Atomic Mass Number - What is Atomic Mass Number?

The total number of neutrons and protons in the nucleus (core) of an atom, makes the mass number of the atom (abbreviated A).
This number is an approximation of the mass of the atom.
Electrons contribute very little to the mass of the atom, so that they are not included in the mass number.
A stable helium atom may have a number of mass equal to three (two protons + one neutron) or equal to four (two protons and two neutrons).
The element Bismuth, which has 83 protons, requires 126 neutrons in order to be stable, so that its mass number is 209 (83 + 126 = 209).