Video Transcript
Which of the following is a correct statement about the stability of atomic
nuclei? (A) An atomic nucleus that is stable can spontaneously become unstable. (B) An atomic nucleus that is stable cannot spontaneously become unstable.
This is an interesting question because it asks us to consider the circumstances
under which an atomic nucleus would become unstable. Recall that a stable nucleus can only change if something from outside of the nucleus
causes the nucleus to change. An unstable nucleus, on the other hand, can change because of processes that occur
entirely within the nucleus.
The key to answering this question is understanding the meaning of the word
“spontaneously” in the two answer options. Recall that when we say a nucleus spontaneously decays, this means that the decay
occurs as part of a process that happens entirely within the nucleus with no outside
influence. Since a stable nucleus can only change as a result of something external to the
nucleus, then we see that a stable nucleus is a nucleus that cannot spontaneously
decay, while an unstable nucleus is a nucleus that can spontaneously decay.
So, if a nucleus that is stable could spontaneously become unstable, as option (A)
claims, this would mean that a nucleus that could not decay could spontaneously
become a nucleus that could decay. That is, this change could happen as part of a process happening entirely within that
nucleus.
It is useful to recall that the time at which an unstable nucleus will decay is
unpredictable. A nucleus that is unstable may not decay immediately. The nucleus may exist for a very long time without decaying. It is very important that we do not say “This nucleus has not decayed yet, so it is
stable.” But rather we should say “This nucleus could decay at any moment, so it is
unstable.” Therefore, if a nucleus could ever decay without an influence from outside the
nucleus acting on it, we would have to say that the nucleus had always been an
unstable nucleus.
So the answer as to whether the nucleus could spontaneously change from being stable
to being unstable must be no. This possibility would not be consistent with the meanings of stable and unstable as
they apply to atomic nuclei. This means that the correct answer must be option (B). An atomic nucleus that is stable cannot spontaneously become unstable.
