Question

Two atoms have the same number of protons but different numbers of neutrons. What can you conclude about these two atoms?

Answer 1

That they are isotopes, i.e. the same element, but different atomic masses.

Explanation:

The number of nuclear protons gives `Z`, the atomic number. The atomic number defines the identity of the element: `Z` = 6, `C`; `Z` = 27, `Co`; `Z` = 47, `Ag`.

Of course while there may be the same number of nuclear protons, the nucleus may contain different numbers of neutrons; these give rise to different isotopes; same element, and same chemistry, but different atomic mass. Most elements have a number of isotopes. The quoted atomic mass that appear on the Periodic Table are the weighted average of the isotopes.