What are isotopes? - Leaving Cert Physics - Question b - 2009

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This difference in neutron count results in different mass numbers for these isotopes.

In simpler terms, all isotopes of an element share the same atomic number, which defines the element, but they have different mass numbers due to the varying number of neutrons in the nucleus. For example, Carbon-12 and Carbon-14 are both isotopes of carbon: they contain six protons, but Carbon-12 has six neutrons while Carbon-14 has eight. This fundamental distinguishing feature is what classifies them as isotopes.