Atoms of the same element that have the same number of protons (same atomic number) but different numbers of neutrons (different atomic masses). Isotopes have the same chemical properties but have different physical and nuclear properties. Examples of isotopes are plutonium-238, plutonium-239, plutonium-240, and plutonium-241. Each acts chemically like plutonium but they have 144, 145, 146, and 147 neutrons, respectively.
