Elements and atoms | Atoms, compounds, and ions | Chemistry | Khan Academy

How elements relate to atoms. The basics of how protons, electrons and neutrons make up an atom.

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Chemistry on Khan Academy: Did you know that everything is made out of chemicals? Chemistry is the study of matter: its composition, properties, and reactivity. This material roughly covers a first-year high school or college course, and a good understanding of algebra is helpful.

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We humans have known,
for thousands of years,

just looking at our
environment around us,

that there are
different substances.

And these different
substances tend

to have different properties.

And not only do they have
different properties,

one might reflect
light in a certain way,

or not reflect light, or
be a certain color, or at

a certain temperature, be
liquid or gas, or be a solid.

But we also start
to observe how they

react with each other in
certain circumstances.

And here's pictures of
some of these substances.

This right here is carbon.

And this is in
its graphite form.

This right here is lead.

This right here is gold.

And all of the ones that I've
shown pictures of, here--

and I got them all from this
website, right over there--

all of these are in
their solid form.

But we also know that
it looks like there's

certain types of air, and
certain types of air particles.

And depending on what
type of air particles

you're looking at, whether it
is carbon or oxygen or nitrogen,

that seems to have different
types of properties.

Or there are other things
that can be liquid.

Or even if you raise the
temperature high enough

on these things.

You could, if you raise
the temperature high enough

on gold or lead, you
could get a liquid.

Or if you, kind of, if
you burn this carbon,

you can get it to
a gaseous state.

You can release it
into the atmosphere.

You can break its structure.

So these are things that we've
all, kind of, that humanity

has observed for
thousands of years.

But it leads to a
natural question

that used to be a
philosophical question.

But now we can answer
it a little bit better.

And that question is, if you
keep breaking down this carbon,

into smaller and
smaller chunks, is there

some smallest chunk, some
smallest unit, of this stuff,

of this substance, that still
has the properties of carbon?

And if you were
to, somehow, break

that even further,
somehow, you would

lose the properties
of the carbon.

And the answer is, there is.

And so just to get
our terminology,

we call these
different substances--

these pure substances that
have these specific properties

at certain
temperatures and react

in certain ways-- we
call them elements.

Carbon is an element.

Lead is an element.

Gold is an element.

You might say that
water is an element.

And in history, people
have referred to water

as an element.

But now we know that water is
made up of more basic elements.

It's made of oxygen
and of hydrogen.

And all of our elements
are listed here

in the Periodic
Table of Elements.

C stands for carbon-- I'm just
going through the ones that

are very relevant to humanity,
but over time, you'll

probably familiarize
yourself with all of these.

This is oxygen.

This is nitrogen.

This is silicon.

Au is gold.

This is lead.

And that most basic unit, of any
of these elements, is the atom.

So if you were to keep
digging in, and keep

taking smaller and
smaller chunks of this,

eventually, you would
get to a carbon atom.

Do the same thing over
here, eventually you

would get to a gold atom.

You did the same thing
over here, eventually,

you would get
some-- this little,

small, for lack of a
better word, particle,

that you would call a lead atom.

And you wouldn't be
able to break that down

anymore and still
call that lead,

for it to still have
the properties of lead.

And just to give you an idea--
this is really something

that I have trouble
imagining-- is

that atoms are
unbelievably small, really

unimaginably small.

So for example, carbon.

My hair is also
made out of carbon.

In fact, most of me
is made out of carbon.

In fact, most of all living
things are made out of carbon.

And so if you took my hair--
and so my hair is carbon,

my hair is mostly carbon.

So if you took my
hair-- right over here,

my hair isn't yellow,
but it contrasts nicely

with the black.

My hair is black,
but if I did that,

you wouldn't be able to
see it on the screen.

But if you took my
hair, here, and I

were to ask you, how many
carbon atoms wide is my hair?

So, if you took a cross
section of my hair, not

the length, the
width of my hair,

and said, how many carbon
atoms wide is that?

And you might
guess, oh, you know,

Sal already told me
they're very small.

So maybe there's 1,000 carbon
atoms there, or 10,000,

or 100,000.

I would say, no.

There are 1 million
carbon atoms,

or you could string 1
million carbon atoms

across the width of
the average human hair.

That's obviously
an approximation.

It's not exactly 1 million.

But that gives you a sense
of how small an atom is.

You know, pluck a
hair out of your head,

and just imagine
putting a million things

next to each other,
across the hair.

Not the length of the hair,
the width of the hair.

It's even hard to see
the width of a hair,

and there would be a
million carbon atoms,

just going along it.

Now it would be pretty
cool, in and of itself,

that we do know that there
is this most basic building

block of carbon, this most basic
building block of any element.

But what's even neater is
that, those basic building

blocks are related
to each other.

That a carbon atom is made
up of even more fundamental

particles.

A gold atom is made up even
more fundamental particles.

And depending-- and
they're actually

defined by the arrangement of
those fundamental particles.

And if you were to change the
number of fundamental particles

you have, you could change the
properties of the element, how

it would react, or you could
even change the element itself.

And just to understand
it a little bit better,

let's talk about those
fundamental elements.

So you have the proton.

And the proton is actually
the defining-- the number

of protons in the
nucleus of an atom,

and I'll talk about the
nucleus in a second-- that

is what defines the element.

So this is what
defines an element.

When you look at the
periodic table right here,

they're actually written
in order of atomic number.

And the atomic
number is, literally,

just the number of
protons in the element.

So by definition,
hydrogen has one proton,

helium has two protons,
carbon has six protons.

You cannot have carbon
with seven protons.

If you did, it
would be nitrogen.

It would not be carbon anymore.

Oxygen has eight protons.

