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Today is a very big day for all
of you.

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Your first semester at MIT is
half over.

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And, in addition to that,
you have a lot of challenges

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remaining ahead of you for the
rest of this semester.

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I am going to help make the
chemical challenges fun for you.

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That is what I am here to do.
I love chemistry.

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I love teaching 5.112.
This is the best part of my

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week.
So, here we are.

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Do you guys know that chemistry
can be incredibly fun?

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Let me just show you how fun
chemistry can be.

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And that is just the gravy.
The fun part is actually making

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the molecules.
Any of you see this picture in

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U.S.A.
Today recently?

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Not reading that important
journal, I see.

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Well, this picture did appear
very recently in U.S.A.

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Today in a story that talked
about the recent Nobel Prize in

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chemistry that was awarded to
Professor Richard R.

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Schrock, of our department.
And, in fact,

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I don't know if you realize
this, but Professor Schrock was

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the first person ever awarded
the Nobel Prize in Chemistry at

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MIT for work conducted while at
MIT.

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That is a pretty amazing
accomplishment because we have

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so many tremendous chemists on
the faculty here,

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and he is the first one to
break through in that manner.

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And maybe you are here at a
very special time,

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and we will see a number more
of these in the next few years.

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May with you contributing,
so much the better because you

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are here.
And let me just explain

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something, too.
Chemistry is fun.

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And only on very rare occasions
like this would Professor

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Schrock be drinking champagne at
8:30 in the morning,

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when this picture was taken.
The Reuters reporter,

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Brian, who took this photo,
is actually a pretty good

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friend of Professor Nocera here.
And so he rode over from the

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Schrock household.
And Dick and Dan came in

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together in the same car,
as they often do,

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from Winchester on that
morning, which was a very

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exciting day.
If you come to see me for

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office hours maybe I will tell
you more about it in detail,

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but it was an exciting day.
And I am in here,

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too.
My name is Kit Cummins.

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Kit is my nickname.
My name is Christopher.

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You can call me Christopher or
Kit, or you can call me

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Professor Cummins,
or you can just say hey,

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chemistry guy.
That will be fine.

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I am in this picture primarily
because it is part of being in

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the right place at the right
time, just like I am here now in

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front of you.
But also I am in it because we

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three are three in one of the
areas in our department that

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really focus our research on
inorganic chemistry.

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And what is inorganic
chemistry?

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I will say more about this
during the semester.

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But inorganic chemistry is
really the chemistry of the

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elements of the Periodic Table
inclusively.

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So, a pretty broad topic.
And I like doing chemistry with

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lots of elements.
And so that is why these guys

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and I share an association with
each other.

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And, in fact,
I was a graduate student with

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Professor Schrock.
And I am very,

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very proud to be able to say
that.

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Hopefully, my computer will be
staying on, here.

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And I want to point out,
too, that people who love

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science want to know something
about its history.

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And part of all of that is
knowing the academic family from

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which you derive,
depending on the person you've

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studied with in learning about
chemistry.

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And I studied with Schrock.
And Schrock studied with

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Osborn, a fantastic chemist in
his own right,

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no longer alive,
sadly, to see his progeny,

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Professor Schrock,
win the Nobel Prize.

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And Osborn studied with
Wilkinson, and Wilkinson was

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also a Nobel Prize winner.
So, you do not have to go very

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far back in that lineage to
encounter that recognition,

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the Nobel Prize,
Schrock, and then it skipped

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Osborne, and if you go back to
Wilkinson, there is another one.

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And you can keep going back.
And I am not going to go

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through that whole history today
because I have another history

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that I am going to need to go
through with you today.

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And I am going to bring this
back over so it does not

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distract you.
And this other history that I

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need to impart to you today has
to do with some of the

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contributions of Gilbert Newton
Lewis.

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This is one of the most famous
of all chemists ever.

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And if you like chemistry,
I encourage you to think of the

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word Newton, as it is found
engraved in one of the pillars

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in Killian Court at MIT as the
middle name of Gilbert Newton

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Lewis.
It is conceivable that having

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that middle name weighed heavily
upon Gilbert during his

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lifetime.
But it remains a good moniker.

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And in the future,
when you have kids,

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I encourage you to consider
using it as a middle name.

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It worked well in this
particular case.

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When you get the notes for
today, which incidentally will

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be available on the website
after class today,

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so you don't need to write down
everything I am writing down.

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You are actually going to get a
little bit more in the online

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version than what I am writing
down here.

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And if you find that you forgot
something that I said during

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class, just email me.
I will tell you what I said.

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It is not a problem.
But if you want to write down

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notes while I am speaking just
to make notes that you can refer

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to later to refresh your memory,
that is fine.

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I think your taking in
information in lots of different

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ways.
We are going to use the

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computer and we are going to use
the blackboard today.

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And you are going to have
available stuff from the

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Internet a little bit later,
too.

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And all of that is good.
I think you should try to

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reinforce and not worry if some
of the information is a little

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redundant.
And never be afraid to ask

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questions, especially,
I like to take questions by

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email or if you come knock on my
office door.

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In 1916, Gilbert Lewis
published a landmark paper in

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chemistry in the journal of the
American Chemical Society which

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remains the flagship journal of
the American Chemical Society.

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And this paper was entitled
"The Atom and the Molecule."

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Besides being an incredibly
important and influential paper

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in the history of chemistry,
"The Atom and the Molecule" was

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also the basis for --

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Like I said,
besides being a hugely

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influential manuscript,
"The Atom and the Molecule"

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also laid the foundations for
Lewis's thinking about

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electronic structure theory that
shaped the way we still today

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talk about valence and chemical
bonding.

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So, an incredibly important
paper from 1916.

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In the lecture notes today,
you will find the specific

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volume and page number for that
reference in the journal of the

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American Chemical Society.
And I encourage you in your

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copious free time to download
that paper.

