- All right, so we've learned
that the atom is made up
of protons, neutrons,
and electrons.
And protons and neutrons are
in the nucleus, very small.
The electrons are very far away.
We've learned that we can
find out the masses of atoms
by adding up and figuring out an
average atomic mass, abundance
times mass, abundance plus
abundance times mass divided
by the total abundance.
And we use that an
average atomic mass so we
don't have to count
each individual atom.
It works really well.
The next thing we're going to
talk about is the electrons.
So here we go.
We have a question.
Where are the electrons?
And they are very far away.
They are floating really,
really, really, really, really,
really, really, really,
really, really, really, really,
really far out from the nucleus.
Again, the analogy, if I
make myself into a nucleus,
my first electron
is at the Moon.
And there's nothing in between.
Halfway to the Moon, and
there's nothing in between.
So what are they
responsible for?
And it turns out they
are responsible for all
of chemistry.
You want to know what
something's going to do,
you have to know what
it's electrons are doing.
Now, why is that?
Why would these things
be responsible for all
of chemistry?
Well, if you think
about it here--
I'm an atom, right.
And I have only a
little bit of space.
So even though I'm
a nucleus here,
I only can make my
electrons this far.
But let's say I'm coming
up, and I'm coming up.
And I've got these electrons.
And they're sitting out
here, floating around
my nucleus, whee, whee, whee.
And they're out here.
Now, what happens when
another atom comes up
to me from the other side?
What are they
going to see first?
Are they going to see
my electrons first?
Are they going to
see my nucleus first?
They're going to see
my electrons first.
Because you come up.
These electrons are
floating really far away.
The nucleus is buried inside.
It's this little, tiny thing.
So you're going to see
my electrons first.
So when one atom comes up to
another atom, what do they see?
They see each other's electrons.
And so that's why knowing
what happens to the electrons
is critically important
because if you
know what happens
to the electrons,
you know how these
atoms are going
to interact with each
other because that's
what they interact with.
The nucleus is buried.
The electrons are
really far out there.
And they're the thing
that you see first.
OK, so what was the
charge on an electron?
If you remember, the charge
on an electron was negative.
And do they attract
or repel one another?
Well, just like the
same side of a magnet,
north to north,
south to south, they
are going to repel each other.
That's going to be really
important later on,
to realize that
electrons don't really
want to be near each other.
They will if they have to.
But they don't really want to.
They're going to
repel each other
and try to get away
from each other.
So what we want to
do is figure out
where do these electrons live?
We've seen these little
drawings like this.
We've got the nucleus here.
And you just draw the electron
in a big ring around here.
And is that actually
what happens?
Are the electrons just
floating around in a ring
or do they do other things?
Well, it turns out it's
actually relatively complicated.
We're going to simplify
it a little bit.
They don't live
anywhere they want.
They can't just kind
of float around and be
wherever they want to be.
They can only exist in very
specific places in the atom.
It's kind of like a bookshelf.
So a good bookshelf
is a decent analogy.
You look down here.
And let's say we're on
the first bookshelf.
We take a book out.
We put a book back in.
We can only put
it on that shelf.
If we tried to hover
that book just a little
above that first
shelf and we let go,
the book would fall down to the
bookshelf, to the actual shelf,
because it can't exist
in between two shelves.
A book can only exist on
a particular bookshelf.
Well, it turns out electrons
are pretty similar.
They're like the books.
They can only exist
on individual shelves.
They can't exist in
between those shelves.
But we have a funny bookshelf.
I'm going to redraw it slightly.
We have a funny bookshelf,
that on the first shelf
two electrons can exist.
On the second shelf,
it's a little bit bigger.
And up to eight
electrons can exist.
And as we keep going up,
and we get more shelves
on our bookshelf, more and more
electrons can exist in them.
And so we have to
figure out, not
only where are the electrons,
but how many of them
are on each shelf.
How do we know how
many are on each shelf?
Do you have to memorize that
there's two in this one, eight
in this one, 10--
No.
