Question

What must two atoms have the same if they represent the same element?

Answer 1

They must have the same number of protons........

Explanation:

The identity of an element is detemined by `Z`, `"the atomic number"`, which gives the number of protons, massive nuclear particles of formal positive charge. And thus the Periodic Table of Elements reports elements on the basis of `Z`.

`Z=1, H`, `Z=2, He`, `Z=3, Li`,....... `Z=24, Cr`, ......`Z=35, Br`,

Of course, a given element, may have differing number of neutrons, which are massive particles of NEUTRAL charge, present in its nucleus. This gives rise to the phenomenon of isotopes. Also see this old answer.