Compounds and chemical formulas | Middle school chemistry | Khan Academy

- [Lecturer] Almost all
the matter in the universe

is made from a few, about
100 different elements,

kind of like how all the English words

and sentences in all the books

and poems are made from just 26 letters,

and of course, some spaces.

But what I love about this analogy

is we can take it one step further.

Just like how letters can
combine in interesting ways

to form lots and lots of words,

these elements can combine
in some interesting ways

to form substances,
what we call compounds,

and that's what we wanna
talk about in this video.

Well, first things first,
how did we figure out

that elements can combine
to form compounds?

Well, when we took naturally
occurring substances

with metals in them, which
we call ores, for example,

cinnabar is an ore

because it contains mercury metal in it.

And when we heated it, we were
able to extract that metal,

we were able to get that metal,

so we were able to get
the mercury out of it,

and some other substances as well.

From this, we were able to guess that,

hey, look, cinnabar is
made from the combination

of different elements.

So cinnabar is probably your compound.

And there are other examples.

For example, if you take the ore of gold,

which we call calaverite

Yeah, these ores have very fancy names.

Then if you heat that up,

we will be able to extract
metal, the gold out of it

and some other elements as well.

And again, you can see
therefore calaverite,

this ore is a combination
of these elements,

and that's how we are able to guess

that elements can combine
together to form compounds.

And that's also the reason
why the periodic table

does not have these compounds,

because they are not
fundamentally new elements.

These are the elements,

and these are formed by
combination of elements.

But of course, let's be very concrete now.

What exactly are compounds?

Well, we define compounds are substances

which are made from chemical combinations

of two or more different elements.

For example, if you look
at water, it's a compound

because it's made from
the chemical combination

of two elements, hydrogen and oxygen.

But what does it mean?

What does it mean to say

that they are a chemical combination?

Well, for that, let's zoom into it.

If you could zoom into water

and look at the smallest bit of water,

then we would see something like this.

We would find that one
oxygen atom is combined,

it's connected to two
other hydrogen atoms.

This is what we call a molecule of water.

But what keeps these atoms together?

Why are they connected like this?

Well, it turns out that
atoms can have a force

of attraction between them.

And this force of attraction

is what we call a chemical bond.

Again, don't worry too much

about where this chemical bond
comes from and all of that.

We'll study more about that
in high school chemistry.

But what's important is that
when atoms combine like this,

that's what we call as
a chemical combination.

And so look, water is
a chemical combination

of two or more different elements.

And so it's made of billions

and trillions of these molecules.

Let's take another example.

If we consider carbon dioxide,

which we usually find in
smoke, then we'll find

that it's made of two
elements, carbon and oxygen.

And again, if we could zoom into it

and look at the smallest
bit of carbon dioxide,

this is what we would find.

We would again find that look,
one carbon atom is connected

to two other oxygen
atoms forming a molecule

of carbon dioxide.

And again, it's a chemical combination.

And that's why carbon
dioxide is a compound.

It's called dioxide because
there are two oxygen atoms.

Okay, let's take one last example.

If you were to look at the
smallest bit of oxygen,

you would see something like this.

Two oxygen atoms are combined
together to form a molecule.

So do you think oxygen is a compound?

Well, remember, compounds
require chemical combination

of two or more different elements.

This is a chemical combination
of the same element,

and therefore this is not a compound.

Okay, now that we understand
what compounds are,

let's finally see how to
write their chemical formulas.

So if we go back to water,

we saw that oxygen atom is connected

to two other hydrogen atoms, right?

So guess what?

This ratio stays the same.

One oxygen atom will always be connected

to two other hydrogen atoms

when it comes to a water molecule.

And so the way we write this

is we write the chemical formula as H2O.

Notice that the two over here
is written as a subscript,

which represents that there are
two hydrogen atoms connected

to one oxygen atom.

We'll not write it as H2O2, or HO.

This would be all wrong.

And fun fact, H2O2 is
called hydrogen peroxide,

which is poisonous,

which is ironic because
H2O, which is water,

it gives us life, but add
one more oxygen atom to it,

it becomes poisonous.

Okay, similarly, if you were
to look at the chemical formula

for carbon dioxide,

because I have one carbon
attached to two oxygen atoms,

and that will always be the
fixed ratio for carbon dioxide.

The way we'll write this is CO2.

Again, notice we have written
two over here as a subscript

because one carbon is
attached to two oxygen atoms.

And we can have much bigger molecules.

If you have ever put, sugar in lemonade,

then you worked with a
molecule called sucrose.

It is a big molecule.

And let's now look at its
chemical formula, C11H22O11.

So this tells us

that there are 11 carbon atoms

attached to 22 hydrogen atoms,
attached to 11 oxygen atoms.

This is one single molecule of sucrose.

All right, putting it all together,

all matter in this universe

is made from a few, about
100 different elements.

And when these different elements combine

together chemically, we get compounds.

The ratio of the elements is always fixed

in these compounds,

and that's represented by
these chemical formulae.

And these compounds can be
represented by these molecules,

which act like a single unit over here.