Question

Why do all atoms of an element have the same atomic number, although they may have different mass num- bers?

Answer 1

They are isotopes.

Step-by-step explanation:

Isotopes mean atoms of the same element with the same number of protons but a different number of neutrons. Protons give the identity of an element, so that cannot change; only the neutron numbers can change, and that affects the mass number because adding or subtracting neutrons will decrease or increase the mass number.

Hope this makes sense.