What defines an isotope?

An isotope is specifically defined as atoms of the same element that have the same number of protons but differing numbers of neutrons. This variation in neutron count means that isotopes can have different mass numbers, which affects their atomic weight. While the chemical properties of isotopes are generally similar due to having the same number of electrons and the same arrangement of electrons in their outer shells, it is the difference in neutrons that categorizes them as isotopes. Therefore, the defining characteristic of an isotope lies in the differing neutron counts while retaining the same element identity. This distinction is critical in fields such as chemistry, nuclear physics, and radiometric dating.