Video Transcript
An atom of calcium has a mass
number of 42 and an atomic number of 20. How many neutrons are there in its
nucleus?
An atom consists of a nucleus
surrounded by an electron cloud. In the nucleus, we can find protons
and neutrons. And electrons occupy the electron
cloud. We won’t need it for this question,
but it’s important to remember that atoms are neutral. So they have the same number of
positive protons and negative electrons. The next thing we need to ask
ourselves is, what do we mean by an atom of calcium?
Calcium is the name of an
element. And we can find information about
calcium on the periodic table of elements. The first thing you might notice is
that the number in the box for calcium is the same as the atomic number of the atom
of calcium. The atomic number is simply the
number of protons in an atom or ion of a particular element. So the first thing we realize is
that we have 20 protons in our atom of calcium. And while it doesn’t make a
difference in this question, we also have 20 electrons.
The other piece of information in
the question is that this atom has a mass number of 42. The mass number is the number of
protons and neutrons in the atom or ion. So this tells us that if we take 20
and add the number of neutrons, we’ll get 42. All this sets us up nicely to
answer the question. How many neutrons are there in the
nucleus of the atom of calcium? The number of neutrons must be 42
minus the number of protons, which is 20. Therefore, the number of neutrons
in an atom of calcium with a mass number of 42 is 22.
