Video Transcript
Which of the following statements
about the stability of atomic nuclei is correct? (A) Isotopes of an element that
have atomic nuclei that are more likely to be unstable are those that have a greater
number of neutrons in the nucleus compared to other isotopes of that element. (B) The number of neutrons in an
atomic nucleus has no effect on how likely the nucleus is to be unstable.
When an atomic nucleus is stable,
it will not spontaneously decay. We can compare atomic nuclei in two
ways: the number of protons in a nucleus and the number of neutrons in a
nucleus. Nuclei that contain different
numbers of protons are nuclei of different elements. For example, all hydrogen nuclei
contain one proton, and all helium nuclei contain two protons. Nuclei that contain the same number
of protons but contain different numbers of neutrons are different isotopes of the
same element.
A helium nucleus can contain
anywhere from zero neutrons to eight neutrons. There are then nine isotopes of
helium. Most helium nuclei contain two
neutrons. The commonest isotope of helium has
two neutrons in its nucleus.
The question asks if the number of
neutrons compared to the number of protons in an isotope of an element has any
relation to how likely it is that the isotope will be stable. Option (B) states that the number
of neutrons in a nucleus makes no difference to how stable the nucleus is. In fact, the number of neutrons in
a nucleus can make a difference to whether or not a nucleus is stable. It is the case that if we compare
different isotopes of different elements, we find that, in general, the isotopes
that have greater numbers of neutrons compared to protons are more likely to be
unstable.
It is important to understand that
this does not mean that it is always the case that isotopes with more neutrons than
protons must be unstable, only that such isotopes are more likely to be
unstable. For example, all calcium nuclei
have 20 protons. Calcium has many isotopes. There are five isotopes of calcium
that have 19 or fewer neutrons. All isotopes of calcium that have
19 or fewer neutrons are unstable.
There are 14 isotopes of calcium
with 27 or more neutrons. All isotopes of calcium that have
27 or more neutrons are unstable. And for isotopes of calcium with
between 20 and 26 neutrons, the isotope of calcium that has 20 neutrons is
stable. The isotope of calcium that has 21
neutrons is unstable. The isotope of calcium that has 22
neutrons is stable. The isotope of calcium that has 23
neutrons is stable. The isotope of calcium that has 24
neutrons is stable. The isotope of calcium that has 25
neutrons is unstable. And the isotope of calcium that has
26 neutrons is stable.
The stability of isotopes of other
elements are in general more or less like that for calcium, although most elements
have fewer isotopes than calcium. Isotopes with more neutrons
compared to protons are, in general, more likely to be unstable. It is also important to understand
that for a given element, an isotope of that element is either stable or
unstable. For example, for calcium, the
isotope with 20 neutrons is stable. This means that all calcium nuclei
with 20 neutrons are stable. For calcium, the isotope with 21
neutrons is unstable. This means that all calcium nuclei
with 21 neutrons are unstable.
So we conclude that the correct
option is (A). Isotopes of an element that have
atomic nuclei that are more likely to be unstable are those that have a greater
number of neutrons in the nucleus compared to other isotopes of that element.
