --> 00:28:31,900
word for covalent bonds is
the idea of being shared.

609
00:28:31,900 --> 00:28:37,130
The two electrons, for example,
we see in the h 2

610
00:28:37,130 --> 00:28:39,830
molecule, they don't belong to
one or the other atom, they're

611
00:28:39,830 --> 00:28:41,050
actually shared.

612
00:28:41,050 --> 00:28:44,040
And what we'll see later, is
that the sharing is not always

613
00:28:44,040 --> 00:28:46,180
equal -- in the case
of h 2, it is

614
00:28:46,180 --> 00:28:47,420
completely equal sharing.

615
00:28:47,420 --> 00:28:50,950
In some cases, because of things
like electronegativity,

616
00:28:50,950 --> 00:28:53,970
one atom will take away more of
the electron density than

617
00:28:53,970 --> 00:28:56,490
the other atom, but they're
still shared, even if they're

618
00:28:56,490 --> 00:28:58,710
not always evenly shared.

619
00:28:58,710 --> 00:29:01,350
So, in talking about covalent
bonds, we should be able to

620
00:29:01,350 --> 00:29:04,920
still apply a more general
definition of a chemical bond,

621
00:29:04,920 --> 00:29:08,270
which should tell us that the
h 2 molecule is going to be

622
00:29:08,270 --> 00:29:11,990
lower in energy than if we
looked at 2 separate hydrogen

623
00:29:11,990 --> 00:29:12,870
atom molecules.

624
00:29:12,870 --> 00:29:15,480
So, let's see if that's
actually the case.

625
00:29:15,480 --> 00:29:18,150
So if I tell you that the energy
for single hydrogen

626
00:29:18,150 --> 00:29:22,540
atom is negative 13 12
kilojoules per mole.

627
00:29:22,540 --> 00:29:25,240
If we want to talk about two
hydrogen atoms, then we just

628
00:29:25,240 --> 00:29:29,130
need to double that, so that's
going to be negative 2 6 2 4

629
00:29:29,130 --> 00:29:32,230
kilojoules per mole that we're
talking about in terms of a

630
00:29:32,230 --> 00:29:33,940
single hydrogen atom.

631
00:29:33,940 --> 00:29:37,310
So, let's compare this to the
energy of the h 2 molecule,

632
00:29:37,310 --> 00:29:39,630
and we find that that's
negative 3,048

633
00:29:39,630 --> 00:29:40,380
kilojoules per mole.

634
00:29:40,380 --> 00:29:45,130
So, in fact, yes, we did confirm
that these covalent

635
00:29:45,130 --> 00:29:48,610
bond, at least in the case of
hydrogen, we have confirmed by

636
00:29:48,610 --> 00:29:51,510
the numbers that we are at a
lower energy state when we

637
00:29:51,510 --> 00:29:55,660
talk about the bonded atom
versus the individual atom.

638
00:29:55,660 --> 00:29:58,460
And when we talk about covalent
bonds, there's 2

639
00:29:58,460 --> 00:30:00,990
properties that we'll mostly
focus on, and that's going to

640
00:30:00,990 --> 00:30:04,860
be thinking about the bond
strength or the energy by

641
00:30:04,860 --> 00:30:07,450
which it stabilized
when it bonds.

642
00:30:07,450 --> 00:30:10,140
And we can also talk about the
bond length, so we might be

643
00:30:10,140 --> 00:30:13,270
interested in what the bond
length is, what the distance

644
00:30:13,270 --> 00:30:15,450
between these two nuclei are.

645
00:30:15,450 --> 00:30:18,910
And we can actually better
visualize this if we plot how

646
00:30:18,910 --> 00:30:21,540
that energy changes
as a function of

647
00:30:21,540 --> 00:30:24,180
internuclear distance.

648
00:30:24,180 --> 00:30:27,000
And when I say internuclear
distance, we actually call

649
00:30:27,000 --> 00:30:28,670
this r here.

650
00:30:28,670 --> 00:30:31,020
It's kind of ironic that we put
this in the same lecture

651
00:30:31,020 --> 00:30:34,320
as we talk about atomic radii,
which we also call r, but

652
00:30:34,320 --> 00:30:36,260
they're two different r's,
so you need to keep them

653
00:30:36,260 --> 00:30:38,760
separated in terms of what
you're talking about.

654
00:30:38,760 --> 00:30:42,400
When we're talking about r for
internuclear distance, we're

655
00:30:42,400 --> 00:30:45,180
talking about the distance
between two different nuclei

656
00:30:45,180 --> 00:30:48,900
in a bond, in a covalent bond.