It turns out, we're
going to find out
that it's pretty easy to
figure all this stuff out
if you follow the
periodic table.
One thing you'll
notice on this diagram
here is they're talking
about increasing energy.
Why is there increasing energy?
Well, because down here,
they say, hey, the nucleus.
Now, what's in the
nucleus, protons, neutrons,
and electrons.
So there's these
little balls down here
in the nucleus that
are positively charged.
And our electrons up here
are negatively charged.
All those little guys
are negatively charged.
Now, just like opposite
poles of a magnet,
these guys attract each other.
The negative
electron is attracted
to the positive proton.
And, well, if they're
attracted why don't they just
come up and hit each other?
And it just turns
out they can't.
We won't deal with
the details of that.
But they can't.
They can only exist
in this level.
They can't go down
to the nucleus
even though they want to.
So they're attracted
to each other.
So as I go further and
further away, I'm less happy.
If this negative electron,
wouldn't he rather
be closer to this
positive charge down here?
He would.
And so as you move him
further and further away,
he's unhappier and unhappier.
And, in general, that
results in increasing energy.
So those electrons at the
higher, higher levels, higher
bookshelves, they're
increasing energy.
You can think about it,
too, as a bookshelf.
If you want put something
on a really high bookshelf,
you've got to use a little
more energy to do that.
So that's what we're
talking about here.
So what we're talking
about next is how
do we know how they arrange?
If we look again at our periodic
table, we can talk about them.
So let's pick hydrogen.
It's got one electron.
Which shelf does it exist on?
Well, we go over
maybe to chlorine.
Now, it's one of
the halogens, one
of the kind of light
purple ones, number 17.
Where is that going to be?
Where are 17
electrons going to go
and how do I know where 17
electrons are going to go?
It seems like a lot
to keep track of.
But it turns out
electrons behave very,
very well and very, very
predictably, at least
in this circumstance.
And so we can predict them.
Now, one of the
things your text does
is it has this
little table here.
And it says one way to know
about electron arrangement
for the first 20 elements
is to memorize this table.
And I look at this table.
And that is not a table
that I want to memorize.
Do you want to memorize it?
You are welcome to.
I'm not offended if you
memorize this table.
But there are a lot of
information on there.
And it's not something
I really want to do.
And it turns out you don't
need to because it turns out
that the periodic table
tells you everything
you need to know about
how the electrons are
going to arrange themselves.
So I'm to going to try to keep
this periodic table on here
and also draw below it.
So I'm going to draw
a nucleus again.
It's got positive charges in it.
It's also got these
neutrons in it,
which are negatively charged.
Now, what we're going to do is
start drawing our electrons.
Now, electrons exist in
these little bookshelves.
I'm drawing it sideways.
So if we start with
the first bookshelf,
and we got our second bookshelf,
and we get our third bookshelf,
and we get our fourth bookshelf.
The bookshelves
are getting bigger.
One thing they also
do is get a little bit
closer together as they get
bigger and bigger, which
I didn't draw very well.
They get a little
closer and closer
together as they get
bigger and bigger.
So let's start at
the very beginning
of the periodic table.
We're going to start with
hydrogen. It's got one proton.
We know that from just looking
at the periodic table, which
means that for most neutral
atoms the number of protons
is equal to number of electrons.
So the hydrogen
has one electron.
Remember, we used
that e minus symbol.
So it's got one electron.
Where is that one
electron going to go?
Well, he comes in and he says,
well, hmm, where should I go?
And he thinks to himself,
well, I can go way out here,
or I can go in here.
Or I could go to
any of these shelves
because everything's empty.
I could go anywhere I want to.
Where is he going to want to be?
Is he going to
want to be far away
from all these positive
charges on the left?
Or is he going to want
to be close to all
these positive
charges on the left?
Negative and positive
attracted to one another.
He is not going to want to go
out to this one, way out there.
He's going to want
to go down here.
And so our first electron
goes in our first bookshelf.
