Atoms of the same element that differ in mass are called:

1. Soddy found that mesothorium was chemically identical to radium despite a different atomic mass. 2. That result, with similar findings for other elements, showed an element could have atoms of different masses that were chemically indistinguishable. 3. These different types are called isotopes, atoms of the same element that differ in mass. 4. Isomers are different arrangements of the same formula, which is a separate idea. _Source: OpenStax Chemistry 2e (CC BY 4.0), Ch 2 'Atoms, Molecules, and Ions', sections 2.1-2.6_