657
00:30:48,900 --> 00:30:52,000
So, if we look at this graph
where what we're charting is

658
00:30:52,000 --> 00:30:54,820
the internuclear distance, so
the distance between these two

659
00:30:54,820 --> 00:30:58,700
hydrogen atoms, as a function
of energy, what we are going

660
00:30:58,700 --> 00:31:00,810
to see is a curve that looks
like this -- this is the

661
00:31:00,810 --> 00:31:04,430
general curve that you'll see
for any covalent bond, and

662
00:31:04,430 --> 00:31:07,020
we'll explain where that
comes from in a minute.

663
00:31:07,020 --> 00:31:10,430
I want to point out that the
zero energy is defined as when

664
00:31:10,430 --> 00:31:15,040
you have a naked proton where
the electron has popped out --

665
00:31:15,040 --> 00:31:17,970
that's what we've defined as
zero energy up to this point

666
00:31:17,970 --> 00:31:20,740
when we're talking about single
atoms. So, for starters

667
00:31:20,740 --> 00:31:22,880
we'll keep that as our zero
energy, we're going to change

668
00:31:22,880 --> 00:31:26,150
it soon to make something that
makes more sense in terms of

669
00:31:26,150 --> 00:31:28,610
bonding, but we'll keep
that as zero for now.

670
00:31:28,610 --> 00:31:32,370
So, we see that the two h atoms
separate have a certain

671
00:31:32,370 --> 00:31:34,650
energy that's lower than
when the electron's

672
00:31:34,650 --> 00:31:36,040
not with the atom.

673
00:31:36,040 --> 00:31:38,650
And then even lower
down, we have our

674
00:31:38,650 --> 00:31:42,120
bonded hydrogen molecule.

675
00:31:42,120 --> 00:31:43,940
So, we can think about the
different kinds of

676
00:31:43,940 --> 00:31:45,620
interactions that are
taking place.

677
00:31:45,620 --> 00:31:48,820
I said what hold the bonds
together, what holds two atoms

678
00:31:48,820 --> 00:31:52,510
together is the attractive force
we have between each

679
00:31:52,510 --> 00:31:55,200
electron and the
other nucleus.

680
00:31:55,200 --> 00:31:57,840
That's the huge force that we're
talking about in terms

681
00:31:57,840 --> 00:32:01,120
of making a bond stable, but
there are also repulsive

682
00:32:01,120 --> 00:32:03,360
forces, so you can imagine
we're going to have

683
00:32:03,360 --> 00:32:06,800
electron-electron repulsion
between the two electrons if

684
00:32:06,800 --> 00:32:08,460
we're bringing them
closer together.

685
00:32:08,460 --> 00:32:11,370
And the real killer is if we get
too close we're even going

686
00:32:11,370 --> 00:32:14,750
to have nuclear-nuclear
repulsion between the nuclei

687
00:32:14,750 --> 00:32:16,320
of the two atoms.

688
00:32:16,320 --> 00:32:19,090
So, this makes this chart shown
in pink make a lot more

689
00:32:19,090 --> 00:32:22,720
sense, because if we're way
out at very far distances,

690
00:32:22,720 --> 00:32:25,310
essentially what we have here
is we're talking about two

691
00:32:25,310 --> 00:32:28,270
separate atoms. They're not
interacting at all so that's

692
00:32:28,270 --> 00:32:31,870
why the energy is the same as
that for two individual atoms,

693
00:32:31,870 --> 00:32:33,430
that's what we're
dealing with.

694
00:32:33,430 --> 00:32:36,350
As we get closer together,
we start get lower

695
00:32:36,350 --> 00:32:38,040
and lower in energy.

696
00:32:38,040 --> 00:32:40,550
The reason is because the
predominant force at this

697
00:32:40,550 --> 00:32:43,400
point is going to be the
attraction that's being felt

698
00:32:43,400 --> 00:32:46,930
between the nuclei and the
electrons in each of the

699
00:32:46,930 --> 00:32:51,120
atoms. At some point you're
going to hit a well here,

700
00:32:51,120 --> 00:32:56,140
which is the point where it's
most stabilized or at it's

701
00:32:56,140 --> 00:32:56,520
lowest energy.

702
00:32:56,520 --> 00:32:58,790
So, when we think about a bond
length, this is going to be

703
00:32:58,790 --> 00:33:01,470
the length of our bond here,
that makes sense because it's

704
00:33:01,470 --> 00:33:03,320
going to want to be at
that distance that

705
00:33:03,320 --> 00:33:05,140
minimizes the energy.

