- [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.