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It is really great.
All the American Chemical

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Society journal articles are
available, from the very

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beginning.
And you can download it and

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read it.
And, having had this

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introduction today,
I think it will probably prove

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to be a very fascinating
exercise for you,

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if you choose to do so.
It also was the basis of a book

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that Gilbert Newton Lewis later
wrote on valance and on atoms

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and molecules.
That is also a classic in

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chemistry.
And, should you be shopping for

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chemistry books on e-bay or
something, you might find a rare

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copy of it.
And if you do and choose not to

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buy it, please alert me and I
will buy it.

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But Gilbert Newton Lewis,
"The Atom and the Molecule."

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In this paper you are going to
see, if you do choose to

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download it, that he was
thinking about polarity.

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And he was also thinking about
the relationship between the

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polarity of a molecule relates
to the polarity of a substance.

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And I mean, when I say a
polarity of a substance,

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that that would be a sample,
you could say a water molecule

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down here and liquid water up
here.

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And you will see this kind of
sigmoidal graph that Lewis made

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in thinking about how polar
condensed phase substances were

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as compared with the structure
of the molecules that comprise

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the substance.
Down here, in this region,

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you will find that there are
substances that are not terribly

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polar.
And what I have written here is

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the organic shorthand for the
hexane molecule.

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Liquid hexane is an alkane,
akin to methane,

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which is natural gas.
And so, if you are not familiar

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with this way of writing organic
molecules, I will just tell you

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briefly that this terminus is CH
three or a methyl group,

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and then it is CH two,
CH two, CH two,

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CH two, and then CH three,
another methyl group.

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So, that is some
organic shorthand.

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And I will go through that kind
of shorthand occasionally so

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that you become familiar with
it.

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And that is the hexane
molecule, a very non-polar

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molecule composed of elements,
carbon and hydrogen,

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that have very similar
electronegativity.

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And then he goes on up the
curve here, and you find this

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familiar molecule,
benzene, considered next,

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as being somehow more polar
than hexane, and liquid benzene

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accordingly in its solvating
properties in the condensed

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phase, being a more polar
medium, as it were.

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And then, as we continue
progress up this scale,

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he writes next molecules like
the diethyl ether molecule.

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No doubt you are familiar with
ether and its applications as an

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anesthetic.
The first use of ether as an

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anesthesia in an operation in
medicine was carried out just

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across the river in Boston.
There is this historical Ether

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Dome there that you can go and
look at.

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But, nonetheless,
we are just getting up a little

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bit more polar than benzene.
And then next you find

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molecules like this one.
That contains an organic

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functional group called an
ester.

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That is ethyl acetate,
which is a substance that you

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might have smelled if you ever
built models,

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because some of the glues that
you may have used would contain

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it, or the paints.
And so, it is a system that,

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in terms of its molecular
structure, is more polar.

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And, when you use it in the
condensed phase,

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it behaves in a manner that you
would describe as more polar.

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And then, finally,
you get up to some very polar

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systems, like the water
molecule.

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Like the ammonia molecule.
And, finally,

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Lewis talks about a species
like this, which is sodium twice

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sulfur.
That is actually called sodium

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sulfide.
And it is a salt analogous to

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table salt, sodium chloride.

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And being a salt,
it is obviously very polar

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because there is quite a large
degree of charge separation

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between the atoms in the
molecule.

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And, accordingly,
if you take that salt and heat

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it up until you have got it as
hot as its melting point and

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make it liquid,
that molten salt has

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individually charged ions moving
around in solution.

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And that provides a very polar
medium, which would be excellent

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at dissolving polar things.
You will notice here that on

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the non-polar side we have
elements, carbon and hydrogen,

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that are very close to each
other in electronegativity.

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And then, as we examine this
relationship,

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we start putting in elements
like oxygen or nitrogen,

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which are very electronegative
close to the upper right-hand

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side of the Periodic Table.
That introduces polarity both

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in the molecule and in
substances comprised of the

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molecule.
And then all the way over to

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the involved atoms,
sodium and sulfur are very

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different in electronegativity
and, therefore,

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have very strongly separated
charges.

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And, in thinking about this,
Lewis proposed the notion of a

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continuum.
And his musing on this are

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really quite lucid in that
paper, but the idea was that a

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00:16:18,000 --> 00:16:22,000
molecule, or a part of a
molecule even,

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you can analyze molecules in
terms of smaller pieces of them

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that can be identified within
the larger molecule,

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can lie on either side of a
continuum in reference to a

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number of different properties.
And the section of class that

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we are launching into today
deals with acid-base properties.

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Let's mention that first,
acid and base.

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00:16:57,000 --> 00:17:02,000
These are two ends of a
continuum.

235
00:17:02,000 --> 00:17:09,000
But he also recognized that you
could have a molecule that would

236
00:17:09,000 --> 00:17:15,000
be an oxidant.
And the counterpart to that is

237
00:17:15,000 --> 00:17:20,000
a reductant.
And then, here is a term that

238
00:17:20,000 --> 00:17:27,000
maybe those of you who have
studied organic chemistry in

239
00:17:27,000 --> 00:17:33,000
more detail than most would
recognize.

240
00:17:33,000 --> 00:17:36,000
This is electrophile.

241
00:17:44,000 --> 00:17:50,000
Electrophile has a counterpart
called a nucleophile.

242
00:18:00,000 --> 00:18:06,000
And then, the fourth one that I
would like to mention is

243
00:18:06,000 --> 00:18:08,000
acceptor.

244
00:18:13,000 --> 00:18:15,000
And, of course,
the counterpart to that is

245
00:18:15,000 --> 00:18:16,000
donor.

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00:18:21,000 --> 00:18:25,000
Please remember those four
pairs because when you can

247
00:18:25,000 --> 00:18:31,000
understand and make predictions
about those four properties as a

248
00:18:31,000 --> 00:18:38,000
function of molecular structure,
then you will be a chemist.

