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- [Lecturer] Almost all
the matter in the universe

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is made from a few, about
100 different elements,

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kind of like how all the English words

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and sentences in all the books

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and poems are made from just 26 letters,

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and of course, some spaces.

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But what I love about this analogy

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is we can take it one step further.

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Just like how letters can
combine in interesting ways

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to form lots and lots of words,

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these elements can combine
in some interesting ways

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to form substances,
what we call compounds,

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

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Well, first things first,
how did we figure out

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that elements can combine
to form compounds?

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Well, when we took naturally
occurring substances

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

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cinnabar is an ore

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because it contains mercury metal in it.

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

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we were able to get that metal,

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

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and some other substances as well.

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From this, we were able to guess that,

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hey, look, cinnabar is
made from the combination

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of different elements.

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So cinnabar is probably your compound.

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And there are other examples.

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For example, if you take the ore of gold,

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which we call calaverite

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Yeah, these ores have very fancy names.

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Then if you heat that up,

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

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and some other elements as well.

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And again, you can see
therefore calaverite,

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this ore is a combination
of these elements,

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and that's how we are able to guess

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that elements can combine
together to form compounds.

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And that's also the reason
why the periodic table

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does not have these compounds,

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because they are not
fundamentally new elements.

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These are the elements,

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and these are formed by
combination of elements.

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

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What exactly are compounds?

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Well, we define compounds are substances

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which are made from chemical combinations

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of two or more different elements.

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

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because it's made from
the chemical combination

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of two elements, hydrogen and oxygen.

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But what does it mean?

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What does it mean to say

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that they are a chemical combination?

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

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If you could zoom into water

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and look at the smallest bit of water,

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then we would see something like this.

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We would find that one
oxygen atom is combined,

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it's connected to two
other hydrogen atoms.

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This is what we call a molecule of water.

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But what keeps these atoms together?

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Why are they connected like this?

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Well, it turns out that
atoms can have a force

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of attraction between them.

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And this force of attraction

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is what we call a chemical bond.

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Again, don't worry too much

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

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

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

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that's what we call as
a chemical combination.

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And so look, water is
a chemical combination

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of two or more different elements.

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And so it's made of billions

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and trillions of these molecules.

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Let's take another example.

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If we consider carbon dioxide,

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

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

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And again, if we could zoom into it

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and look at the smallest
bit of carbon dioxide,

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this is what we would find.

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

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to two other oxygen
atoms forming a molecule

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of carbon dioxide.

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And again, it's a chemical combination.

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And that's why carbon
dioxide is a compound.

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

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Okay, let's take one last example.

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

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you would see something like this.

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Two oxygen atoms are combined
together to form a molecule.

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So do you think oxygen is a compound?

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Well, remember, compounds
require chemical combination

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of two or more different elements.

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This is a chemical combination
of the same element,

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and therefore this is not a compound.

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Okay, now that we understand
what compounds are,

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

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So if we go back to water,

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we saw that oxygen atom is connected

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to two other hydrogen atoms, right?

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So guess what?

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This ratio stays the same.

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One oxygen atom will always be connected

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to two other hydrogen atoms

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when it comes to a water molecule.

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And so the way we write this

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is we write the chemical formula as H2O.

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

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which represents that there are
two hydrogen atoms connected

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to one oxygen atom.

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

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This would be all wrong.

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And fun fact, H2O2 is
called hydrogen peroxide,

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which is poisonous,

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which is ironic because
H2O, which is water,

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

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it becomes poisonous.

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

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for carbon dioxide,

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because I have one carbon
attached to two oxygen atoms,

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

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The way we'll write this is CO2.

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

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because one carbon is
attached to two oxygen atoms.

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And we can have much bigger molecules.

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If you have ever put, sugar in lemonade,

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then you worked with a
molecule called sucrose.

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It is a big molecule.

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

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So this tells us

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that there are 11 carbon atoms

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

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This is one single molecule of sucrose.

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All right, putting it all together,

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all matter in this universe

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is made from a few, about
100 different elements.

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And when these different elements combine

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together chemically, we get compounds.

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The ratio of the elements is always fixed

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in these compounds,

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and that's represented by
these chemical formulae.

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And these compounds can be
represented by these molecules,

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which act like a single unit over here.