When is an atom considered an isotope?

Answer 1

Atoms have the same number of protons, but different numbers of neutrons. This means that the mass number changes, even though the atomic number stays the same. Therefore, the two atoms will still belong to the same element, even though their number of nucleons vary.

Answer 2

An atom is considered an isotope when it has the same number of protons as other atoms of the same element but a different number of neutrons, leading to a different Atomic Mass. This results in isotopes having the same chemical properties but different physical properties, such as stability and radioactivity.