- [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.