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# Worked example: Identifying isotopes and ions

## Video transcript

- [Narrator] An isotope
contains 16 protons, 18 electrons, and 16 neutrons. What is the identity of the isotope? And I encourage you to
pause the video and see if you can figure it out
and I'll give you a hint, you might want to use
this periodic table here. All right, so I'm assuming
you've had a go at it. So, an element is defined by the number of protons it has. So if someone tells you
the number of protons, you should be able to look at
a periodic table and figure out what element they are talking about. So, because it is 16
protons, well we can go right over here to the atomic
number, what has 16 protons, well anything that has 16
protons by definition is going to be sulfur right over here. So I could write a big S. Now, the next thing we
might want to think about is the mass number of
this particular isotope. Remember, an isotope, all
sulfur atoms are going to have 16 protons, but
they might have different numbers of neutrons. So, the sulfurs that have
different number of neutrons, those would be different isotopes. So, this case we have
16 protons and we have 16 neutrons, so if you add
the protons plus the neutrons together, you're going
to get your mass number. So 16 plus 16 is 32. Now let's figure out if there's
going to be any charge here. Well, the protons have a positive charge. The electrons have a negative charge. If you have an equal amount
of protons and electrons, then you would have no charge. But in this case, we have
a surplus of electrons. We have two more electrons
than protons and since we have a surplus of the
negative charged particles we, and we have two
more, we're going to have a negative two charge and
we write that as two minus. So this is actually an
ion, it has a charge. So this is the isotope
of sulfur that has a mass number of 32, the protons
plus the neutrons are 32, and it has two more electrons
than protons which gives it this negative charge. Let's do another example
where we go the other way. Where we are told, we are
given some information about what isotope and
really what ion we're dealing with because this
has a negative charge and we need to figure out
the protons, electrons, and neutrons. Well, the first thing that
I would say is, well look, they tell us that this is fluorine. As soon as you know what
element we're dealing with, you know what it's atomic
number is when you look at the periodic table and
you can figure out the number of protons. Remember, your atomic number
is the number of protons and that's what defines the element. That's what makes this one fluorine. So let's go up to the, our periodic table and we see fluorine right
over here has an atomic number of nine. That means any fluorine has nine protons. So, let's scroll back down. So, must because it is
fluorine, we know we have nine protons. Now what else can we figure out? Well, we know we have a
negative charge right here and this is, you can use
as a negative one charge and so we have one more
electron than we have protons. And so since we have nine
protons, we're going to have 10 electrons. And then finally how many neutrons? Well, remember, the
neutrons plus the protons add up to give us this mass number. So, if you have nine protons,
well how many neutrons do you have to add to
that to get to 18, well you're going to have
to have nine neutrons. Nine plus nine is 18.