Question

How is the atomic mass number of an element determined from its isotopes?

Answer 1

Well, it is the WEIGHTED AVERAGE its various isotopes....

Explanation:

`Z_"the atomic number"` represents the number of positively charged, massive, nuclear particles, and `Z` specifies the identity of the element....`Z=1; "hydrogen"`, `Z=2; "helium"`, `Z=3; "lithium"`, …… `Z=23; "vanadium...."`.

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 `Z=1`..one nuclear proton. But hydrogen has a number of isotopes....`""^1H`, `"i.e protium"`, whose nuclei contain only the ONE PROTON. And there are smaller quantities of `""^2H`, `"i.e deuterium"`, whose nuclei contain the ONE PROTON (necessarily, why so), AND ALSO ONE neutron. And hence there are TWO massive nucular particles, and the superscript gives the mass number.

An even smaller number of hydrogen atoms, contain TWO NEUTRONS to give `""^3H`, `"i.e tritium"`. The weighted average of the isotopic masses gives the mass number reported on the Periodic Table. As `Z` increases, most elements have several stable isotopes...and their non-integral atomic masses reflects this proportion.