706
00:33:05,140 --> 00:33:08,010
But as we keep getting closer,
even though as we get closer,

707
00:33:08,010 --> 00:33:10,390
the attraction is going to get
stronger between the two

708
00:33:10,390 --> 00:33:11,990
nuclei and the electrons.

709
00:33:11,990 --> 00:33:14,610
We're also going to start to
have the repulsive forces

710
00:33:14,610 --> 00:33:17,640
become more prominent here, and,
in fact, they take over

711
00:33:17,640 --> 00:33:21,050
at some point, becoming the more
prevalent of the forces,

712
00:33:21,050 --> 00:33:22,910
so as you get closer, the
electron-electron repulsions,

713
00:33:22,910 --> 00:33:26,950
and eventually the
nucleus-nucleus repulsion is

714
00:33:26,950 --> 00:33:29,070
going to mean that your energy
is just absolutely

715
00:33:29,070 --> 00:33:32,550
skyrocketing, so it just keeps
going up and up as you get

716
00:33:32,550 --> 00:33:36,300
closer to zero here.

717
00:33:36,300 --> 00:33:39,030
So, when we want to talk about
the information that we can

718
00:33:39,030 --> 00:33:41,480
get out of looking at a chart
like this, well, the first

719
00:33:41,480 --> 00:33:43,720
thing I did tell you was that
this is going to be the bond

720
00:33:43,720 --> 00:33:46,890
length, so the distance r where
the energy is lowest,

721
00:33:46,890 --> 00:33:49,720
but we can also talk about
something called dissociation

722
00:33:49,720 --> 00:33:54,170
energy, that's going to be this
distance right here or

723
00:33:54,170 --> 00:33:56,520
the energy that is this value.

724
00:33:56,520 --> 00:33:59,790
And the dissociation energy is
very intuitive in terms of

725
00:33:59,790 --> 00:34:02,570
what it means, it means how much
energy you need to put

726
00:34:02,570 --> 00:34:05,320
into the molecule in order to
disassociate it into its

727
00:34:05,320 --> 00:34:09,650
individual atoms. And so we can
actually think about how

728
00:34:09,650 --> 00:34:12,460
do we calculate what the
dissociation energy should be

729
00:34:12,460 --> 00:34:16,270
for h 2, so let's go
ahead and do this.

730
00:34:16,270 --> 00:34:20,760
So, if we talk about
dissociating h 2, we're going

731
00:34:20,760 --> 00:34:25,260
from the h 2 molecule, and
breaking this bond right in

732
00:34:25,260 --> 00:34:28,540
half, so we now have
two individual

733
00:34:28,540 --> 00:34:31,090
hydrogen atoms here.

734
00:34:31,090 --> 00:34:35,020
So we need to take the energy
for the two atoms, which we

735
00:34:35,020 --> 00:34:39,070
know is -- so let's take our
dissociation energy is going

736
00:34:39,070 --> 00:34:48,360
to be equal to negative 2 6 2 4
kilojoules per mole, and we

737
00:34:48,360 --> 00:34:52,420
want to subtract the energy of
the hydrogen molecule itself,

738
00:34:52,420 --> 00:35:00,560
so that's going to be negative
3 0 4 8 kilojoules per mole.

739
00:35:00,560 --> 00:35:03,770
So, what we get for the
disassociation energy for a

740
00:35:03,770 --> 00:35:11,880
hydrogen atom is 424 kilojoules
per mole.

741
00:35:11,880 --> 00:35:14,680
So what that means is that's how
much energy we would have

742
00:35:14,680 --> 00:35:17,890
to put in to a hydrogen molecule
in order to get it to

743
00:35:17,890 --> 00:35:25,110
split apart into
its two atoms.

744
00:35:25,110 --> 00:35:28,280
So, another way to talk about
dissociation energy is simply

745
00:35:28,280 --> 00:35:32,150
to call it bond strength, it's
the same thing, they're equal

746
00:35:32,150 --> 00:35:32,690
to each other.

747
00:35:32,690 --> 00:35:36,150
If we know that this is it the
dissociation energy for a

748
00:35:36,150 --> 00:35:39,250
hydrogen atom, we can also say
the bond strength for hydrogen

749
00:35:39,250 --> 00:35:43,090
molecule is 424.

750
00:35:43,090 --> 00:35:45,520
So, there's actually another way
to graph it where we can

751
00:35:45,520 --> 00:35:48,550
directly graph the dissociation
energy or the

752
00:35:48,550 --> 00:35:49,620
bond strengths.

