Question

The letter m in the symbol for the radioactive isotope technetium-99m indicates that when a nucleus of this isotope decays, it's atomic mass will _________.

a. decrease by 4
b. increase by 2
c. decrease by 2
d. stay the same

Answer 1

Stays the same. The m is needed because this isn't really a radioactive decay; the Tc-99m is just the Tc-99 nucleus in an unusually long-lived excited state, like an H atom with the electron in a higher orbit. After a few hours, the nucleus falls down to the ground state, emitting a fat gamma ray. But there's no transmutation of the nucleus into some other nuclide, as in normal radioactive decay.