Molecules, crystals, and diatomic elements | Middle school chemistry | Khan Academy

- [Instructor] If you zoom into water,

you'll find a molecule of water

where you have one oxygen atom

attached to two hydrogen atoms.

You'll find trillions
and trillions of them.

Similarly, if you're to zoom into methane,

which we also call natural gas,

you will now find molecules of methane,

which happens to be one carbon atom

attached to four hydrogen atoms.

Again, you'll find trillions
and trillions of them.

So the big question now
is what would you get

if you were to zoom into
table salt, which is NaCl?

Well, our guess could be,
we'll find molecules of NaCl,

where one Na is attached to one Cl.

Trillions and trillions of them, right?

Wrong. That's not what we'd get.

We'll get something completely different.

But what? Well, let's find out.

So what will we find if
we were to zoom into NaCl?

We'll get something like this

where you'll find trillions
of Na's and trillions of Cl

all connected to each other.

We call this a crystal of sodium chloride,

a crystal of NaCl.

You probably heard of this word crystal.

It means we have an
orderly repeating pattern.

This looks very different, right?

So let's compare it with
what we saw earlier.

If we were to go back to
the molecules of water,

what we find is that an
individual oxygen atom

is connected to two other hydrogen atoms,

and, you know, that connection

is due to a force of attraction,

which we call a chemical bond.

The same thing is happening over here.

There's a chemical bond
because of which a carbon atom

is attached to four hydrogen atoms, right?

But when it comes to a crystal of NaCl,

look, we don't have individual molecules

like what we see over here.

We don't find them.

Instead, we find trillions
and trillions of Na's

connected to trillions and trillions of Cl

in that chemical bond.

So there are no individual molecules.

You only have a crystal.

But wait a second, wait
a second, wait a second.

Again, if you go back
to molecule of water,

the reason why we call it H2O

is because you have two hydrogen atoms

connected to one oxygen atom

to form a single unit molecule, right?

Same is the case over here.

It's because you have one
carbonate has to four hydrogens,

that's why we say the,
you know, this is CH4.

But what about over here?
Why do we call it NaCl?

I mean, if you don't have
individual molecules,

what does it mean to say NaCl?

Well, what it means is that

if you're to take a chunk of this crystal,

then you'll find equal
amounts of Na and Cl.

In other words, you'll have
them in the ratio one is to one.

For every one Na, you'll find one Cl.

That is what that means over here.

It doesn't mean that you'll
find a molecule of NaCl

where one Na is attached to one Cl.

No, no, no. You find a crystal.

But the ratio of Na and
Cl is one is to one.

That's what it means over here.

And we can take another
example of the a salt.

There are other salts as well.

So for example, if you
take potassium oxide,

which also happens to be a salt, it is K2O

where K is potassium
and O is oxygen, okay?

But what does it mean to say K2O?

Does it mean that there
are molecules of K2O?

No, this is, again, a crystal.

It's a crystal of potassium oxide.

But what does it mean for us to say K2O?

Well, what it means now is that
if you were to take a chunk

of this crystal,

you will find that there
are twice as many potassium

as oxygen.

You'll have two potassium
atoms for every oxygen atom.

That's what it means to say it is K2O.

So putting it all together,

what we have so far is that we can have

two kinds of compounds.

We have compounds that are
made of individual molecules,

and we have compounds
which are made of crystals.

Now, of course,

these are just two general
categories of compounds,

but there are more advanced
forms of compounds as well.

We'll not worry too much about them.

But anyways, the compounds
that are made of molecules

are called molecular compounds,

and the ones that are made of crystals

are called crystalline compounds.

So look, not all compounds
are made of molecules.

That's the important thing.

They can also be crystalline.

They can also be made of crystals.

But here's an interesting question.

Not all compounds are molecules, right?

But what about the other way around?

Are all molecules
compounds? Well, let's see.

Let's take an example. What if
we were to zoom into oxygen?

Well, we'll find that two atoms of oxygen

are combined together to form O2,

which is a molecule of oxygen.

And of course, you'll find
trillions and trillions of them.

But the big question is
"Is this a compound?"

The answer is no. It's not a compound.

Because remember, compounds
are chemical combination

of two or more different elements.

Here, there's a chemical
combination of the same element.

Oxygen and oxygen is getting combined.

So it's a molecule, but
it is not a compound.

In fact, we call this as diatomic element.

Because di- because there are two,

and there are other examples
of diatomic elements.

Nitrogen, fluorine,
chlorine, and many more.

But of course, one thing to
remember is that these elements

are usually diatomic when they're alone,

like when you have oxygen
gas or chlorine gas.

But of course, when
they do form a compound,

they can split apart.

Again, go back to molecule of water.

You can have one oxygen atom
combined to two hydrogen atoms.

Anyways, if you were
to put it all together,

we have elements which are the
fundamental building blocks

of matter.

Some elements are made
of individual atoms.

We usually call them monoatomic.

Mono- for single, individual,
like gold or mercury.

And other elements can
be formed as diatomic,

like molecules of nitrogen or oxygen.

And then two or more different elements

chemically combined together,

we get compounds.

We can have molecular compounds

where you find individual molecules,

or we can have crystalline compounds

when there are no molecules at all.

And finally, we categorize this all under

what we call pure substances

because they're made of
purely the same stuff.

For example, water is purely
made of water molecules.

Sodium chloride is purely made of NaCl.

And well, oxygen, for example,

is purely made of oxygen molecules.

So because they're all made
of purely the same stuff,

these are all pure substances.