lib.utexas.edu/.../Lewis_Symbols.mp4

Hi everyone. In this video we're
going to discuss Lewis Symbols,

but in order to do that properly
we absolutely need to review

valence electrons.

So for valence electrons, if you recall,
they are your outermost electrons.

So if I were to draw an atom I would
have to start with a nucleus.

So the nucleus is at the very center,
that's where your protons

and your neutrons are stored.

It is tiny, it is miniscule,
it is very very small.

So what's cool about an atom is its
very first shell,

so the first one that's right around
the nucleus,

that one has two electrons in it
so we'll just put one here and one here,

but because the nucleus is so small,
that first shell is also just so tiny

that it can be very tightly compact aound
the nucleus.

It's so small that it can actually
only hold two electrons,

because from there on out,
every single other shell around it

wants to have eight electrons in it
and so it will just keep growing

and growing and every single shell
will have eight electrons.

So now if you recall, we're talking
about this in the context of

valence electrons.

So for my example here, I have a
nuclus in the center and I have

one, two, three shells around it.

So there first two, this one and this
one, these are considered your core shells.

They host your core electrons so they
hold on very tight to the nucleus,

but on the outside here, we have
our outermost shell and this is the one

that houses our valence electrons.

So when we go to a periodic table
and try to determine the number of

valence electrons, we're actually talking
about the electrons that are sitting

in the outermost shell.

So now what's really neat about this
is because we have an inner shell

that's so small and only houses
two electrons,

we actually have to kind of manipulate
the periodic table when

we're determining valence electrons
on our own accord.

So what we're going to do here is
look at our periodic table and specifically

look at this very first row.

So we said it can only have two electrons
in its outermost shell and so

what we need to do is just come over here
and we're going to actually move helium.

So I'm going to put it right here
above this second group and we're

going to cross out helium over here.

So now what I'm going to do is go
starting from the left,

we're going to go across and number
our valence electrons.

So for this first group,
our alkali metals,

they only have one valence electron,
just one.

So if were we to draw it out in
our diagram,

here is our nucleus, there's our shell,
they have just one electron

sitting in the outermost.

Then we go to the second group,
our alkali earth metals.

They have two valence electrons.

So one and two, and so on and
so forth.

So we go to Boron which has three,
then four, five, six, seven, and eight.

Hopefully this is all a review,
but we need to understand that in order

to actually draw our Lewis symbols.

So let's start off with hydrogen.,

Hydrogen has the symbol H and we know,
because we just discussed it,

that hydrogen has one valence electron.

It's sitting here in the first group.

So I'm going to come down here and
just draw it out.

So this is a Lewis symbol for hydrogen.

I'm saying that it has one valence
electron.

So let's say I go down in the
alkali metals and go to lithium.

Here's lithium. It also has just one
valence electron so I would draw it

in the same manner as hydrogen.

We can go on and on and on
with sodium and potassium and rubidium,

but essentially you just draw
one electron to the side of it.

So let's move on the second group,
our alkali earth metals.

So let's start with helium here,
and remember we moved helium.

So now this is the only tricky part about
Lewis symbols.

The first two dots always go together.

So remember how we said that
we can have two electrons

in that innermost shell?

That's why.

So we draw it out here with the
two electrons next to each other

so helium has two dots.
Boom, boom. Just like that.

Let's go to the next one, Beryllium.

So we have berylium here.
Berylium also has two dots.

One and two. Two valence electrons,
you always draw the first two together,

and we could keep going.

Magnesium has two,
one and two.

So let's keep going to Boron.

Boron has three valence electrons.

Remember, the first two always go
together -- one two.

For the third one you can put it
anywhere you want.

It could go here, it could go here,
or it could go here.

It doesn't matter where you go,
but you just have to be consistent.

Oh you need to erase.

And you can only have three total.

So you have two together and then three
on the outside.

We would do the same thing for aluminum
and then so on and so forth.

Okay? Now let's go to carbon.
Carbon has four valence electrons.

So one, two go together.
The rest, it doesn't matter.

Three and four. I could have put one
up here in this spot, it doesn't matter,

we just want to be consistent and only
have four total.

And so now we have nitrogen.

NItrogen has five valence electrons so
one, two, three, four, five.

Again, it doesn't matter where you
put them, it just matters that the

first two go together.

So now for oxygen I could start
down here.

One, two, then you just go around.
Three, four, five --

I have one more valence electron so
you just have to double up.

It doesn't matter where, just pick one
and double up.

Then fluorine, that's seven, so one,
two, three, four, five, six, and seven.

Okay for neon, our first noble gas,
we have neon here, we go

one, two, three, four, five, six,
seven, and eight.

Okay, so it has eight electrons,
they're all together.

As long as you keep the first two
together that's all that matters.

Alright, now I'm going to draw out some
for silicon and I want you

to try to decide which is
the correct answer.

So for silicon, we know it has
four valence electrons,

it's positioned right underneath carbon,
here it is, so four valence electrons.

So let's draw out three
different structures.

You're either going to answer multiple
choice answer A for this,

so it's going to look like this one,
answer B is if you think silicon

looks like this.

And answer C is if you think silicon
looks like this.

Alright, did you get an answer?
Hopefully you picked answer C

because that's absolutely correct.

In order to do this, all you had to do
is draw the first two electrons together,

one, two, and then three, four.

It doesn't matter where you put them,

this would have also been a
correct answer.

So the last thing I want to do is
introduce one more piece of information

which is called a lone pair of electrons.

Now, some people will refer to it
as a lone pair,

some people will write it out as l.p.,
but all of it means the same thing.

It is just saying that you have two
electrons on some kind of symbol

so here' s your thing.

Here's Silicon, two electrons that
are together,

they're a pair of electrons so they're
sitting like this.

If I were to draw an energy level,
I would have one electron here

and one electron here.

I don't expect you to know what
this means, but just know that

those two first electrons go together.

Remember, here's your nucleus,
here's your inner shell,

here's one, here's two.

So it's just two electrons that are
together paired up in an energy level

and so that's why we put these
together like this.

Right, we put the first two together.

This is considered a lone pair of electrons.

Anytime you have two electrons together,
if this one was a correct symbol over here

which it is not, this technically has
one, two lone pairs of electrons

so that's how you count a pair.

So I want to ask you a quick question.

How many lone pairs do you think
sulfur has?

Okay, we're just looking for the number
of lone pairs. Go.

Alright, did we get an answer,
hopefully you did.

So the first thing you had to do was
go to your periodic table

and find sulfur so we're going to go
all way up the top and sulfur sits

right underneath oxygen and so
it has six valence electrons

so we'll go all the way down here.

SO the six valence electrons,
we have to draw that out.

First two go together, one two,
three, four, five, and six.

And so now all we're doing is looking for
two electrons that are grouped together

so here's one lone pair and
here's two lone pairs.

So hopefully put the answer two because
that's correct.

Take care of yourself, drink water,
and have a great day.