Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.
- ‘Normally the proportion of the two isotopes of carbon is simply controlled by the rate of photosynthesis in plant tissues.’
- ‘Radioactive elements have different isotopes that decay at different rates.’
- ‘Beta radiation is the emission of an electron from the nucleus of a radioactive isotope.’
- ‘The nuclei of the hydrogen isotopes are the proton, the deuteron, and the triton.’
- ‘When uranium is bombarded with neutrons, the two isotopes have differing nuclear reactions.’
1913: coined by F. Soddy, from iso- equal + Greek topos place (because the isotopes occupy the same place in the periodic table of elements).