249
00:18:38,000 --> 00:18:41,000
I mean that is a large part of
what it is all about,

250
00:18:41,000 --> 00:18:46,000
understanding all the different
chemical properties that will

251
00:18:46,000 --> 00:18:50,000
make it interested in terms of
acid-base, oxidant-reductant,

252
00:18:50,000 --> 00:18:55,000
electrophile-nucleophile,
and acceptor and donor.

253
00:18:55,000 --> 00:19:00,000
And the currency of chemistry
can be seen, with reference to

254
00:19:00,000 --> 00:19:05,000
this, to be the electron.
That is what chemistry is all

255
00:19:05,000 --> 00:19:08,000
about.
And so, we will be talking a

256
00:19:08,000 --> 00:19:13,000
lot about atoms and molecules
with reference to their

257
00:19:13,000 --> 00:19:17,000
properties and how the
properties stem from knowing

258
00:19:17,000 --> 00:19:21,000
something about the electrons in
the molecule.

259
00:19:21,000 --> 00:19:28,000
And I think you are going to
find this to be quite fun.

260
00:19:35,000 --> 00:19:38,000
With respect to the properties
of the medium,

261
00:19:38,000 --> 00:19:43,000
let's talk about a substance
like hydrogen chloride.

262
00:19:48,000 --> 00:19:53,000
This is just to illustrate what
I was saying over there.

263
00:19:53,000 --> 00:19:57,000
If you take a molecule like
hydrogen chloride,

264
00:19:57,000 --> 00:20:00,000
and it is a gas,
of course.

265
00:20:00,000 --> 00:20:04,000
And if you make a solution of
it in hexane,

266
00:20:04,000 --> 00:20:10,000
hexane is going to be our
solvent, and then what you find

267
00:20:10,000 --> 00:20:16,000
is that in this low polarity
medium hexane solution,

268
00:20:16,000 --> 00:20:21,000
liquid hexane,
the HCl molecule remains intact

269
00:20:21,000 --> 00:20:27,000
and floats around in solution,
interacting weakly with hexane

270
00:20:27,000 --> 00:20:34,000
molecules that surround it.
But if you take this HCl

271
00:20:34,000 --> 00:20:40,000
molecule and put it into a
solvent like water,

272
00:20:40,000 --> 00:20:47,000
then something on the other end
of the continuum transpires

273
00:20:47,000 --> 00:20:53,000
because we get ionization to
produce hydrogen ions.

274
00:20:53,000 --> 00:20:59,000
And they go off and are
separated in solution from

275
00:20:59,000 --> 00:21:04,000
chloride ions.
And we will talk more about

276
00:21:04,000 --> 00:21:09,000
just what is going on when you
separate H plus from Cl

277
00:21:09,000 --> 00:21:13,000
minus and let them
diffuse apart in aqueous

278
00:21:13,000 --> 00:21:15,000
solution.
But you can see,

279
00:21:15,000 --> 00:21:19,000
I think, the dichotomy.
Either the proton shares in the

280
00:21:19,000 --> 00:21:24,000
electrons of the chloride,
or it just pops off and ionizes

281
00:21:24,000 --> 00:21:28,000
and goes out into solution.
And Lewis wanted to understand

282
00:21:28,000 --> 00:21:32,000
this.
And, to understand it,

283
00:21:32,000 --> 00:21:37,000
he developed a theory.
And his theory was the "cube

284
00:21:37,000 --> 00:21:42,000
theory." In this part we get to
draw some cubes.

285
00:21:42,000 --> 00:21:47,000
And he decided that if you
thought of the nucleus,

286
00:21:47,000 --> 00:21:53,000
which he calls the kernel,
of an atom as being located at

287
00:21:53,000 --> 00:21:59,000
the center of the system,
then if you had an atom like

288
00:21:59,000 --> 00:22:05,000
neon, which is a noble gas with
eight valance electrons,

289
00:22:05,000 --> 00:22:11,000
that these valance electrons
would want to get away from each

290
00:22:11,000 --> 00:22:19,000
other as far as possible because
they repel each other.

291
00:22:19,000 --> 00:22:21,000
They are all negatively
charged.

292
00:22:21,000 --> 00:22:24,000
This could be,
here, the neon atom.

293
00:22:24,000 --> 00:22:28,000
Here, on that cube,
is how he would represent the

294
00:22:28,000 --> 00:22:33,000
electronic structure of the neon
atom, with the eight electrons

295
00:22:33,000 --> 00:22:37,000
here represented by
peach-colored circles,

296
00:22:37,000 --> 00:22:42,000
each the vertex of a cube.
So, they got far away from each

297
00:22:42,000 --> 00:22:44,000
other.
And you can see that in the

298
00:22:44,000 --> 00:22:48,000
historical archives,
some of his original sketch

299
00:22:48,000 --> 00:22:52,000
books containing these ideas are
there, and now you will know

300
00:22:52,000 --> 00:22:55,000
what they mean.
You will see that he progressed

301
00:22:55,000 --> 00:23:01,000
beyond understanding the atom to
understanding the molecule.

302
00:23:01,000 --> 00:23:07,000
And here is one of the
molecules that he talks about.

303
00:23:22,000 --> 00:23:28,000
Here we have a molecule
containing 14 valance electrons,

304
00:23:28,000 --> 00:23:34,000
all arranged in that manner.
And this could be,

305
00:23:34,000 --> 00:23:39,000
for example,
the I two molecule.

306
00:23:39,000 --> 00:23:45,000
And let me draw it in terms of
a dot structure,

307
00:23:45,000 --> 00:23:46,000
--

308
00:23:52,000 --> 00:23:55,000
-- like that.
And there was known at the

309
00:23:55,000 --> 00:24:01,000
time, partly because of the
contributions of Lewis himself,

310
00:24:01,000 --> 00:24:05,000
something called the Rule of
Eight.

311
00:24:05,000 --> 00:24:10,000
And that was a reference to
this notion that most atoms want

312
00:24:10,000 --> 00:24:14,000
to have eight electrons in their
valance shell,

313
00:24:14,000 --> 00:24:20,000
and it is also something that
we call today the Octet Rule.

