What defines isotopes of an element?

Isotopes of an element are defined as atoms that have the same number of protons but different numbers of neutrons. This distinction is crucial because the number of protons determines the element's identity, while variations in neutron count result in different atomic masses for those isotopes. For example, carbon has isotopes like carbon-12 and carbon-14, where both have 6 protons, but carbon-12 has 6 neutrons, and carbon-14 has 8 neutrons.

The presence of different numbers of neutrons in isotopes contributes to variations in nuclear stability and can affect the nuclear properties, such as radioactivity, without changing the chemical behavior of the element significantly, which is primarily dictated by electron arrangement and proton number.