753
00:35:49,620 --> 00:35:53,000
So I said before when we were
talking about single atoms, we

754
00:35:53,000 --> 00:35:55,770
always define the zero energy
as when an electron was

755
00:35:55,770 --> 00:36:00,120
actually ejected, but now, when
we talk about chemical

756
00:36:00,120 --> 00:36:03,010
reactions taking place, it's
very, very rare that we're

757
00:36:03,010 --> 00:36:05,940
actually going to be talking
about anything that gets to

758
00:36:05,940 --> 00:36:06,660
this point here.

759
00:36:06,660 --> 00:36:10,990
It's much more relevant to set
our zero point energy as the

760
00:36:10,990 --> 00:36:14,790
separation of a bond in terms of
talking about the reactions

761
00:36:14,790 --> 00:36:16,850
that we'll usually be
dealing with here.

762
00:36:16,850 --> 00:36:20,300
So, let's change our graph where
we now have this zero

763
00:36:20,300 --> 00:36:23,700
point set as the two individuals
hydrogen atoms,

764
00:36:23,700 --> 00:36:27,780
and then we see that our h 2
molecule is at the negative of

765
00:36:27,780 --> 00:36:30,640
the dissociation energy, or the
negative what that bond

766
00:36:30,640 --> 00:36:31,600
strength is.

767
00:36:31,600 --> 00:36:33,630
So we know what that number
would be, it would be negative

768
00:36:33,630 --> 00:36:37,710
424 kilojoules per mole
that we see here.

769
00:36:37,710 --> 00:36:41,380
So, what this let's us do now
is directly compare, for

770
00:36:41,380 --> 00:36:45,420
example, the strength of a bond
in terms of a hydrogen

771
00:36:45,420 --> 00:36:48,760
atom and hydrogen molecule,
compared to any kind of

772
00:36:48,760 --> 00:36:51,350
molecule that we want to
graph on top of it.

773
00:36:51,350 --> 00:36:54,040
So, let's, for example,
look at nitrogen.

774
00:36:54,040 --> 00:36:58,020
So n 2, we can do the chart
here in green, so it's the

775
00:36:58,020 --> 00:37:02,120
green dotted line, and what
we see is that we have now

776
00:37:02,120 --> 00:37:04,000
defined this energy
as where the 2

777
00:37:04,000 --> 00:37:05,870
nitrogen atoms are separated.

778
00:37:05,870 --> 00:37:08,710
So what we can actually directly
compare is the

779
00:37:08,710 --> 00:37:12,230
dissociation energy or the
bond strength of nitrogen

780
00:37:12,230 --> 00:37:14,350
versus hydrogen.

781
00:37:14,350 --> 00:37:16,960
So, if we think about this,
which would you say has a

782
00:37:16,960 --> 00:37:17,930
stronger bond?

783
00:37:17,930 --> 00:37:21,810
Is it going to be hydrogen
or nitrogen?

784
00:37:21,810 --> 00:37:23,530
Yup, it's going to
be nitrogen.

785
00:37:23,530 --> 00:37:26,140
And the reason we can see that
by looking at this graph is

786
00:37:26,140 --> 00:37:28,940
that we see that nitrogen when
it's bonded is in an even

787
00:37:28,940 --> 00:37:31,200
lower well than we
saw for hydrogen.

788
00:37:31,200 --> 00:37:33,420
It's going to be a stronger
bond because it's more

789
00:37:33,420 --> 00:37:38,130
stabilized when it when it comes
together as a molecule.

790
00:37:38,130 --> 00:37:42,020
We can also think about the
distance, the bond distance.

791
00:37:42,020 --> 00:37:43,350
So, which would you
say is going to be

792
00:37:43,350 --> 00:37:44,350
shorter in this case?

793
00:37:44,350 --> 00:37:47,980
Is a hydrogen bond shorter, or
is a nitrogen-nitrogen triple

794
00:37:47,980 --> 00:37:50,310
bond going to be shorter?

795
00:37:50,310 --> 00:37:52,790
Um-hmm, again, we can
get this information

796
00:37:52,790 --> 00:37:54,370
directly from our graph.

797
00:37:54,370 --> 00:37:57,450
We see that the radius is
shorter, so that means that

798
00:37:57,450 --> 00:38:00,460
the nitrogen-nitrogen bond
is going to be shorter.

799
00:38:00,460 --> 00:38:03,340
We can know this information
even if we just knew that the

800
00:38:03,340 --> 00:38:05,880
bond was stronger, we wouldn't
need to look at a graph here,

801
00:38:05,880 --> 00:38:08,890
because it turns out that if you
have a stronger bond, that

802
00:38:08,890 --> 00:38:11,650
also means that you have a
shorter bond -- those ywo are

803
00:38:11,650 --> 00:38:12,260
correlated.