314
00:24:20,000 --> 00:24:25,000
So, when you see the Rule of
Eight mentioned in his paper,

315
00:24:25,000 --> 00:24:31,000
it is the familiar Octet Rule.
And you can see here that one

316
00:24:31,000 --> 00:24:37,000
of the key features is two
electrons shared equally.

317
00:24:37,000 --> 00:24:40,000
And when electrons are shared
equally like that,

318
00:24:40,000 --> 00:24:45,000
you have a molecule with
symmetrical charge distribution,

319
00:24:45,000 --> 00:24:48,000
charge referring to the
electrons where they are in

320
00:24:48,000 --> 00:24:51,000
space.
And then, he showed that you

321
00:24:51,000 --> 00:24:55,000
could think about this another
way.

322
00:25:05,000 --> 00:25:10,000
You could consider some kind of
a geometric distortion of the I

323
00:25:10,000 --> 00:25:12,000
two molecule, like this.

324
00:25:12,000 --> 00:25:17,000
Distorted, not only because of
my drawing, but because of the

325
00:25:17,000 --> 00:25:22,000
way the atoms themselves are
thought to be rearranging in

326
00:25:22,000 --> 00:25:24,000
this process.

327
00:25:34,000 --> 00:25:39,000
Notice that what has happened
here is that the I two

328
00:25:39,000 --> 00:25:44,000
molecule is doing something
electronically such that one

329
00:25:44,000 --> 00:25:49,000
iodine is starting to pull away
from the other.

330
00:25:49,000 --> 00:25:55,000
This one over here is carrying
with it the electron that came

331
00:25:55,000 --> 00:25:58,000
from over here.
And, in that case,

332
00:25:58,000 --> 00:26:04,000
what we have is a molecule in
which we are developing partial

333
00:26:04,000 --> 00:26:08,000
negative charge on the
right-hand side and partial

334
00:26:08,000 --> 00:26:14,000
positive charge on the left-hand
side.

335
00:26:14,000 --> 00:26:18,000
As I was talking up here,
I was thinking about what I was

336
00:26:18,000 --> 00:26:21,000
going to say here,
because the ideal is that

337
00:26:21,000 --> 00:26:25,000
molecules, even if they are
inherently non-polar like the I

338
00:26:25,000 --> 00:26:29,000
two molecule,
in their equilibrium structure

339
00:26:29,000 --> 00:26:33,000
they can have electron
fluctuations that lead to

340
00:26:33,000 --> 00:26:37,000
partial charges in the manner
shown here.

341
00:26:37,000 --> 00:26:39,000
And Lewis was working
beautifully toward the

342
00:26:39,000 --> 00:26:42,000
development of a theory like
this that would allow the

343
00:26:42,000 --> 00:26:45,000
description of electronic
structure to be made graphically

344
00:26:45,000 --> 00:26:48,000
apparent.
And this is something that

345
00:26:48,000 --> 00:26:51,000
chemists really like to do,
is we like to be able to see

346
00:26:51,000 --> 00:26:53,000
stuff.
I hope I am going to be able to

347
00:26:53,000 --> 00:26:56,000
show you some really cool stuff
this semester.

348
00:26:56,000 --> 00:26:58,000
And, in fact,
even some really cool stuff

349
00:26:58,000 --> 00:27:03,000
before we finish here today.
So, that is the development of

350
00:27:03,000 --> 00:27:08,000
charge as represented using the
Lewis cube theory in an I two

351
00:27:08,000 --> 00:27:12,000
molecule.
Next, one thing you could do is

352
00:27:12,000 --> 00:27:17,000
say, let's just continue this
process, and the right-hand

353
00:27:17,000 --> 00:27:20,000
iodine with its eight electrons
pops off.

354
00:27:20,000 --> 00:27:25,000
That would give you an ionized
situation in which you had an I

355
00:27:25,000 --> 00:27:32,000
minus floating away from
an I plus molecule.

356
00:27:32,000 --> 00:27:35,000
And that situation is drawn in
the notes, and because I am

357
00:27:35,000 --> 00:27:39,000
running out of space on this
board I am not going to draw

358
00:27:39,000 --> 00:27:42,000
that one up.
Instead I am going to go onto

359
00:27:42,000 --> 00:27:46,000
another triumph of the Lewis
cube theory, which was its

360
00:27:46,000 --> 00:27:50,000
ability to explain different
chemical bond orders.

361
00:28:02,000 --> 00:28:05,000
By the way, Lewis never got the
Nobel Prize.

362
00:28:05,000 --> 00:28:08,000
He certainly should have.
A dramatic oversight.

363
00:28:08,000 --> 00:28:13,000
He was born in Massachusetts.
He was a Harvard professor for

364
00:28:13,000 --> 00:28:16,000
a while and did not like it,
and came to MIT to be a

365
00:28:16,000 --> 00:28:20,000
professor for a while.
And, although he liked that,

366
00:28:20,000 --> 00:28:23,000
that was not far enough away
from Harvard,

367
00:28:23,000 --> 00:28:27,000
so he went to Berkeley,
where he became the department

368
00:28:27,000 --> 00:28:31,000
head and was very successful
over a long period of time in

369
00:28:31,000 --> 00:28:37,000
learning about chemistry and in
teaching chemistry.

370
00:28:37,000 --> 00:28:40,000
I will invite you,
and you will see this in the

371
00:28:40,000 --> 00:28:45,000
notes, too, to go ahead and see
if you can find a good biography

372
00:28:45,000 --> 00:28:49,000
of Lewis because it is very
instructive.

373
00:28:49,000 --> 00:28:53,000
Anyway, here is another
molecule considered by Lewis's

374
00:28:53,000 --> 00:28:55,000
cube theory.

375
00:29:00,000 --> 00:29:05,000
How many electrons are being
shared by the two nuclei or

376
00:29:05,000 --> 00:29:09,000
atoms involved?
Four electrons shared.

377
00:29:09,000 --> 00:29:15,000
We can circle them here.
That is four electrons shared.