We have a name for
that first bookshelf.
We call it n equals 1, the
first bookshelf, n equals 1.
So that first electron
goes down in there.
So hydrogen, it's first
electron goes in there.
It doesn't want to be anywhere
else because it's not as happy.
It wants to be as close to these
positive charges as possible.
And so we say that the
configuration of hydrogen
is that there is one
electron in the first level,
not very exciting.
All righty.
So let's move on to helium.
Helium has how many electrons?
Well, if we look at
helium, it's right here
on the periodic table.
It's number two.
It's got two
protons, which means
it also has two electrons.
Where do you think that first
electron is going to go?
It's going to go in exactly
the same spot as did
the hydrogen, right.
It's going to be as
close as possible.
Now, that second
electron comes in.
Where is it going to want to go?
It's going to want to
go far away or close?
Well, the same
thing-- it's attracted
to those positive charges.
So it's going to say, hey,
I'm going to come down here.
And I want to be as
close as possible
to those positive charges.
And I'm going to rest
on this first bookshelf.
So two electrons there
in that first bookshelf.
And the configuration-- this
is what we call a simplified
configuration.
It's just 2, 2 in
the first bookshelf.
Where it gets a little more
confusing perhaps for some, is
what happens when
we go to lithium?
Well, you'll notice in order
to get to lithium, what do we
have to do?
We have to go to a new
row on the periodic table.
And what does that mean?
When we go to a new row
on the periodic table,
it means our last row is full.
So if you look back
at this table here,
you'll notice that
they have 1 for here,
hydrogen; 2 for helium.
And then they go and they don't
put any more in that first row.
Well, how am I
supposed to know that?
Well, you're supposed to know
that because the periodic table
tells you.
You got to the end of
that first row, which
means you're full now.
And you can't fit
anymore in here.
So this row is full when
there's two electrons on it.
But the periodic
table tells you that.
So because we went to
the second row or period
on the periodic table, we know
that some of our electrons
have to go into the
second bookshelf.
Our first two, they're going
to go as close as possible.
Lithium has three electrons.
The first two are going to
go as close as possible.
They're going to stay down here.
And that n equals 1.
An additional electron
is going to come in.
Is he going to
want to go far away
or is he going to
want to go close?
He wants to go close.
He wants to go down here.
But it's full.
So he settles for second
best, which is this one here.
And he hunkers down
in that second row.
And now what's
our configuration?
Lithium, well, there's two in
that first row, these two here.
And then there's one in
that second bookshelf.
I'm going to put a
comma, and we say
the simplified configuration
of lithium is 2, 1--
2 in the first row,
1 in the second.
And these rows, like I said,
we can call them n equals 1.
We can also call them shells.
So we say this is
the first shell.
What do you think this
one's called, n equals 2
and it's the second shell.
But we're going to use
the simplified, just 2, 1,
as we go forward.
All righty.
So what's next?
Well, let's move on--
not every element on
the periodic table.
Let's move over
to nitrogen here.
Nitrogen has seven electrons
because it has seven protons.
So where are those
electrons going to go?
Where do you think those
first two are going to go?
The first two are going
to go in that first row.
The next one, well, if we go
across the periodic table,
did we finish a row here?
We didn't.
And so it means
they're all going
to go into the same shell.
They're going to
go into n equals 2.
How many of them?
Well, there's seven total,
two in the first row,
which means there's going to
be five in the second row.
You can always count
that just my counting
across the periodic table, 1,
2, 3, 4, and 5, to nitrogen.
So I'm going to put five total
electrons in my second shell,
n equals 2.
And now I can see very
visually that my configuration
of nitrogen is that there's two
in the first shell, n equals 1.
And there's five in the
second shell, n equals 2.
Now, what do you think
happens when we get to neon?
It's got 10 electrons.
Two are going to go
on that first one
because they want to be
as close as possible.
And eight are going to
go in the second one.
I've got 1, 2, 3, 4 5, 6, 7, 8.