804
00:38:12,260 --> 00:38:15,810
And something that we'll see
later on is that triple bonds,

805
00:38:15,810 --> 00:38:18,880
for example, are going to be
stronger than a corresponding

806
00:38:18,880 --> 00:38:21,130
double bond or a corresponding
single bond.

807
00:38:21,130 --> 00:38:23,720
So, if we talked about a
nitrogen-nitrogen single

808
00:38:23,720 --> 00:38:26,550
versus double versus triple
bond, the triple bond will be

809
00:38:26,550 --> 00:38:29,600
the shortest and it will
be the strongest.

810
00:38:29,600 --> 00:38:32,300
So, that's basically the idea
of how we are going to be

811
00:38:32,300 --> 00:38:33,870
thinking about covalent bonds.

812
00:38:33,870 --> 00:38:37,420
It's also important, once we
start talking about molecules,

813
00:38:37,420 --> 00:38:39,840
to have a way to represent them,
and also to be able to

814
00:38:39,840 --> 00:38:43,410
look at a shorthand notation
for a certain molecule and

815
00:38:43,410 --> 00:38:45,310
understand what the bond is.

816
00:38:45,310 --> 00:38:48,370
So, for example, down here I
wrote that it was n 2 and that

817
00:38:48,370 --> 00:38:51,300
it was h 2, but when I re-wrote
the molecules up

818
00:38:51,300 --> 00:38:54,490
here, you saw that it's an h
h single bond where it's a

819
00:38:54,490 --> 00:38:56,630
nitrogen-nitrogen triple bond.

820
00:38:56,630 --> 00:39:00,140
So any chemist should be able
to just look at n 2 and know

821
00:39:00,140 --> 00:39:02,500
that it's a triple bond, but
that's not something that

822
00:39:02,500 --> 00:39:04,950
we've learned how did to do
yet, so let's go ahead and

823
00:39:04,950 --> 00:39:07,950
start a new topic that's going
to allow us to have some sort

824
00:39:07,950 --> 00:39:11,150
of sense of what the valence
electron configuration, which

825
00:39:11,150 --> 00:39:13,770
includes whether something's a
single or double or a triple

826
00:39:13,770 --> 00:39:17,850
bond can be figured out for
any given molecule.

827
00:39:17,850 --> 00:39:20,320
So, to do this, what I'm going
to do is introduce the topic

828
00:39:20,320 --> 00:39:21,510
of Lewis structures.

829
00:39:21,510 --> 00:39:24,290
We're going to really get into
this next class, but I just

830
00:39:24,290 --> 00:39:27,250
want to introduce it to you to
give us a start, and many of

831
00:39:27,250 --> 00:39:30,310
you have used Lewis structures
in high school, but we'll be

832
00:39:30,310 --> 00:39:33,340
doing some much more challenging
Lewis structures,

833
00:39:33,340 --> 00:39:37,140
I can assure you, in
this class here.

834
00:39:37,140 --> 00:39:41,160
So, a Lewis structure is
basically an organizing

835
00:39:41,160 --> 00:39:46,740
property of bonding, of
molecules, which is the idea

836
00:39:46,740 --> 00:39:49,890
that when we're thinking about
bonding, the key is to achieve

837
00:39:49,890 --> 00:39:53,420
a full valence shell in each of
the individual atoms. So we

838
00:39:53,420 --> 00:39:58,090
want to have in an h h bond, for
example, a full shell for

839
00:39:58,090 --> 00:40:01,550
each of the hydrogen atoms.
And G.N. Lewis is the

840
00:40:01,550 --> 00:40:05,050
scientist that is credited, and
who did, in fact, come up

841
00:40:05,050 --> 00:40:08,690
with this idea for the way to
represent this, so the other

842
00:40:08,690 --> 00:40:11,630
parts of this idea, another way
to phrase it is that the

843
00:40:11,630 --> 00:40:14,300
electrons are going to be
distributed in such a way that

844
00:40:14,300 --> 00:40:18,650
we have what are called full
octets for each of the atoms,

845
00:40:18,650 --> 00:40:21,790
and basically that's the same
thing as saying we have a full

846
00:40:21,790 --> 00:40:24,130
valence shell, and this is
something that Lewis was able

847
00:40:24,130 --> 00:40:28,320
to recognize very, very early,
way before we had quantum

848
00:40:28,320 --> 00:40:31,390
mechanics to describe what these
orbitals were, but it

849
00:40:31,390 --> 00:40:34,630
makes sense a full valence shell
means for most atoms

850
00:40:34,630 --> 00:40:38,820
that we have a full s orbital
plus a full p orbital, so

851
00:40:38,820 --> 00:40:42,200
we're going to have a total of
four orbitals that are each

852
00:40:42,200 --> 00:40:45,300
filled with eight electrons, so
that's why we see that we

853
00:40:45,300 --> 00:40:48,570
need an octet here.