378
00:29:15,000 --> 00:29:18,000
Therefore, that is not a single
bond.

379
00:29:18,000 --> 00:29:23,000
We call that a double bond.
And so, that might be a

380
00:29:23,000 --> 00:29:27,000
representation of,
for example,

381
00:29:27,000 --> 00:29:32,000
the O two molecule.
Here we have I two,

382
00:29:32,000 --> 00:29:37,000
here we have distorted I two
with charges developing,

383
00:29:37,000 --> 00:29:41,000
and here we have O two.
O two is a very

384
00:29:41,000 --> 00:29:45,000
interesting diatomic molecule.
We are going to talk about that

385
00:29:45,000 --> 00:29:49,000
later in the semester,
but for now let's just leave it

386
00:29:49,000 --> 00:29:53,000
at that.
But then, even though this was

387
00:29:53,000 --> 00:29:57,000
quite a successful theory,
the cube theory,

388
00:29:57,000 --> 00:30:00,000
it has certain problems,
certain shortcomings,

389
00:30:00,000 --> 00:30:04,000
such as, we can do single bonds
with it quite well,

390
00:30:04,000 --> 00:30:09,000
and now we can do double bonds
with the cube theory,

391
00:30:09,000 --> 00:30:13,000
but what do you do if you have
to make a triple bond?

392
00:30:13,000 --> 00:30:18,000
That was a problem for Lewis.
That should be a problem for

393
00:30:18,000 --> 00:30:21,000
you, too.
Cubes just don't fit together

394
00:30:21,000 --> 00:30:24,000
that way.
They have edges and faces,

395
00:30:24,000 --> 00:30:29,000
and that is it.
And they have vertices.

396
00:30:29,000 --> 00:30:33,000
You can do one vertex.
So, triple bond is a problem.

397
00:30:33,000 --> 00:30:37,000
And we know that there are
molecules that have triple

398
00:30:37,000 --> 00:30:39,000
bonds.
And we cannot handle them with

399
00:30:39,000 --> 00:30:43,000
the cube theory,
so we are going to have to

400
00:30:43,000 --> 00:30:46,000
something a little different,
for that reason.

401
00:30:46,000 --> 00:30:51,000
But there is also another
reason that had to do with just

402
00:30:51,000 --> 00:30:55,000
considering the chemistry of
light elements like hydrogen and

403
00:30:55,000 --> 00:30:58,000
helium.
Those elements tend to

404
00:30:58,000 --> 00:31:03,000
associate themselves with only
two electrons.

405
00:31:03,000 --> 00:31:08,000
And so, Lewis decided,
well, maybe the rule of two is

406
00:31:08,000 --> 00:31:14,000
more fundamental than the rule
of eight, and that we should be

407
00:31:14,000 --> 00:31:18,000
thinking about pairs of
electrons somehow.

408
00:31:18,000 --> 00:31:24,000
Because he looked at what was
known about chemistry and

409
00:31:24,000 --> 00:31:29,000
realized that in most molecules,
the number of electrons is

410
00:31:29,000 --> 00:31:35,000
always even, is always divisible
by two.

411
00:31:35,000 --> 00:31:39,000
That is something interesting
about two electrons at a time

412
00:31:39,000 --> 00:31:43,000
being important.
You take that together with the

413
00:31:43,000 --> 00:31:47,000
triple bond problem,
and Lewis started thinking in

414
00:31:47,000 --> 00:31:51,000
terms of electron pairs,
and this is where this

415
00:31:51,000 --> 00:31:55,000
"electron pair theory" comes
from.

416
00:32:00,000 --> 00:32:04,000
Although he spent a lot of time
on the cube theory,

417
00:32:04,000 --> 00:32:09,000
he was perfectly willing to
cast that theory aside in favor

418
00:32:09,000 --> 00:32:14,000
of something that maybe was
going to work a little better,

419
00:32:14,000 --> 00:32:18,000
the electron pair theory.
And how does that look?

420
00:32:18,000 --> 00:32:23,000
Well, of course,
his initial pictures still had

421
00:32:23,000 --> 00:32:28,000
the cube, which just shows that
once the human mind fixes on

422
00:32:28,000 --> 00:32:33,000
something, it fixes on it with
great tenacity and it can be

423
00:32:33,000 --> 00:32:39,000
hard to shake your way of
thinking about things.

424
00:32:39,000 --> 00:32:42,000
But hopefully you will be able
to do that.

425
00:32:42,000 --> 00:32:45,000
He still decided,
let's arrange these electron

426
00:32:45,000 --> 00:32:50,000
pairs on a cube with the nucleus
of the atom supposed to be at

427
00:32:50,000 --> 00:32:54,000
the center, and let's keep these
electrons, now,

428
00:32:54,000 --> 00:32:58,000
nicely collected in pairs.
And let's get these pairs as

429
00:32:58,000 --> 00:33:03,000
far away from each other as
possible, for the same reasons

430
00:33:03,000 --> 00:33:08,000
we used to be putting eight
electrons as far away from each

431
00:33:08,000 --> 00:33:13,000
other as possible,
namely, electron repulsion.

432
00:33:13,000 --> 00:33:17,000
Now, the problem of electron
repulsion between the pairs was

433
00:33:17,000 --> 00:33:20,000
not really solved.
And Lewis decided that the pair

434
00:33:20,000 --> 00:33:25,000
theory worked so well that he
was not going to worry about

435
00:33:25,000 --> 00:33:27,000
that, and so we won't worry
about it yet,

436
00:33:27,000 --> 00:33:31,000
either.
You have your four pairs of

437
00:33:31,000 --> 00:33:37,000
electrons arranged at the center
of four of the edges of the

438
00:33:37,000 --> 00:33:43,000
cube, and that geometry is what
we call a tetrahedron,

439
00:33:43,000 --> 00:33:47,000
which I am drawing the edges of
here in pink.

