Question

Two atoms have the same number of protons but

Answer 1

We can say they're the ISOTOPES of an element.

Isotopes of an element means the atoms of the same element has the same number of protons but different number of neutrons. They're still the same element because their number of proton is the same, which is like an ID card number, each element has a specific number of protons.

Each element has different amount of Isotopes. Some may have a lot, probably up to twenty, and some only have one stable isotope. For example, in hydrogen, there's the hydrogen atom with 0 neutrons, 1 neutron and 2 neutrons. They're the Isotopes of hydrogen.

Different isotopes of an element has a different atomic mass, as the number of neutron is different. Additionally, they also have different abundance in the environment. Maybe certain Isotopes can be found more in the earth.