How is the atomic mass number of an element determined from its isotopes?
Well, it is the WEIGHTED AVERAGE its various isotopes....
On the other hand, the nucleus of a given element MAY contain different numbers of neutrons. Interactions between protons, and neutrons result in the strong nuclear force, an attractive force the which, at NUCLEAR ranges, is strong enuff to overcome the electrostatic force of repulsion. The nucleus of a given atom may contain different numbers of neutrons, and this gives rise to the phenomenon of isotopes, the which we will illustrate by reference to hydrogen.
For hydrogen, as we far as we know the most abundant element in the universe, has
An even smaller number of hydrogen atoms, contain TWO NEUTRONS to give