Radioactive isotopes of an atom are:
a. more scarce
b. more stable
c. less stable
d. more abundant

c.less stable

Explanation:

A radioactive isotope is an isotope of an atom that has an unstable nucleus. This generally happens when the nucleus has a much larger number of neutrons compared to the number of protons: in this case, the nucleus tends to decay, emitting part of the neutrons (but also protons or electrons, depending on the type of decay), becoming a lighter nucleus which is more stable. Therefore, a radioactive isotope is generally not very stable.

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