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- [Instructor] If you zoom into water,

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you'll find a molecule of water

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where you have one oxygen atom

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attached to two hydrogen atoms.

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You'll find trillions
and trillions of them.

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Similarly, if you're to zoom into methane,

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which we also call natural gas,

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you will now find molecules of methane,

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which happens to be one carbon atom

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attached to four hydrogen atoms.

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

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So the big question now
is what would you get

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

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

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where one Na is attached to one Cl.

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Trillions and trillions of them, right?

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

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We'll get something completely different.

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But what? Well, let's find out.

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

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We'll get something like this

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

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all connected to each other.

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We call this a crystal of sodium chloride,

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a crystal of NaCl.

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You probably heard of this word crystal.

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It means we have an
orderly repeating pattern.

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This looks very different, right?

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

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

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what we find is that an
individual oxygen atom

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is connected to two other hydrogen atoms,

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and, you know, that connection

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is due to a force of attraction,

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which we call a chemical bond.

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The same thing is happening over here.

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

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is attached to four hydrogen atoms, right?

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But when it comes to a crystal of NaCl,

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look, we don't have individual molecules

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like what we see over here.

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We don't find them.

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Instead, we find trillions
and trillions of Na's

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connected to trillions and trillions of Cl

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in that chemical bond.

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So there are no individual molecules.

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You only have a crystal.

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

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Again, if you go back
to molecule of water,

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the reason why we call it H2O

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is because you have two hydrogen atoms

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connected to one oxygen atom

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to form a single unit molecule, right?

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Same is the case over here.

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

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

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

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

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what does it mean to say NaCl?

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Well, what it means is that

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

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

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

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

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That is what that means over here.

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

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where one Na is attached to one Cl.

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No, no, no. You find a crystal.

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

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That's what it means over here.

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And we can take another
example of the a salt.

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There are other salts as well.

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So for example, if you
take potassium oxide,

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which also happens to be a salt, it is K2O

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where K is potassium
and O is oxygen, okay?

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But what does it mean to say K2O?

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Does it mean that there
are molecules of K2O?

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No, this is, again, a crystal.

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It's a crystal of potassium oxide.

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But what does it mean for us to say K2O?

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

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of this crystal,

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you will find that there
are twice as many potassium

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as oxygen.

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

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

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So putting it all together,

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what we have so far is that we can have

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two kinds of compounds.

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We have compounds that are
made of individual molecules,

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and we have compounds
which are made of crystals.

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Now, of course,

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these are just two general
categories of compounds,

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but there are more advanced
forms of compounds as well.

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We'll not worry too much about them.

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But anyways, the compounds
that are made of molecules

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are called molecular compounds,

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and the ones that are made of crystals

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are called crystalline compounds.

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So look, not all compounds
are made of molecules.

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That's the important thing.

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They can also be crystalline.

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They can also be made of crystals.

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But here's an interesting question.

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Not all compounds are molecules, right?

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But what about the other way around?

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

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

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Well, we'll find that two atoms of oxygen

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are combined together to form O2,

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which is a molecule of oxygen.

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

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

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

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Because remember, compounds
are chemical combination

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of two or more different elements.

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

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Oxygen and oxygen is getting combined.

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

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In fact, we call this as diatomic element.

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Because di- because there are two,

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and there are other examples
of diatomic elements.

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Nitrogen, fluorine,
chlorine, and many more.

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

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are usually diatomic when they're alone,

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like when you have oxygen
gas or chlorine gas.

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But of course, when
they do form a compound,

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they can split apart.

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Again, go back to molecule of water.

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

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Anyways, if you were
to put it all together,

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we have elements which are the
fundamental building blocks

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of matter.

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Some elements are made
of individual atoms.

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We usually call them monoatomic.

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

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And other elements can
be formed as diatomic,

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like molecules of nitrogen or oxygen.

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And then two or more different elements

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chemically combined together,

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we get compounds.

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We can have molecular compounds

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where you find individual molecules,

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or we can have crystalline compounds

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when there are no molecules at all.

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And finally, we categorize this all under

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what we call pure substances

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because they're made of
purely the same stuff.

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For example, water is purely
made of water molecules.

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Sodium chloride is purely made of NaCl.

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And well, oxygen, for example,

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is purely made of oxygen molecules.

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

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these are all pure substances.