854
00:40:48,570 --> 00:40:52,320
And the idea is that when you do
these Lewis dot structures,

855
00:40:52,320 --> 00:40:55,180
we're representing electrons
with dots, which we'll see in

856
00:40:55,180 --> 00:40:57,260
a minute, and each
dot is going to

857
00:40:57,260 --> 00:40:59,600
represent a valence electron.

858
00:40:59,600 --> 00:41:02,080
So, hopefully, you remember
what we mean by valence

859
00:41:02,080 --> 00:41:04,570
electrons versus
core electrons.

860
00:41:04,570 --> 00:41:07,340
Core electrons are all those
electrons held in really tight

861
00:41:07,340 --> 00:41:10,640
with the nucleus in the inner
shells, whereas the valence

862
00:41:10,640 --> 00:41:12,830
electrons are only those
electrons that are in the

863
00:41:12,830 --> 00:41:16,720
outer-most shell, or at your
highest value of n of the

864
00:41:16,720 --> 00:41:19,110
principal quantum number.

865
00:41:19,110 --> 00:41:22,740
So, Lewis structures are really
a model for a way to

866
00:41:22,740 --> 00:41:26,220
think about what the valence
electron configuration is, and

867
00:41:26,220 --> 00:41:29,130
as I said, it's not based on
quantum mechanics, it's

868
00:41:29,130 --> 00:41:33,140
something that Lewis observed
far, far before quantum

869
00:41:33,140 --> 00:41:34,860
mechanics were discovered.

870
00:41:34,860 --> 00:41:38,230
So he came up with the ideas
that led to the idea of Lewis

871
00:41:38,230 --> 00:41:41,230
structures in the very
early 1900's.

872
00:41:41,230 --> 00:41:44,440
So you might ask well, why are
we using this model if it

873
00:41:44,440 --> 00:41:47,000
clearly doesn't take into
account quantum mechanics?

874
00:41:47,000 --> 00:41:49,420
And the reason that we use it
is that it is incredibly

875
00:41:49,420 --> 00:41:52,580
accurate, and allows us to very,
very quickly predict and

876
00:41:52,580 --> 00:41:55,810
to predict accurately, in most
cases, what the electron

877
00:41:55,810 --> 00:41:58,760
configuration of molecules
are going to be.

878
00:41:58,760 --> 00:42:00,530
So this is really useful.

879
00:42:00,530 --> 00:42:02,730
We don't always want to go
and solve the Schrodinger

880
00:42:02,730 --> 00:42:05,790
equation, and in fact, once we
start talking about molecules,

881
00:42:05,790 --> 00:42:08,270
I can imagine none of you, as
much as you love math or

882
00:42:08,270 --> 00:42:10,110
physics, want to be trying
to solve this Schrodinger

883
00:42:10,110 --> 00:42:12,310
equation in that case either.

884
00:42:12,310 --> 00:42:15,260
So, what Lewis structures allow
us to do is over 90% of

885
00:42:15,260 --> 00:42:18,180
the time be correct in terms
of figuring out what the

886
00:42:18,180 --> 00:42:20,130
electron configuration is.

887
00:42:20,130 --> 00:42:22,150
And we won't just use
them in this class.

888
00:42:22,150 --> 00:42:25,760
If you actually go to any of the
chemistry labs at MIT, if

889
00:42:25,760 --> 00:42:28,670
you go over to building 18 and
look in the organic labs where

890
00:42:28,670 --> 00:42:31,670
they're synthesizing new
molecules or making up new

891
00:42:31,670 --> 00:42:35,680
reactions, what you'll see if
you open anyone's notebook,

892
00:42:35,680 --> 00:42:39,210
their lab notebook, assuming
they keep a nice lab notebook,

893
00:42:39,210 --> 00:42:42,150
is that they will have Lewis
structures drawn in there that

894
00:42:42,150 --> 00:42:44,280
explain the reactions that
they're going to be

895
00:42:44,280 --> 00:42:45,680
doing for that day.

896
00:42:45,680 --> 00:42:48,900
And I mean this means way past
all the chemistry they've

897
00:42:48,900 --> 00:42:51,160
taken, they're now graduate
students or they're now

898
00:42:51,160 --> 00:42:53,900
professors, and they're still
writing out Lewis structures.

899
00:42:53,900 --> 00:42:56,420
Now they're writing a more
abbreviated form, which you'll

900
00:42:56,420 --> 00:42:59,950
probably get to if you take
organic chemistry, but really

901
00:42:59,950 --> 00:43:02,130
it's the exact same idea.

