The elements with the most isotopes, with 36 each, are xenon (Xe) with nine stable isotopes (identified from 1920 to 1922) and 27 radioactive isotopes (identified from 1939 to 1981), and cesium (Cs) with one stable isotope (identified in 1921) and 35 radioactive isotopes (identified from 1935 to 1983).

The element with the fewest number of isotopes is hydrogen (H), with three isotopes, including two stable ones—protium (identified in 1920) and deuterium, often called heavy water, (identified in 1931)—and one radioactive isotope—tritium (first identified in 1934, although not considered a radioactive isotope in 1939).