- [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.