902
00:43:02,130 --> 00:43:05,220
And this goes all the
way back to 1902.

903
00:43:05,220 --> 00:43:09,240
In fact, Lewis was an American
scientist, so he was trained

904
00:43:09,240 --> 00:43:13,340
in America, and he actually was
a professor here at MIT

905
00:43:13,340 --> 00:43:18,660
from 1905 all the way to about
1911 or 1912, and these are

906
00:43:18,660 --> 00:43:23,590
some notes from 1902, and you
can't see them very well, but

907
00:43:23,590 --> 00:43:27,520
this was essentially an early
form of Lewis structures, and

908
00:43:27,520 --> 00:43:29,640
this was called the
cubicle atom.

909
00:43:29,640 --> 00:43:32,030
So, basically what he's showing
in these cubes is that

910
00:43:32,030 --> 00:43:34,510
there are eight spaces that
need to be filled up

911
00:43:34,510 --> 00:43:35,960
to have a full cube.

912
00:43:35,960 --> 00:43:38,700
So in order to fill them, he
would have to have eight

913
00:43:38,700 --> 00:43:41,350
electrons or an octet
around the cubes.

914
00:43:41,350 --> 00:43:45,460
So what we're seeing is this
is notes from 1902 -- he

915
00:43:45,460 --> 00:43:48,750
actually didn't publish any of
this work or these ideas that

916
00:43:48,750 --> 00:43:52,120
led to Lewis structures until
1916, but his early class

917
00:43:52,120 --> 00:43:55,590
notes were used as evidence
about how long ago he actually

918
00:43:55,590 --> 00:43:57,080
came up with the idea of it.

919
00:43:57,080 --> 00:43:59,690
So it's really neat to think
that your counterparts 100

920
00:43:59,690 --> 00:44:02,230
years ago right here at MIT
could have been sitting in a

921
00:44:02,230 --> 00:44:05,570
class where they had Lewis as
their lecturer, and he's

922
00:44:05,570 --> 00:44:07,980
putting forth these ideas --
these are actually his lecture

923
00:44:07,980 --> 00:44:11,160
notes, even though it wasn't
even published yet, and giving

924
00:44:11,160 --> 00:44:13,680
this idea of Lewis structure,
which is exactly what we keep

925
00:44:13,680 --> 00:44:16,720
using today in order to make
a lot of these predictions.

926
00:44:16,720 --> 00:44:19,910
So, let's see how some of this
works, and hopefully your

927
00:44:19,910 --> 00:44:22,540
counterparts from 100 years
ago would also be able to

928
00:44:22,540 --> 00:44:25,700
think about how this works, even
if they don't have the

929
00:44:25,700 --> 00:44:29,410
quantum mechanics behind
the individual electron

930
00:44:29,410 --> 00:44:32,500
configurations for atoms. So
I said that we want to be

931
00:44:32,500 --> 00:44:35,280
talking about valence electrons
here, so that means

932
00:44:35,280 --> 00:44:38,700
if we're talking about, for
example, the octet rule for an

933
00:44:38,700 --> 00:44:42,350
f f molecule where we have two
fluorine atoms, we need to

934
00:44:42,350 --> 00:44:45,610
write the valence electrons
as dots around them.

935
00:44:45,610 --> 00:44:48,220
So let's do a quick clicker
question, and you tell me how

936
00:44:48,220 --> 00:44:52,100
many valence electrons
does fluorine have?

937
00:44:52,100 --> 00:44:54,850
Remember, valence electrons
are different from core,

938
00:44:54,850 --> 00:44:57,030
they're only the outer-most
electrons in

939
00:44:57,030 --> 00:45:03,410
the outer-most shell.

940
00:45:03,410 --> 00:45:04,960
So, 10 seconds on this,
this should

941
00:45:04,960 --> 00:45:17,510
be fast. OK, great.

942
00:45:17,510 --> 00:45:19,340
Good job on the clicker
questions today.

943
00:45:19,340 --> 00:45:21,840
So we have seven valence
electrons.

944
00:45:21,840 --> 00:45:24,570
So, let's go back to the notes,
and let's fill these

945
00:45:24,570 --> 00:45:26,570
in, seven electrons.

946
00:45:26,570 --> 00:45:28,780
Another way you could have
known them was to look at

947
00:45:28,780 --> 00:45:32,040
Lewis' notes here, where if look
at this box carefully you

948
00:45:32,040 --> 00:45:35,510
see there are seven dots around
the cube, so there are

949
00:45:35,510 --> 00:45:38,080
his seven valence electrons.