440
00:33:47,000 --> 00:33:52,000
You have a tetrahedral
disposition of four pairs of

441
00:33:52,000 --> 00:33:55,000
electrons, now,
for an atom.

442
00:33:55,000 --> 00:34:00,000
This might again be the neon
atom.

443
00:34:00,000 --> 00:34:11,000
But the nice thing about this
choice is that if you want to

444
00:34:11,000 --> 00:34:24,000
you can make single bonds with
this theory, and you can make

445
00:34:24,000 --> 00:34:28,000
double bonds --

446
00:34:33,000 --> 00:34:39,000
-- with this theory,
like that.

447
00:34:39,000 --> 00:34:49,000
And then, due to the properties
of the tetrahedron,

448
00:34:49,000 --> 00:35:00,000
you can also make triple bonds.
This is marvelous.

449
00:35:00,000 --> 00:35:05,000
You can see that the famous
chemist Gilbert Newton Lewis was

450
00:35:05,000 --> 00:35:09,000
talking about electronic
structure in terms of

451
00:35:09,000 --> 00:35:12,000
symmetrical things,
like tetrahedra and cubes,

452
00:35:12,000 --> 00:35:17,000
as a way of arranging electrons
reasonably in space.

453
00:35:17,000 --> 00:35:20,000
This could, again,
be our iodine molecule.

454
00:35:20,000 --> 00:35:24,000
This could be,
now, our O two molecule.

455
00:35:24,000 --> 00:35:29,000
And, if we continue that
progression in the Periodic

456
00:35:29,000 --> 00:35:35,000
Table, what is this molecule?
Dinitrogen, N two,

457
00:35:35,000 --> 00:35:41,000
the major component of our
atmosphere, another interesting

458
00:35:41,000 --> 00:35:47,000
molecule that we will be talking
about as the semester progresses

459
00:35:47,000 --> 00:35:51,000
forward.
But chemical language and

460
00:35:51,000 --> 00:35:57,000
electronic structure theory are
very much still permeated with

461
00:35:57,000 --> 00:36:03,000
this affection for symmetry and
the problems that you can solve

462
00:36:03,000 --> 00:36:10,000
using symmetry to analyze
electronic structure theory.

463
00:36:10,000 --> 00:36:15,000
Let's talk next about another
molecule because we want to talk

464
00:36:15,000 --> 00:36:20,000
about Lewis's development of the
electron pair theory,

465
00:36:20,000 --> 00:36:26,000
but also we want to understand
how that relates to acid-base

466
00:36:26,000 --> 00:36:29,000
properties.
And so, let me draw here

467
00:36:29,000 --> 00:36:32,000
another molecule.

468
00:36:37,000 --> 00:36:41,000
And when you look at this
molecule, which is the aluminum

469
00:36:41,000 --> 00:36:45,000
trichloride molecule,
in which I am drawing out

470
00:36:45,000 --> 00:36:50,000
explicitly all of the electrons,
not forgetting these two.

471
00:36:50,000 --> 00:36:54,000
And having drawn the aluminum
trichloride molecule

472
00:36:54,000 --> 00:36:59,000
and looked at it like that,
you can ask yourself is the

473
00:36:59,000 --> 00:37:03,000
number of electrons divisible by
eight?

474
00:37:03,000 --> 00:37:06,000
Yes it is.
We have a 24 valance electron

475
00:37:06,000 --> 00:37:10,000
system here.
It is divisible by eight.

476
00:37:10,000 --> 00:37:14,000
Of course, it is also divisible
by two.

477
00:37:14,000 --> 00:37:18,000
You can also ask yourself is
this molecule,

478
00:37:18,000 --> 00:37:23,000
as written, one that satisfies
the rule of eight for all four

479
00:37:23,000 --> 00:37:25,000
of the atoms?
No.

480
00:37:25,000 --> 00:37:30,000
We have a problem there because
this thing is electron

481
00:37:30,000 --> 00:37:33,000
deficient.

482
00:37:42,000 --> 00:37:45,000
And if you were Lewis,
you would want to know,

483
00:37:45,000 --> 00:37:50,000
well, if it is electron
deficient, this molecule must be

484
00:37:50,000 --> 00:37:54,000
doing something about that to
get more electrons.

485
00:37:54,000 --> 00:37:58,000
Which atom is missing
electrons, here?

486
00:37:58,000 --> 00:38:02,000
The aluminum.
How can we give aluminum more

487
00:38:02,000 --> 00:38:04,000
electrons, in a structure like
this?

488
00:38:04,000 --> 00:38:08,000
Can we get it up to the rule of
eight?

489
00:38:13,000 --> 00:38:14,000
Double bonds,
awesome.

490
00:38:14,000 --> 00:38:18,000
Beautiful.
We can make a double bond.

491
00:38:18,000 --> 00:38:22,000
And, if we do that,
we can allow aluminum to

492
00:38:22,000 --> 00:38:27,000
satisfy the rule of eight,
or allow the rule of eight to

493
00:38:27,000 --> 00:38:33,000
satisfy aluminum.
Whichever way you want to look

494
00:38:33,000 --> 00:38:36,000
at it.
And see what we have done,

495
00:38:36,000 --> 00:38:39,000
here?
We have taken one of the

496
00:38:39,000 --> 00:38:42,000
electron pairs,
say, this one,

497
00:38:42,000 --> 00:38:49,000
and we have moved it in between
the aluminum and the chlorine,

498
00:38:49,000 --> 00:38:52,000
so it joins the other one in
there.

499
00:38:52,000 --> 00:38:57,000
And we get a double bond.
And now, everybody has an

500
00:38:57,000 --> 00:39:02,000
octet.
But what is the problem with

501
00:39:02,000 --> 00:39:05,000
that?
The problem with that is that

502
00:39:05,000 --> 00:39:08,000
chlorine is very
electronegative,

503
00:39:08,000 --> 00:39:13,000
and it does not want its
electrons stolen away by the

504
00:39:13,000 --> 00:39:16,000
very electropositive aluminum
center.

