Which isotope of the element Cadmium is most abundant, the one with 64 neutrons or the one with 65 neutrons? Why?

In general, nuclei with even mass numbers are both more stable and more abundant than those with odd mass numbers. Cd has an atomic number of Z = 48, so without looking at actual data, one would predict that 112-Cd (64 neutrons, and an even mass number) should be more abundant than 113-Cd, (odd mass number).

In fact, although 113-Cd is a naturally occurring isotope of Cd, it is radioactive, albeit with a very long halflife of 7.7*10^15 years. It makes up 12.2% of natural Cd. In contrast, 112-Cd is stable, and makes up 24.1% of natural Cd, in keeping with the "rule-of-thumb" prediction.

Natural Cd is made up of 8 different isotopes, ranging in mass number from 106 to 116. Roger the Mole's argument cannot be used to determine the relative abundance of 112-Cd and 113-Cd.

The atomic number of Cd is 48, so all of its isotopes contain 48 protons.
The the choices are between isotopes with masses of 112 and 113.
The natural atomic mass of Cd is 112.4110 g/mol, which is closer to 112 than it is to 113, so the 112 isotope (the one with 64 neutrons) must be more abundant.