1913

Isotopes

Ernest Rutherford (1871–1937), Francis William Aston (1877–1945), Frederick Soddy (1877–1956), James Chadwick (1891–1974)

Early work on radioactivity produced significant advances in both physics and chemistry and opened whole new fields of research. In one breakthrough, British physicists Frederick Soddy and Ernest Rutherford proved that radioactive elements turned into entirely different elements as they decayed. Later Soddy showed that the radium that Marie and Pierre Curie had isolated from uranium ores was actually produced by radioactive decay of the uranium atoms. This led to the idea that there is more than one form of an element, even though their chemical properties seem identical.

In 1913, at the suggestion of his friend Dr. Margaret Todd, Soddy proposed the term isotope for these alternate forms (from the Greek iso- meaning “equal” and topos meaning “place,” since these alternate forms occupy the same place on the periodic table). An element (which has a fixed atomic number) could somehow have more than one atomic mass. British physicist J.J. Thomson later showed that even nonradioactive elements could have isotopes, and he and British chemist Francis William Aston provided proof through early mass spectrometry. This led Rutherford to propose that some neutral particle with almost the same weight as a proton might be making up the difference. (The neutron was experimentally detected in 1932, by British physicist James Chadwick.)

Isotopes have been of great importance in chemistry ever since. Kinetic isotope effects have given information about reaction mechanisms. The different magnetic behavior of various isotopes in NMR (nuclear magnetic resonance) experiments led to a huge branch of analytical chemistry, and radioactive tracers are used to diagnose and treat disease. As mass spectrometry has grown more powerful, the nonradioactive “cold” isotopes have been used more and more as distinctive labels for such experiments, instead of the radioactive ones.

An arrangement of isotope possibilities. The number of protons increases off into the distance, and the number of neutrons increases to the right. The dark-blue blocks in the middle are stable isotopes, while the lighter blue are unstable ones that have been observed. Gray blocks are possible isotopes that have never yet been seen to exist.