Answer
It is highly unlikely that any other naturally occurring atom besides carbon-12 would have an exact integral mass, expressed in unified atomic mass units (u).
Work Step by Step
Atomic masses are based on the average mass of isotopes: Most elements have multiple isotopes, which are atoms with the same number of protons but different numbers of neutrons. The atomic mass of an element is based on the weighted average of the masses of its isotopes, taking into account their natural abundance. Therefore, it is unlikely that the average mass of all isotopes of an element would be a whole number.
