The term Frederick Soddy used for atoms of one element with different masses was “isotope.” Isotopes have the same number of protons, which defines their element, but different numbers of neutrons, producing different mass numbers.
Soddy developed the idea in 1913 while studying radioactive decay. Radioactive series appeared to contain many more distinct atomic species than the periodic table had available places. Soddy proposed that chemically identical atoms could occupy the same position in the table while differing in mass and radioactive behavior.
The word isotope means “same place,” from the Greek isos, meaning equal, and topos, meaning place. Scottish physician Margaret Todd suggested the name to Soddy, who then popularized it. He received the 1921 Nobel Prize in Chemistry partly for his work on isotopes and radioactive substances.
A common confusion is between an isotope and a nuclide. A nuclide is any specific nuclear species; isotopes are nuclides of the same element with different mass numbers. An ion, by contrast, differs in its number of electrons, not its number of neutrons.