950
00:45:38,080 --> 00:45:41,660
So, we see is when we use the
octet rule to look at fluorine

951
00:45:41,660 --> 00:45:45,000
molecule, we're combining two
fluorine atoms, and what we

952
00:45:45,000 --> 00:45:48,380
end up with is an f f molecule
where they're sharing two

953
00:45:48,380 --> 00:45:51,360
electrons, so making
that covalent bond.

954
00:45:51,360 --> 00:45:54,890
But that each individual
fluorine atom has eight

955
00:45:54,890 --> 00:45:56,860
electrons, or full
octet around it.

956
00:45:56,860 --> 00:45:59,710
We can think about where those
electrons came from, so we got

957
00:45:59,710 --> 00:46:03,610
seven from the blue electrons
here, seven as shown in green

958
00:46:03,610 --> 00:46:07,470
here, but each individual
fluorine atom has eight, even

959
00:46:07,470 --> 00:46:10,640
though two of those are being
shared between both of them.

960
00:46:10,640 --> 00:46:13,250
So, the octet rule is a general
rule that you'll for

961
00:46:13,250 --> 00:46:16,700
all of the atoms. There are some
exceptions, which we'll

962
00:46:16,700 --> 00:46:20,160
get to later, but the only a
big exception here is with

963
00:46:20,160 --> 00:46:23,090
hydrogen, which has a special
stability that's associated

964
00:46:23,090 --> 00:46:24,470
with two electrons.

965
00:46:24,470 --> 00:46:26,420
This should make a lot of sense,
because we know that a

966
00:46:26,420 --> 00:46:32,940
hydrogen has 1 s as it's
outer-most or valence orbital,

967
00:46:32,940 --> 00:46:37,690
so it can be filled up just
with two 1 s electrons.

968
00:46:37,690 --> 00:46:40,320
And we give different names,
depending on what kind of

969
00:46:40,320 --> 00:46:43,280
electrons we're dealing with,
so, for example, with h c l

970
00:46:43,280 --> 00:46:45,760
here, we can talk about
having bonded

971
00:46:45,760 --> 00:46:48,060
versus lone pair electrons.

972
00:46:48,060 --> 00:46:50,990
So, in terms of the c l atom,
we need to talk about each

973
00:46:50,990 --> 00:46:52,470
atom individually.

974
00:46:52,470 --> 00:47:01,030
How many bonding electrons
does c l have?

975
00:47:01,030 --> 00:47:01,540
All right.

976
00:47:01,540 --> 00:47:05,000
Let's see, we've got a mixed
response here, it turns out it

977
00:47:05,000 --> 00:47:06,540
has two bonding electrons.

978
00:47:06,540 --> 00:47:09,290
I heard some people say one,
and that's a good guess,

979
00:47:09,290 --> 00:47:11,300
remember they're actually
sharing.

980
00:47:11,300 --> 00:47:14,630
So these two electrons, they
belong to chlorine, they also

981
00:47:14,630 --> 00:47:17,250
belong to hydrogen, but they
do, in fact, belong to

982
00:47:17,250 --> 00:47:17,920
chlorine as well.

983
00:47:17,920 --> 00:47:20,750
There's no one person owning
them, so they both have two

984
00:47:20,750 --> 00:47:22,480
electrons here that
are bonding.

985
00:47:22,480 --> 00:47:26,510
So how many lone pair electrons
do we have?

986
00:47:26,510 --> 00:47:26,850
OK.

987
00:47:26,850 --> 00:47:29,810
I hear six and three, so both
are sort of right, we have 6

988
00:47:29,810 --> 00:47:31,400
lone pair electrons,
which means that we

989
00:47:31,400 --> 00:47:35,180
have three lone pairs.

990
00:47:35,180 --> 00:47:39,560
So, in terms of thinking about
how to draw a Lewis structure,

991
00:47:39,560 --> 00:47:42,740
I won't go through this today
or any day in terms of just

992
00:47:42,740 --> 00:47:45,230
reading through the rules, you
can read that yourself.

993
00:47:45,230 --> 00:47:47,760
But what we'll do is go through
each of these rules in

994
00:47:47,760 --> 00:47:48,900
terms of an example.

995
00:47:48,900 --> 00:47:52,060
So, what will start with on
Monday is doing the most

996
00:47:52,060 --> 00:47:54,670
simple example of methane
using these

997
00:47:54,670 --> 00:47:56,130
Lewis structure rules.

998
00:47:56,130 --> 00:47:59,400
So, don't forget to study this
weekend and get those extra

999
00:47:59,400 --> 00:48:02,840
practice problems from
the course website.