Why is it that only isotopes tend to be radioactive?
Every natural element consists of isotopes.
Even hydrogen, the smallest atom, has three isotopes:
Of these, the first two are stable. The third, tritium, is radioactive.
Thus there are two kinds of isotopes — stable isotopes and radioactive isotopes.
To be stable, a nucleus should contain about the same number of neutrons as protons.
The chart above shows that there are many more radioactive isotopes than stable isotopes.