Why is the relative atomic mass of an element rarley a whole number?

Answer 1

Partly because many elements occur naturally as at least two isotopes, which have different mass numbers, and partly because may isotopes have actual masses that are different from their (always integral) mass numbers. This latter effect is most pronounced among ferrous metal atoms, which have the strongest nuclear binding energies of all atoms and consequently have less actual mass than the sum of the actual masses of all their constituent subatomic particles.