Atoms of the same element that have different numbers of neutrons are called isotopes. They retain the same number of protons, which keeps them the same element, but their different neutron counts give them different mass numbers.
Carbon provides a clear example. Carbon-12, carbon-13, and carbon-14 each contain six protons, but they contain six, seven, and eight neutrons respectively. Carbon-12 and carbon-13 are stable, while carbon-14 is radioactive and decays over time. That radioactive behavior makes carbon-14 useful for estimating the age of once-living archaeological material.
The term isotope comes from Greek words meaning “same place,” because isotopes occupy the same position on the periodic table. Margaret Todd suggested the word to chemist Frederick Soddy in 1913. Isotopes usually have nearly identical chemical behavior because chemical reactions depend mainly on electrons, although their different masses can produce measurable isotope effects.
Isotopes are often confused with ions and allotropes. Ions differ in electron number and carry a charge; allotropes are different structural forms of the same element, such as diamond and graphite. Isomers are compounds with the same formula but different arrangements.