I had to squeeze
that last one in.
There's eight in the second row.
So we can get to neon.
That's going to be 2, 8.
Now, what you think happens is
we get to the end of that row?
What does the period
table tell us?
Hello.
We're at the end of a row.
Neon is full.
That second shell,
n equals 2, is full.
So any more electrons are going
to have to go into n equals 3.
So when there's sodium,
with 11 electrons,
its first 10 are going to
go as close as possible.
But that 11th electron
has to go up here.
And so for sodium, you've
got two in the first,
eight in the second,
and one in the third.
And we just keep doing that.
We go to argon, what
you think happens?
We get 2, 8, 8.
If you count across,
argon, 8 in that row.
Then what happens?
That row is full.
And so let's go to potassium.
What is potassium?
Oops.
Sorry, my picture
was covering up.
What does potassium look like?
Well, potassium
has 19 electrons.
Where are they going to go?
Well, two are going to
go in the first row.
Eight are going to
go in the second row.
Eight are going to
go in the third row.
You know that because that's
what we just had for argon.
So it's going to be 2, 8, 8.
Well, let's add that up.
That's 8 plus 8 is 16, 2 is 18.
We need 19 electrons.
That must mean there's
one in the third row.
So let's do that, 2, 3,
4, 5, 6, 7, 8, and 1.
And that's what
potassium looks like.
We can do that up
through calcium.
It turns out it gets a little
more complicated after that.
So we're not going to do these
simplified configurations
for anything past calcium.
But it's really easy.
You follow the periodic table.
When you get to the end of
the row, that row is full.
And you have to go
to the next one.
You have to go from n equals
1, to n equal 2, to n equals 3.
So those are what we
call simplified electron
configurations.
So question for you.
What's the electron
configuration of sulfur?
Take a moment.
Look at your periodic table.
Pause the video.
And see if you can
come up with that one.
All righty.
So we look at our
periodic table.
Sulfur is in the third
row, the third period.
It is number 16.
And so what do we do?
We know there's two
in the first row.
We count across, lithium,
beryllium, boron, carbon,
nitrogen, oxygen, fluorine,
neon; eight in the second row.
And we count across, sodium,
magnesium, aluminum, silicon,
phosphorus, and sulfur;
six in the third row.
So we have a
configuration of 2, 8, 6.
And hopefully that's what
you answered for sulfur.
So chemistry determined
by the electrons.
Why?
Because one atom comes
up to the other atom,
it's the first thing they
see because the electrons are
so far away from the nucleus.
This atom's electrons interacts
with this atom's electrons.
Now, we now know that
there's different shells.
There's n equals 1.
There's n equals 2.
There's n equals 3.
And so we've got
one atom over here--
whee-- with electrons.
And n equals 1, n
equals 2, n equals 3.
We got 1, 2, 3, 4, 5, 6, 7, 8.
It looks its got
one or two out here.
And n equals 3.
And it comes up to
another atom on the right.
Let's make this guy green.
And he only got less electrons.
So he's only got 2, 3, 4, 5, 6.
So he's got a configuration
of 2, 6; two in the first row,
six in the second.
This one has a
configuration of 2, 8, 2.
If you want to
test yourself, you
can look back at
the periodic table
and try to figure out
what atoms these are.
So how are they going
react with each other?
Are all those electrons
on our black ones
going to react with all those
electrons on the green one?
As they come close
to each other--
what I'm going to do is I'm
going to highlight this guy.
And we're going to start moving
him closer to the other one,
do, do, do, do,
do, do, do, do, do.
What happened?
Who interacts first?
Who interacts first?
It's these outer electrons,
right, these two here--
and it got buried by the text
there-- but the six here.
They are the ones who
are going to touch
each other first because they're
furthest away from the nucleus.
So not only are
electrons the things
that determine the chemistry,
it's actually mostly
just those outermost
electrons that
determine the chemistry
because those are
the ones that atoms see first.
