Lewis diagrams for molecules | Chemistry | Khan Academy

- [Instructor] Let's
draw Lewis dot structures

for certain molecules, it's a
lot of fun to do that, okay?

Now the first thing we need to do

to draw these structures
is to identify the number

of valence electrons, okay?

And we've talked about these
valence electrons in our

previous videos, but this
is such a central concept,

so just let's quickly recap.

Valence electrons represents the number

of electrons in the outermost shell, okay?

And if we bring up our
periodic table, then

the valence electrons for
all the elements belonging

to a particular group is given this way.

So the first group will
have the elements of,

the first group will have
one valence electron.

The elements of the second
group will have two electrons

in its outermost shell,
two valence electrons,

and so on and so forth.

When it comes to group 18,
helium is an exception.

It has two valence electrons,

but the rest of them will
have eight valence electrons,

eight electrons, and its
outermost shell, okay?

And just to get a sneak peek
of why this is the case,

here are some structures
of some of the elements.

You can see hydrogen, one
electron in its outermost shell.

Lithium, one electron
in its outermost shell.

Sodium, which has total of 11 electrons,

but one electron in its outermost shell.

Same thing will continue.

Okay, but what about the
elements of these group?

What about their valence electrons?

Well, we don't have to worry about them,

because we are drawing
structures for molecules

that are formed by covalent bonds.

Meaning bonds formed by
sharing of electrons.

Now, metals, which I've
drawn in red, okay?

They usually do not participate
in covalent bonds, okay?

It's only the non-metals
that usually do that.

And the non-metals are in green.

And you can see all the valence electrons

for the non-metals we already have them.

So we only got to worry about them

and not worry too much about these, okay?

All right, with that recap,

let's now draw the Lewis
structure for hydrogen.

So the first step is how
many valence electrons

does hydrogen have?

It has one valence electron.

So let's write that, one valence electron.

And now, hydrogen would
love to have two electrons

because it'll make it stable
in its outermost shell.

And how do I know that?
Because if I look at helium,

look, helium has two electrons

and it doesn't bond with anybody.

Because helium is stable,
all noble gases are stable.

That's how I know hydrogen will love

to have two electrons
in its outermost shell.

Now that means it needs one more electron.

And here's the thing, if
it needs one more electron,

it's gonna say, "Hey, I'm
gonna share one electron."

If you need one, you share one, okay?

So hydrogen says, "Hey, I'm willing

to share my one valence electron."

So it's gonna share one electron.

And if somebody else is willing

to share their one valence electron,

it's gonna form a nice
bond with them, okay?

That's why it forms a bond with itself.

And so now we can draw
the structure this way.

So here's the hydrogen, and
here is the valence electron.

And it's ready to share that.

Here's another hydrogen,
here's the valence electron

it's ready to share that.

And look, they're gonna form a bond.

This is called the covalent bond.

And by sharing, each atom
has access to two electrons,

making them both stable.

That's why they stay
together, okay? Covalent bond.

How do we draw the Lewis
structure, the final structure?

Well, we are just gonna
represent this bond

with a single line.

So a single line represents
a bond between two electrons.

Okay?

This is the Lewis dot
structure for hydrogen.

Okay, let's take another example.

Let's take oxygen.

If I go over here, oxygen
has six valence electrons,

that's where we start.

Oxygen has six valence electrons.

And how many does it
need to attain stability?

Well, if you look at
neon, then you can see

that it has eight electrons
in its outermost shell,

and I know neon is very stable
so that helps me understand

that eight electrons
in the outermost shell

gives you stability.

In fact, that is the case
for most other elements.

For hydrogen it's two, but
for most other elements,

they require eight electrons
in its outermost shell.

We call this the octet rule, all right?

So since oxygen requires eight electrons

in it's outermost shell, it has six.

So it needs two more.

If you need to, you share
two, that's the rule, okay?

So it's going to share two electrons.

And if somebody else is willing

to share their two electrons,

it's gonna form a bond with them.

That's why it forms a
bond with itself, okay?

So how do we do that?

Well, again, we're gonna draw two.

So we're gonna draw the Lewis structure.

We're gonna draw two oxygen atoms.

And now since it has six valance electron,

here's how I show the
valence electrons, okay?

One, two, three, four, five, six.

Why do I draw it this way?

Because just by drawing
it this way, again,

you can see just from the dot structure

that there's one pair,
there's a second pair,

oh, there's no pair here, oh,
there's no pair over here.

