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- [Instructor] In other
videos we have talked

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about that the type of
element that we are dealing

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with is defined by the number of protons

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in an atom's nucleus.

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So for example, any atom
with exactly one proton

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in its nucleus is by definition hydrogen.

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Any atom with six protons in its nucleus

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is by definition carbon,

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any atom with 17 protons in its nucleus

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is by definition chlorine,

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

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

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

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of that element types nucleus.

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And that defines what
type of element it is.

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

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and realize that you can
still have different versions

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of the same element and these
versions in chemistry speak

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are known as isotopes.

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Now how can you have different
versions of the same element

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if the number of protons
defines what the element is?

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Well, the versions the various
isotopes are going to happen

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based on the number of neutrons you have.

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

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

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Let me write it over here, chlorine 35.

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It's sometimes written like this,

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

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and this isotope notation
that you see over here

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where we have 35 in the top left,

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that 35 is the sum of this version,

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this isotope of chlorines
protons and neutrons.

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This number 35 is this isotope
of chlorines mass number.

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So it has a total of 35
protons and neutrons,

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how many neutrons does this
version of chlorine have?

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Well it's going to have 17 protons.

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17 protons, I know that
because we are dealing

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with chlorine, so how many
neutrons will it have?

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Well 35 minus 17 is 18, 18 neutrons.

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And there's another version
of chlorine that is stable

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and that is chlorine 37.

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Now how many protons
is that going to have?

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Well that's a trick question,

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

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This is going to have 17 protons,

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but then how many neutrons will it have?

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Well the protons plus the neutrons is 37,

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so 17 plus 20 is going to be 37.

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So it's going to be 20 neutrons,

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

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

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same number of protons
which make them chlorine,

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but different number of neutrons.

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Now you can imagine these
different versions are going

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to have different atomic masses,

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

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average atomic mass listed,

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

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

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the stable chlorines that you will find.

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So for example, in nature
75.77% of the chlorine

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found is chlorine 35,

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and then the remaining
24.23% of the chlorine found

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is chlorine 37.

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So when they calculate
this average atomic mass,

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

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

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times the atomic mass,

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atomic mass of chlorine 35 plus,

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and now the weight here would be 24.23%

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times the atomic mass,
atomic mass of chlorine 37.

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And if you were to do this
calculation you would get

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this number right over here, 35.45 unified

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atomic mass units.

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

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

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atomic mass units, and you would be close

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because the mass of a
proton is close to one

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universal atomic mass unit,

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and the mass of a neutron is close to one

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universal atomic mass unit,

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and then the electrons
are have a much, much,

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much smaller mass.

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You can also almost
consider them negligible

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for atomic mass purposes,

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

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

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because not only are the masses
of each individual proton

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or neutron a little bit more
actually than one unified

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atomic mass unit,

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but when you put all those
protons and neutrons together

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in a nucleus, their
combined masses is actually

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a little bit less than
their individual masses

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if you were to just add them up,

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and that's actually
known as a mass defect.

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And so if you actually want
to know the atomic mass

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

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and you will see that it's
actually slightly under

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35 unified atomic mass units.