If, somehow, you were to
add another proton to there,

it wouldn't be oxygen anymore.

It would be fluorine.

So it defines the element.

And the atomic
number, the number

of protons-- and
remember, that's

the number that's
written right at the top,

here, for each of these
elements in the periodic table--

the number of protons is
equal to the atomic number.

And they put that
number up here,

because that is the defining
characteristic of an element.

The other two constituents
of an atom-- I

guess we could
call it that way--

are the electron
and the neutron.

And the model you
can start to build

in your head-- and this model,
as we go through chemistry,

it'll get a little bit more
abstract and really hard

to conceptualize.

But one way to
think about it is,

you have the protons
and the neutrons that

are at the center of the atom.

They're the nucleus of the atom.

So for example, carbon,
we know, has six protons.

So one, two, three,
four, five, six.

Carbon-12, which is
a version of carbon,

will also have six neutrons.

You can have versions
of carbon that

have a different
number of neutrons.

So the neutrons can change,
the electrons can change,

you can still have
the same element.

The protons can't change.

You change the protons, you've
got a different element.

So let me draw a carbon-12
nucleus, one, two, three, four,

five, six.

So this right here is
the nucleus of carbon-12.

And sometimes, it'll
be written like this.

And sometimes, they'll actually
write the number of protons,

as well.

And the reason why we
write it carbon-12--

you know, I counted
out six neutrons--

is that, this is
the total, you could

view this as the total number
of-- one way to view it.

And we'll get a
little bit nuance

in the future-- is that this
is the total number of protons

and neutrons inside
of its nucleus.

And this carbon, by definition,
has an atomic number of six,

but we can rewrite
it here, just so

that we can remind ourselves.

So at the center of a carbon
atom, we have this nucleus.

And carbon-12 will have six
protons and six neutrons.

Another version of
carbon, carbon-14,

will still have six
protons, but then it

would have eight neutrons.

So the number of
neutrons can change.

But this is carbon-12,
right over here.

And if carbon-12 is neutral--
and I'll give a little nuance

on this word in a second
as well-- if it is neutral,

it'll also have six electrons.

So let me draw those six
electrons, one, two, three,

four, five, six.

And one way-- and this is
maybe the first-order way

of thinking about
the relationship

between the electrons
and the nucleus--

is that you can imagine the
electrons are, kind of, moving

around, buzzing
around this nucleus.

One model is, you
could, kind of,

thinking of them as
orbiting around the nucleus.

But that's not quite right.

They don't orbit the
way that a planet, say,

orbits around the sun.

But that's a good
starting point.

Another way is, they're kind
of jumping around the nucleus,

or they're buzzing
around the nucleus.

And that's just
because reality just

gets very strange at this level.

And we'll actually have
to go into quantum physics

to really understand what
the electron is doing.

But a first mental
model in your head

is at the center of this
atom, this carbon-12 atom,

you have this nucleus,
right over there.

And these electrons are
jumping around this nucleus.

And the reason why these
electrons don't just

go off, away from this nucleus.

Why they're kind of
bound to this nucleus,

and they form part
of this atom, is

that protons have
a positive charge

and electrons have
a negative charge.

And it's one of these properties
of these fundamental particles.

And when you start
thinking about,

well, what is a
charge, fundamentally,

other than a label?

And it starts to
get kind of deep.

But the one thing
that we know, when

we talk about
electromagnetic force,

is that unlike charges
attract each other.

So the best way
to think about it

is, protons and
electrons, because they

have different charges,
they attract each other.

Neutrons are neutral.

So they're really just sitting
here inside of the nucleus.

And they do affect the
properties, on some level,

for some atoms of
certain elements.

But the reason why we have the
electrons not just flying off

on their own is
because, they are

attracted towards the nucleus.

And they also have an
unbelievably high velocity.

It's actually hard for-- and
we start touching, once again,

on a very strange
part of physics

once we start talking about what
an electron actually is doing.

But it has enough, I
guess you could say,

it's jumping around enough that
it doesn't want to just fall

into the nucleus, I guess is
one way of thinking about it.

And so I mentioned,
carbon-12 right over here,

defined by the
number of protons.

Oxygen would be defined
by having eight protons.

But once again, electrons can
interact with other electrons.

Or they can be taken
away by other atoms.

And that actually forms a lot of
our understanding of chemistry.

It's based on how many
electrons an atom has,

or a certain element has.

And how those electrons
are configured.

And how the electrons of
other elements are configured.

Or maybe, other atoms
of that same element.

We can start to predict
how an atom of one element

could react with another
atom of that same element.

Or an atom of one element,
how it could react,

or how it could
bond, or not bond,

or be attracted, or
repel, another atom

of another element.

So for example-- and we'll
learn a lot more about this

in the future-- it is possible
for another atom, someplace,

to swipe away an
electron from a carbon,

just because, for
whatever reason.

And we'll talk about certain
elements, certain neutral atoms

of certain elements,
have a larger

affinity for
electrons than others.

So maybe one of those swipes
an electron away from a carbon,

and then this carbon
will be having

less electrons than protons.

So then it would have five
electrons and six protons.

And then it would have
a net positive charge.

So, in this carbon-12,
the first version I did,

I had six protons,
six electrons.

The charges canceled out.

If I lose an electron, then
I only have five of these.

And then I would have
a net positive charge.

And we're going to talk
a lot more about all

of this throughout the
chemistry playlist.

But hopefully, you
have an appreciation

that this is already
starting to get really cool.

Once we can already
get to this really,

fundamental building
block, called the atom.

And what's even neater is that
this fundamental building block

is built of even more
fundamental building blocks.

And these things
can all be swapped

around, to change the
properties of an atom,

or to even go from an
atom of one element

to an atom of another element.