These electrons are lonely,
they want to get paired.

And that's how I know

that these two electrons are
gonna participate in bonding.

It's gonna be shared.

So just by looking at the
dot structure, I can see

that it's going to share two electrons.

That's another way to just confirm

the same thing that we
got over here, okay?

So it's gonna share these two electrons.

This oxygen atom is
gonna do the same thing,

one, two, three, four, five, six.

It's gonna share these two,
which means these two electrons

and these two electrons can form a bond.

Now, just to make it a
little convenient for me

to draw this, I'm gonna show
the lonely electron here,

I'm gonna show the pair over here.

You can draw the pair
wherever you want, okay?

It just makes it so much
convenient to draw bonds over here.

So this means these two electrons

are gonna form a bond over here.

And these two electrons are
gonna form a bond over here.

And there you have it, that
means oxygen is gonna form

a double bond, and you
can count the number of,

because of this covalent
bond, you can count the number

of electrons each oxygen
has access to now.

If you look at this oxygen,

it has one, two, three, four,
five, six, seven, eight,

it has eight electrons access to it.

And this oxygen, one, two, three,

four, five, six, seven, eight,

it has also eight electrons access to.

And so octet rule is satisfied.

And so now how do we draw
the final Lewis structure?

Well, here's the oxygen
atoms, one and two.

So two bonds, two lines,

and then we have to show the lone pairs.

These are called lone pairs

because hey do not participate in bonding,

but it's important to show them.

So here's the lone pairs.

You can show the lone pairs
wherever you want, okay?

But do show them in the
Lewis dot structure.

And there you have it,
that is a dot structure.

All right, your turn,
why don't you try drawing

a Lewis dot structure for N2?

Why don't you pause the
video and give it a try?

All right, here we go.

Again, I look at nitrogen over here,

and I know that nitrogen
has five valence electrons.

That's where you always start,
number of valence electrons.

And octet rule, remember,

most elements follow the octet rule.

So it requires eight electrons

and it's outermost shell for stability.

It has five, so it needs three more.

If you need three, you share three.

So it shares three electrons.

And again, if it can find some other atom

or elements that can
share three electrons,

it'll happily bond with them.

That's why it forms a bond with itself.

And so how are we gonna show that?

Same way.

So we're gonna draw the five
valence electrons over here

in the same way we wrote it over here.

One, two, three, four, five.

And again, you can see just
by drawing the dot structure,

we can see there's one pair over here.

But there are three lonely
electrons, which would love

to participate in bonding.

That's why it shares three electrons.

Just by looking at the dot structure,

you can confirm that.

Same thing over here, one,
two, three, four, five.

So these three electrons
can bond with these three.

And again, just to make it
a little convenient to draw.

Okay, I'm gonna...

Sorry, I'm gonna draw that pair over here,

and I'm gonna draw that pair over here.

All right, and of course,
the more practice we get,

the better we'll be at
conveniently drawing them, okay?

But anyways, now that we
have these three electrons,

they can form a bond with each other.

And so bond number one, bond
number two, bond number three.

So nitrogen forms a
triple bond with itself.

And again, if you count the number

of electrons, this nitrogen has

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

And for this nitrogen, again,

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

Octet rule is satisfied.

So what does the Lewis dot structure,

final structure look like?

Well, I have three bonds, so
it's gonna be a triple bond.

And then do not forget the lone pairs.

Never ever forget the lone pairs, okay?

And this now explains why nitrogen

loves to form three bonds.

Either a triple bond or a
double bond with one element

and a single bond with another element,

or three separate bonds.

And oxygen, for example,
loves to form two bonds.

Double bonds with another
atom or two single bonds.

All right, let's kick
things up a little bit.

How about we draw one for water molecules?

Hmm, things are getting interesting now.

The idea is the same though.

You start with the number of
valence electrons for each one,

and then think about how much they share

and then try to form a bond.

Now of course, because you
have three elements over here,

there could be a little bit confusion

about how they're all connected.

The general rule is the element which is

on the leftmost side
of the periodic table.

Again, if here's a period
table, look at the one

that's on the leftmost side,
which is least electronegative.

Or you can also think
of it as most metallic,

because, you know, all metals
are on the left side, right?

So the one that is on the
leftmost side will usually be

the central atom, right?

What about hydrogen?

Hydrogen is the leftmost,
but it is an exception.

All right? So hydrogen will
always be on this side.

So except for hydrogen, any
other elements that you have,

you have to consider the one
that's on the leftmost side,

that will be your central atom.