505
00:39:16,000 --> 00:39:21,000
There are other ways that
aluminum can ultimately satisfy

506
00:39:21,000 --> 00:39:27,000
its octet, and this is how we
get into the realm of "Lewis

507
00:39:27,000 --> 00:39:32,000
acid-base theory."
And I want to show you one of

508
00:39:32,000 --> 00:39:36,000
the other ways that aluminum can
satisfy its octet,

509
00:39:36,000 --> 00:39:41,000
and then, we are going to move
to the computer so we can

510
00:39:41,000 --> 00:39:44,000
visualize some of these
properties.

511
00:39:44,000 --> 00:39:49,000
If I put my aluminum here --
and do you remember this

512
00:39:49,000 --> 00:39:52,000
molecule up here,
the ammonia molecule?

513
00:39:52,000 --> 00:39:57,000
I am going to say that if I
draw that ammonia molecule in

514
00:39:57,000 --> 00:40:00,000
place, here --

515
00:40:05,000 --> 00:40:10,000
The ammonia molecule comes in,
and it has its eight electrons.

516
00:40:10,000 --> 00:40:13,000
It is sharing six of them with
three hydrogens.

517
00:40:13,000 --> 00:40:18,000
The idea is that it has an
extra pair of electrons on that

518
00:40:18,000 --> 00:40:23,000
nitrogen in the ammonia
molecule, that it can share with

519
00:40:23,000 --> 00:40:26,000
the aluminum.
And now we go ahead and

520
00:40:26,000 --> 00:40:30,000
complete the picture.
And we can have eight electrons

521
00:40:30,000 --> 00:40:33,000
around everybody.

522
00:40:38,000 --> 00:40:42,000
And so the rule of eight is now
satisfied for the three

523
00:40:42,000 --> 00:40:46,000
chlorines, for the aluminum,
and for the nitrogen of the

524
00:40:46,000 --> 00:40:50,000
ammonia molecule.
The ammonia molecule is what we

525
00:40:50,000 --> 00:40:53,000
call a "Lewis base."

526
00:40:58,000 --> 00:41:03,000
And the aluminum trichloride
molecule is what we

527
00:41:03,000 --> 00:41:08,000
call a "Lewis acid,"
for reasons that should now be

528
00:41:08,000 --> 00:41:12,000
quite clear.
And I want to show you how we

529
00:41:12,000 --> 00:41:17,000
can look at these things.
And if you will give me just a

530
00:41:17,000 --> 00:41:22,000
second here, I am going to load
the aluminum trichloride

531
00:41:22,000 --> 00:41:26,000
molecule up onto the screen.

532
00:41:33,000 --> 00:41:38,000
What we are going to do is
remember that there are lots of

533
00:41:38,000 --> 00:41:42,000
different ways in which you can
visualize molecules.

534
00:41:42,000 --> 00:41:48,000
And I want to show you some of
the more interesting ways.

535
00:42:26,000 --> 00:42:33,000
All that is is a picture of the
aluminum trichloride

536
00:42:33,000 --> 00:42:40,000
molecule with spheres drawn at
kind of arbitrary radii.

537
00:42:40,000 --> 00:42:45,000
The size of these spheres is
not representative of any

538
00:42:45,000 --> 00:42:51,000
particular physical quantity,
but we can change that.

539
00:42:51,000 --> 00:42:56,000
And so now, what we are going
to do is draw the thing a

540
00:42:56,000 --> 00:43:02,000
slightly different way.
And what you can see is that

541
00:43:02,000 --> 00:43:07,000
the positions of those nuclei
are in this flat box.

542
00:43:07,000 --> 00:43:13,000
I made the box kind of flat.
It is kind of a slab because

543
00:43:13,000 --> 00:43:19,000
this AlCl three
molecule is conveniently kind of

544
00:43:19,000 --> 00:43:24,000
flat in its structure.
And what are we going to do

545
00:43:24,000 --> 00:43:28,000
now?
We are going to draw a contour

546
00:43:28,000 --> 00:43:31,000
surface.
And I like this.

547
00:43:31,000 --> 00:43:34,000
We are going to use the solid
surface method,

548
00:43:34,000 --> 00:43:37,000
first.
And can you see that now?

549
00:43:37,000 --> 00:43:41,000
These curves that enclose the
four nuclei of the aluminum

550
00:43:41,000 --> 00:43:45,000
trichloride molecule
do have some physical

551
00:43:45,000 --> 00:43:49,000
significance,
because what I have done here

552
00:43:49,000 --> 00:43:53,000
is taken results,
in this case from the quantum

553
00:43:53,000 --> 00:43:57,000
chemistry calculation,
but you could also take it from

554
00:43:57,000 --> 00:44:03,000
an X-ray crystallographic
structure determination study.

555
00:44:03,000 --> 00:44:06,000
And I have drawn a
three-dimensional contour map.

556
00:44:06,000 --> 00:44:11,000
The value of the electron
density at every point on the

557
00:44:11,000 --> 00:44:14,000
surface of one of those ovaloid
shapes is the same.

558
00:44:14,000 --> 00:44:19,000
Just like when you have a
contour map to describe in two

559
00:44:19,000 --> 00:44:22,000
dimensions an interesting
complicated surface,

560
00:44:22,000 --> 00:44:27,000
we can do this in three
dimensions as well.

561
00:44:27,000 --> 00:44:30,000
And one of the things you
notice, when you look at a

562
00:44:30,000 --> 00:44:33,000
surface like this,
is that the value of the

563
00:44:33,000 --> 00:44:37,000
electron density actually goes
down to some low value in

564
00:44:37,000 --> 00:44:39,000
between the nuclei,
for this.

565
00:44:39,000 --> 00:44:43,000
When you see molecules
represented as balls and sticks,

566
00:44:43,000 --> 00:44:47,000
or if you represent molecules
this way, just showing a pair of

567
00:44:47,000 --> 00:44:51,000
dots between the aluminum and
chloride, you may not be getting

568
00:44:51,000 --> 00:44:56,000
a physically complete picture of
the situation.

