Proof that the atomic mass is a approximately equal to the number of protons+neutrons?

Question

"mathematicly"

Answer 1

It isn't provable; it's just true. The number of neutrons in a nucleus is not directly measurable; it is inferred from the charge and the total mass.

Answer 2

Mass of the electron is 1/1800th that of a proton or neutron, so even the heaviest elements at around 100 protons would only have about 5% of their total weight attributed to electrons.

Answer 3

Here's your proof>
http://en.wikipedia.org/wiki/Atomic_mass