Question

Two atoms same atomic mass number but differen atomic number what is difference?

Answer 1

The difference is in the number of neutrons.

Explanation:

Examples would be `""_16^40"S",color(white)(l) _17^40"Cl",color(white)(l)_18^40"Ar",color(white)(l)_19^40"K"`, and `""_20^40"Ca"`.

These nuclei are isobars: they have different numbers of protons but enough neutrons to give them the same atomic mass.