569
00:44:56,000 --> 00:44:59,000
You can get a much more
complete picture if you have

570
00:44:59,000 --> 00:45:02,000
access to the actual electron
density.

571
00:45:02,000 --> 00:45:06,000
And then, you can even go a
little further,

572
00:45:06,000 --> 00:45:09,000
and I want to show you
something very cool.

573
00:45:09,000 --> 00:45:14,000
First I am going to need to
load in a second molecule.

574
00:45:19,000 --> 00:45:22,000
And that will be the product of
the first "Lewis acid-base

575
00:45:22,000 --> 00:45:25,000
reaction" that we have
discussed.

576
00:45:25,000 --> 00:45:29,000
Because, I didn't say it over
there, but that was your first

577
00:45:29,000 --> 00:45:33,000
exposure to a Lewis acid-base
reaction, in which the ammonia

578
00:45:33,000 --> 00:45:36,000
molecule comes in and binds to
the aluminum trichloride

579
00:45:36,000 --> 00:45:40,000
molecule,
in order to satisfy the valence

580
00:45:40,000 --> 00:45:44,000
requirements of the respective
atom types.

581
00:45:44,000 --> 00:45:47,000
And so, here we are going to
look at that molecule,

582
00:45:47,000 --> 00:45:51,000
and we are going to look at its
electron density.

583
00:46:00,000 --> 00:46:04,000
And I think you are going to
like this very much.

584
00:46:26,000 --> 00:46:29,000
Here it comes.
And then I am going to

585
00:46:29,000 --> 00:46:33,000
represent this as a solid
surface.

586
00:46:33,000 --> 00:46:38,000
The value of the electron
density everywhere on this

587
00:46:38,000 --> 00:46:42,000
surface is 0.1 electron density
units.

588
00:46:42,000 --> 00:46:48,000
I want you to start thinking
about how that shape appears

589
00:46:48,000 --> 00:46:53,000
there because I am going to
underscore that a little bit

590
00:46:53,000 --> 00:46:59,000
more in just a second.
But one thing you should notice

591
00:46:59,000 --> 00:47:04,000
is that at the NH three
molecule, which is pictured at

592
00:47:04,000 --> 00:47:10,000
the top of that representation,
what you are seeing is that the

593
00:47:10,000 --> 00:47:15,000
value of the electron density
does not get very small as you

594
00:47:15,000 --> 00:47:19,000
go between the nitrogen and
hydrogen nuclei because both

595
00:47:19,000 --> 00:47:25,000
nitrogen and hydrogen are quite
similar in electronegativity as

596
00:47:25,000 --> 00:47:30,000
compared with the aluminum atom
in this system.

597
00:47:38,000 --> 00:47:46,000
The fascinating thing about
illustrating a molecule in this

598
00:47:46,000 --> 00:47:55,000
way is that we can actually
associate another property with

599
00:47:55,000 --> 00:48:02,000
the color on the surface.
Now what we are doing is

600
00:48:02,000 --> 00:48:06,000
looking at the aluminum
trichloride ammonia system.

601
00:48:06,000 --> 00:48:10,000
And let me see if I can make
this thing gradually spin,

602
00:48:10,000 --> 00:48:15,000
so that I can talk about it
without being pinned to my

603
00:48:15,000 --> 00:48:18,000
computer.
What we have on the surface on

604
00:48:18,000 --> 00:48:23,000
each of these electron density
isosurface enclosed regions is a

605
00:48:23,000 --> 00:48:28,000
color that corresponds with a
value of a function that tells

606
00:48:28,000 --> 00:48:33,000
us about the propensity of
electrons to be paired at that

607
00:48:33,000 --> 00:48:38,000
point in space.
The Lewis theory is now colored

608
00:48:38,000 --> 00:48:42,000
onto the electron density
isosurface of this Lewis acid,

609
00:48:42,000 --> 00:48:47,000
Lewis base complex molecule.
And the values that correspond

610
00:48:47,000 --> 00:48:52,000
to those regions in space where
electrons are most likely to be

611
00:48:52,000 --> 00:48:54,000
found paired up are colored
blue.

612
00:48:54,000 --> 00:48:59,000
Where they are least likely to
be found paired up are colored

613
00:48:59,000 --> 00:49:02,000
kind of red-orange,
here.

614
00:49:02,000 --> 00:49:05,000
And then an intermediate color
is this green.

615
00:49:05,000 --> 00:49:09,000
And so, you can see that you
have electron pairs associated

616
00:49:09,000 --> 00:49:12,000
with each of the three NH bonds,
up at the top.

617
00:49:12,000 --> 00:49:16,000
You have another electron pair
here in blue,

618
00:49:16,000 --> 00:49:19,000
which is very proximate to this
aluminum center,

619
00:49:19,000 --> 00:49:24,000
that is very electron deficient
as we had found right over here

620
00:49:24,000 --> 00:49:27,000
because it lacks an octet.
And then, each of these

621
00:49:27,000 --> 00:49:32,000
chlorines is able to attract
electron pairs.

622
00:49:32,000 --> 00:49:34,000
Because chlorine is very
electronegative,

623
00:49:34,000 --> 00:49:39,000
it attracts the electron pairs
to itself and sort of withholds

624
00:49:39,000 --> 00:49:43,000
them from the aluminum center,
but yet, that development of

625
00:49:43,000 --> 00:49:47,000
charge separation keeps
everything together in what we

626
00:49:47,000 --> 00:49:51,000
can call an ionic interaction,
an electrostatic interaction.

627
00:49:51,000 --> 00:49:55,000
You can see here covalent
bonding, donor-acceptor complex

628
00:49:55,000 --> 00:50:00,000
formation, and electrostatic
contributions to bonding.

629
00:50:00,000 --> 00:50:03,000
And all of this is just an
illustration of the brilliant

630
00:50:03,942 --> 00:50:06,000
ideas of Gilbert Newton Lewis.