Isotopes | Atomic structure and properties | AP Chemistry | Khan Academy

- [Instructor] In other
videos we have talked

about that the type of
element that we are dealing

with is defined by the number of protons

in an atom's nucleus.

So for example, any atom
with exactly one proton

in its nucleus is by definition hydrogen.

Any atom with six protons in its nucleus

is by definition carbon,

any atom with 17 protons in its nucleus

is by definition chlorine,

and so these numbers that I'm
circling on a periodic table

of elements, that's known
as the atomic number,

but it's really just the
number of protons in an atom

of that element types nucleus.

And that defines what
type of element it is.

But in this video we're going
to dig a little bit deeper

and realize that you can
still have different versions

of the same element and these
versions in chemistry speak

are known as isotopes.

Now how can you have different
versions of the same element

if the number of protons
defines what the element is?

Well, the versions the various
isotopes are going to happen

based on the number of neutrons you have.

So for example, there are two
stable isotopes of chlorine,

there's one version of
chlorine known as chlorine 35.

Let me write it over here, chlorine 35.

It's sometimes written like this,

in fact it's often written
like this, chlorine 35

and this isotope notation
that you see over here

where we have 35 in the top left,

that 35 is the sum of this version,

this isotope of chlorines
protons and neutrons.

This number 35 is this isotope
of chlorines mass number.

So it has a total of 35
protons and neutrons,

how many neutrons does this
version of chlorine have?

Well it's going to have 17 protons.

17 protons, I know that
because we are dealing

with chlorine, so how many
neutrons will it have?

Well 35 minus 17 is 18, 18 neutrons.

And there's another version
of chlorine that is stable

and that is chlorine 37.

Now how many protons
is that going to have?

Well that's a trick question,

by definition it's chlorine,
it's going to have 17 protons.

This is going to have 17 protons,

but then how many neutrons will it have?

Well the protons plus the neutrons is 37,

so 17 plus 20 is going to be 37.

So it's going to be 20 neutrons,

and this would be written
out as chlorine, chlorine 37.

So you can see these are two
different versions of chlorine,

same number of protons
which make them chlorine,

but different number of neutrons.

Now you can imagine these
different versions are going

to have different atomic masses,

but here on a periodic table
of elements there's only one

average atomic mass listed,

and the key word here is this
is an average atomic mass.

It's the weighted average of
the masses of the chlorines,

the stable chlorines that you will find.

So for example, in nature
75.77% of the chlorine

found is chlorine 35,

and then the remaining
24.23% of the chlorine found

is chlorine 37.

So when they calculate
this average atomic mass,

what they do is they would
take, or you would take,

if you're calculating it,
so this would be 75.77%

times the atomic mass,

atomic mass of chlorine 35 plus,

and now the weight here would be 24.23%

times the atomic mass,
atomic mass of chlorine 37.

And if you were to do this
calculation you would get

this number right over here, 35.45 unified

atomic mass units.

Now, how do you figure out the
atomic mass of chlorine 35?

You might be tempted to
say it's just 35 unified

atomic mass units, and you would be close

because the mass of a
proton is close to one

universal atomic mass unit,

and the mass of a neutron is close to one

universal atomic mass unit,

and then the electrons
are have a much, much,

much smaller mass.

You can also almost
consider them negligible

for atomic mass purposes,

and so you will get an
atomic mass close to 35.

But it actually turns out
it's a little bit different

because not only are the masses
of each individual proton

or neutron a little bit more
actually than one unified

atomic mass unit,

but when you put all those
protons and neutrons together

in a nucleus, their
combined masses is actually

a little bit less than
their individual masses

if you were to just add them up,

and that's actually
known as a mass defect.

And so if you actually want
to know the atomic mass

of chlorine 35, you can look
that up in a lot of tables,

and you will see that it's
actually slightly under

35 unified atomic mass units.