Atomic Mass

The total number of protons and neutrons in the nucleus is the atomic mass number, the electrons are not included, because their weight is negligible.

The mass of individual elements is useful information. But it is unpractical to work with. They should use an isotope of carbon 12 as a basis and then compare the other atoms to the carbon atoms. Carbon has 12.00 AMU - (Atomic - Mass - Units).

E.g. He(4) would have 1/3 the mass of carbon - 4 AMU

Ni (60) would have 5/1 the mass of carbon - 60 AMU

Most of the atoms have an atomic mass, which is approximately a whole number, but atoms like chlorine (Cl) has an atomic mass of 35.453 AMU.