### Video Transcript

In the table below, which of the
following has a value of eight?

At the far left of the table, we
have a column called “Isotope of Atom.” An isotope is a type of atom. The letter or pair of letters in
the symbol indicates the element and the number superscripted to the left is the
mass number. So, we can read the first entry as
isotope of fluorine with a mass number of 18. The mass number is simply the
number of protons and neutrons that we would find in a nucleus of this particular
isotope.

So, what we seem to have is a table
that’s listing properties for four atoms, which are all different isotopes of
different elements. The first entry is an atom of
fluorine 18, the second atom is an atom of nitrogen 15, and the third atom is an
atom of carbon 13. But for the fourth atom, although
we know it’s an atom of potassium, we don’t know its mass number. It’s just been given the symbol
D. So, it’s one of the unknowns that
we need to figure out. So, what I’m going to do is walk
through how to figure out what A B, C, D, and E are. And then, we’re going to find the
one that has a value of eight.

So, here’s the question: How do we
find out the number of protons in an atom of fluorine 18? We know the mass number of fluorine
18. It’s 18. And that’s the number of protons
and neutrons in the nucleus. But we just want to know the number
of protons. For that, we need the atomic
number. Sometimes when nucleoid symbols
like these isotopes symbols are written down, the atomic number is written in
subscript to the bottom left of the element symbol. But in this question, they haven’t
been so generous. We’re going to have to think of
another way. Thankfully, we can look up the
atomic number of an element on the periodic table.

For fluorine, the atomic number is
nine, which means the number of protons in the atom of fluorine 18 we have here is
nine. Unfortunately, nine isn’t
eight. So, we have to continue our
calculations. Now, since we figured out how to do
atomic numbers, let’s do the atomic numbers for all the atoms and then come back to
the calculations. The atomic number for nitrogen is
seven. So, we figured out one more
value. But unfortunately, it doesn’t have
a value of eight. The atomic number for carbon is
six. And the atomic number for potassium
is 19. So, we could also fill in the
number of protons for these atoms.

Now, let’s move on to figuring out
the number of neutrons. We can easily figure out the number
of neutrons by just taking the atomic number away from the mass number. Since the mass number is the number
of protons and neutrons, all we need to do is take away the number of protons to get
the number of neutrons. So, there are nine neutrons in our
atom of fluorine 18. We just take the mass number, 18,
and take away the atomic number, nine. And the number of neutrons for our
atom of nitrogen 15 is eight, 15 minus seven. So, we found our answer. But I’m gonna fill in the rest of
the table anyway. It’s just good practice.

13 minus six is seven. So, there are seven neutrons in our
atom of carbon 13. And we can move on to figuring out
the number of electrons. Now, we’ve been told we’re dealing
with atoms. And atoms are by definition
neutral. So, they have the same number of
protons and electrons. And we can see that in the example
of our potassium isotope. So, C is six because we have six
electrons in our atom of carbon 13. We have seven electrons in our
nitrogen atom. And there are nine electrons in our
atom of fluorine 18. To figure out the last entry in the
table, which is the mass number for our potassium isotope, all we need to do is add
up the number of protons and the number of neutrons, meaning that our final unknown
has a value of 40.

And our final selection with a
value of eight is the number of neutrons in an atom of nitrogen 15.