So it's the ones furthest
from the nucleus that
do all of our chemistry work.
And we call those
valence electrons.
So valence electrons
are the electrons
in the outermost shell.
And we can get those
from our configurations.
If, for example, we
talk about oxygen,
it's got a
configuration of 2, 6.
The outermost shell has six.
And we say that oxygen
has six valence electrons.
If, on the other hand, we talk
about potassium over here,
we said was 2, 8, 8, 1.
The outermost one is the 1.
And we say that potassium
has one valence electron.
So how many valence
electrons would boron have?
Well, it's
configuration is 2, 3--
2 in the first row, hydrogen
and helium, 1, 2, 3,
in the second row.
And so it would have
three valence electrons.
And so it turns out
we can figure out
the number of valence
electrons, the number
of important electrons,
the ones that
are going to be doing
the chemistry, just
by looking at these simplified
electron configurations.
Now, it gets a little
trickier because we
said our simplified
configurations only
go up to calcium.
What happens if we want to know
how many valence electrons are
in bromine?
What are we going to do then?
Well, it turns out everybody
in the same group--
if you remember, the groups
on the periodic table
are sorted by chemistry.
We now determined
that valence electrons
are what determines chemistry.
So what can you guess about
the number of valence electrons
in a group?
It's always going
to be the same.
So how many valence
electrons in bromine?
We don't know how to
write its configuration.
But we do know how to write
the configuration of fluorine.
I'm going to erase
my oxygen up here.
We do know how to write the
configuration of fluorine,
which is 2, 7, seven valence
electrons in the outer one.
So guess what bromine has?
It has seven valence electrons,
just like fluorine did.
So everybody in the same
group has the same number
of valence electrons.
So over here on the left, we
did potassium just a moment ago,
one valence electron;
lithium, one valence electron;
sodium, one valence electron;
rubidium, one valence electron,
everybody in the same group.
And that makes finding
valence electrons really easy.
What I personally do is I
always pay attention simply
to row number two on
the periodic table.
I always calculate
my valence electrons
by looking at row number two.
So if I've got P
here, phosphorus, I
want to know how many
valence electrons?
I'm always just
going to look and see
how many valence electrons
does nitrogen have?
I count across, one,
two, three, four, five.
So nitrogen has five
valence electrons,
which means phosphorus has
five valence electrons.
Some people will
point out to you
that you can also use these
old group numbers for that.
And that's true.
But I'd rather have you do it
by counting because you don't
always have those old group
numbers available to you
on more modern periodic tables.
So you should be able to find
out valence electrons of things
just by looking at the periodic
table and looking at what group
it's in.
How do we draw these
valence electrons?
We usually draw them as dots.
So if you look back
at calcium, calcium
is in the alkaline earth metals.
It has two valence electrons.
So what we normally
draw is just a pair
of dots on one side
of the calcium.
I could just draw it
on this side as well.
We'll find out later,
there's reasons
for drawing them more on
one side than the other.
But for now, you can draw
them anywhere you want.
Silicon, four valence
electrons, if you look back
at the periodic table.
We can draw it like that.
We could also draw it with
those electrons spaced one
on each side.
And again, we'll see later
on what reasons for doing it.
I'll accept any way you draw
for a quiz or test for now.
And then what are some rules?
In general, we don't draw
more than two dots per side.
Now, I'm going to violate my own
rule later on in the semester.
But for now, we don't draw
more than two dots per side,
in general.
And so if something had
eight valence electrons,
you wouldn't put four on one
side and four on the other.
You put two, two, two, and two.
So, for example, if we
had our noble gas neon,
we'd go 1, 2, 3, 4, 5,
6, 7, 8, just like that.
So now we know about
valence electrons,
those outermost
electrons that are most
responsible for the chemistry.
We get that by looking at
our electron configurations.
And just looking
at that last shell,
the outermost shell, how
many electrons are there?
And those are going to be the
ones that do our work for us.
OK, thanks so much
for being here.
Bye.
