Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Because their proton numbers are equal, their atomic numbers and elemental identities are the same. Their neutron numbers—and therefore their mass numbers—are different.

Nuclei with the same proton number but different neutron numbers are nuclei of different isotopes of that element.

Atoms of one element must have the same number of protons, but they do not all have to contain the same number of neutrons. These different versions of the element are called isotopes.

Isotopes are nuclei of the same element with the same proton number but different neutron numbers. They consequently have different mass numbers.