The periodic table | Middle school chemistry | Khan Academy

- [Instructor] Almost all
the matter in the universe

from the tiniest microbes

to the biggest stars in the universe,

are made from a few about
100 different elements,

which we arrange this
way in a table called

the periodic table.

But wait a second, why are
they arranged like this

and why do they have these colors?

And if you were to zoom into it,

you can see these different numbers.

What do they represent?

Well, let's find out.

First and foremost these
elements have symbols, right?

H for hydrogen, He for helium.

Well, turns out these
symbols are universal,

and that's pretty cool,
which means if you're

to look at this Chinese
periodic table, look,

the symbols stay the same.

It's in Latin H for
hydrogen, He for helium.

Similar is the case for this
periodic table in Hindi, one

of the Indian languages.

So of course the elements
have different names in Hindi

for iron we call loha.

But the the important thing is

that the symbol stays the same.

So the symbols are universal.

But the next question is, what exactly are

the numbers over here?

There are two, right?

Let's start with the number on the top.

So it starts with one for
hydrogen and then two for helium.

And then if you zoom
in, three for lithium,

four for beryllium,
and so on and so forth.

Well, these numbers are identification

numbers of these elements.

And we u name to it, we
call it the atomic number.

So for example, if I say the
element with atomic number 22,

well that's always going to be titanium,

element with atomic number five
is always going to be boron.

And so these are integers,
they are continuous.

And the last element has
the atomic number 118,

which is called Oganesson.

Fun fact elements up to uranium,

which has the atomic number
92 are naturally occurring,

but the elements after that
are pretty much synthesized in

labs as far as we know.

They do not occur in nature.

But what about the second number?

Well, that's called the atomic mass

and it tells us

how massive this element
is compared to hydrogen.

Again, just to give us some examples,

if you look at hydrogen's
atomic mass, it's 1.008 right,

now that is in some unit
called the atomic mass unit.

Don't worry too much about that.

But now if you look at helium,

it's atomic mass is 4.003.

This means an atom of helium
is about four times more

massive compared to the atom of hydrogen.

That's the meaning of atomic mass.

The atom of gold is about 1.97
times more massive compared

to hydrogen and so on and so forth.

Now, one technical detail is

that this number is in average value.

So this is actually an
average atomic mass.

And the reason for that is because these

elements can have variance.

For example, not all the
gold atoms will have the

exact same mass.

Some will be slightly
more massive science,

some will be slightly less massive.

And so this number here
represents the average value.

This would be true for
all the other elements.

They have variance.

And these variance are called isotopes.

And we'll learn more about
them in high school chemistry.

Don't worry too much about it right now.

All right, the next question is,

what's the deal with these colors?

Well, the colors helps us
categorize the elements

into three categories.

The pink ones are called metals.

The blue ones are called non-metals.

And what about these green ones over here?

Well even kind of think they're
someone somewhat in between.

And so we call them metalloids.

And yes, the color schemes
are definitely not universal.

Different periodic tables
might use different colors.

So better way to think about it is we,

we draw a zigzag line over here,

and all the elements to the left

of this zigzag would be metals.

And the elements to the
right would be non-metals.

And these elements which
are in between over here,

those are the metalloids.

And of course, this
distinction doesn't make sense

for these extremely heavy elements

that we synthesize in labs.

Okay, anyways, this brings us

to the most important thing now,

and that is the structure
of this periodic table.

If you count the horizontal rows,

there are 1, 2, 3, 4, 5, 6, 7

horizontal rows, right?

These horizontal rows are

what we call the periods
of the periodic table.

And similarly, if you're to
look at the vertical columns,

you'll see 1, 2, 3, 4, 5.

Okay, let's just wait.

Should wait, 18.

There are 18 vertical columns,

and these are called the
groups of the periodic table.

So let's look at them a
little bit more closely.

So this basically means that
if I look at this element,

say chromium, it belongs
to period four groups six.

If you look at carbon,
it belongs to period two,

group 14, and so on and so forth.

What about these elements?

Well, if you look at the atomic
number carefully, it starts

with 57 and you can see
57 to 70s over here.

So these actually belong to period six,

and this belongs to period seven.

So it's not like a new period.

Okay?

But what's interesting for
us is the groups elements

that belong to the same group tend

to have similar properties.

And that's why some of
these groups have names.

For example, elements belonging

to group one are called alkaline metals.

Elements belonging to group two are called

Alkaline Earth Metals.

Group 17 elements like fluorine, chlorine,

and all they're called halogens.

And group 18 are called the noble gases.

For example, these elements in group one,

the alkaline metals, they're
all soft and squishy.

I mean, look at this.

This is sodium.

You can cut it like this.

It's kind of like clay.

They also have silvery color

and they have very low melting points.

And their behaviors are
similar in chemical reactions.

For example, if you were
to put them in water,

they all react violently.

Look at that.

Another example,

group 11 elements like
copper, silver, and gold.

They're all metals and
they're all very shiny

and super hard.

They are malleable, which
means they can be hammered into

shapes and ductile,

which means they can also
be pulled into wires.

And if you look at group
17 elements, the halogens

like fluorine, chlorine and so on, t

hey are quite chemically reactive.

So in the natural form,
they can be dangerous,

but they all form salts
as chemical reaction.

That's why they're called halogens.

Halogens literally means salt formers.

And you might know about
some of these salts,

sodium chloride for example, table salt,

which we all consume.

But guess what?

Sodium iodide is also a salt.

Sodium fluoride is also a salt,
which is used in toothpaste

because they can help prevent decays,

but halogens can also
kill germs and bacteria,

and therefore they're also found in a lot

of sanitizers like bleach,
swimming pool chlorine,

and other stuff.

And as a final example, if
you look at noble gases,

the group 18 elements, well, first of all,

they're all gases,

but more importantly, they
pretty much resist chemical

reactions altogether,

and they're found in tiny
amounts in our atmosphere.

And if you put these gases in glass tubes

and passive electricity
through them, they will glow.

Yes, neon lights.

Well, of course not
everything that glows is neon.

Different elements can
have different colors,

but that's pretty much it.

So to summarize, elements
have the atomic number,

which is the ID

and the atomic mass number that tells you

how massive it is compared to hydrogen.

They're arranged in seven horizontal rows,

which are called the periods,

and the vertical columns
you are called, the groups

and elements that belong

to the same groups have
similar properties.