We'll take more examples,
it'll make sense,

but at least when it comes to water,

since hydrogen will always be on the side,

the oxygen will be in the center,

so we can immediately draw the structure.

So the oxygen will be in the center,

the two hydrogens will be connected to it.

And we already know their valence

electrons and how much
they're gonna share.

So I'm gonna write one for oxygens.

I'm gonna start with that same technique,

one, two, three, four, five, six.

So it's gonna share these two.

And hydrogen just has one.

And so you can kind of see
these two are gonna form

a bond with these two.

And again, just for convenience,

what I'm gonna do is I'm gonna take this

and put that pair here

so that I can nicely bond of these two.

And there you have it.

So this is gonna be bond number one,

and this will be bond number two.

And that's it, that's the structure.

So dot structure.

So it's gonna be oxygen,
hydrogen, hydrogen,

one bond here, one bond here.

And do not forget the lone pairs.

Okay, again, your turn.

How about you draw one for NH3?

Pause and try.

All right, there are
four elements over here,

but again, because hydrogen
will always be on the side,

we know it's the nitrogen which is going

to be the central atom.

And the three hydrogen
will be connected to it.

So nitrogen will be in the center

and the three hydrogen,

well, we can draw the three
hydrogens wherever we want.

So lemme just draw them over here.

And again, we'll start
by drawing the central,

the valence electrons on the central atom.

So one, two, three, four, five.

I know it has five valence electrons.

So where do I draw them?

I'm gonna draw the fifth one over here.

You can see that, right?

Now I can conveniently
draw the bonds over here

and one valence electron for hydrogen.

It's way to share that.

So one bond over here,

one bond over here,
and one bond over here.

And this will be the lone pair.

And again, you can check,
hydrogen has access

to two electrons, nitrogen has access to

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

Okay, so what's the final
structure going to be?

Well, I have N and I have
single bond H, single bond H,

single bond H.

Am I forgetting something?
Yes, the lone pair.

Do not forget the lone pairs.

All right, final challenge.

Carbon dioxide.

So carbon is a new element for us now.

Why don't you pause the video,
look at the periodic table

and try to draw the Lewis
structure for this one?

All right, since carbon is a new one,

we need to think about carbon.

Carbon has four valence
electrons and it needs four more.

And therefore, so lemme
just write that down.

Four valence electrons,
it needs four more.

If you need four, you share four.

Okay, and now, now is an interesting one.

I don't have hydrogens,
I have carbon and oxygen.

How do I know which is the
element that comes in the center?

But again, CO2 might be simpler,

you might be able to guess it,

but we're gonna look at which
one is the leftmost element.

And you can see carbon is the leftmost

element on the periodic table.

Oxygen is over here, carbon is over here.

So carbon comes in the center.

All right, so carbon will be in the center

and oxygen will be attached to it.

So I'm gonna draw two
oxygen atoms over here.

And now again, I can draw the
Lewis structure for carbon.

How do I do that? I'm gonna
show the four valence electrons.

One, two, three, four,
and that's all there is.

And look, they're all lonely,
they're all four lonely

and therefore they're gonna
share all four of them

to form pairs.

And the same thing for
oxygen as we've done before.

One, two, three, four, five and six.

Where should I draw five and six?

Well, lemme see.

I mean, we can draw it anywhere, right?

So lemme draw the five and six here.

Okay, same thing over here,
one, two, three, four.

I'm gonna draw the fifth
and the sixth one over here.

So we can bond this and we can bond this

and we can kind of bond this
and we can kind of bond this.

It's a little crooked, but it's okay.

We get the idea across.

So this is one pair and then
we can bond it over here

and we can bond it over here.

It's fine, it's fine.

I know it looks a little weird,
but we get the gist of it.

Carbon is gonna form a double
bond with this oxygen atom

and the carbon is also gonna form

a double bond with this oxygen atom.

And let's count carbon
has one, two, three,

four, five, six, seven, eight.

What about oxygen?

One, two, three, four,
five, six, seven, eight.

And the same thing with this one.

And so we can now draw a
better Lewis final structure.

So carbon, oxygen, oxygen.

So two bonds.

So double bond here, a
double bond over here.

Do not forget the lone pairs.

So there are two lone pairs over here.

Lemme draw them over here and here.

And there are two lone pairs over here.

Are there any lone pairs on the carbon?

No, there are no lone pairs on the carbon.

So there you have it.

That was fun, right?