American chemist whose accurate determination of the atomic weights of approximately 25 elements indicated the existence of isotopes (isotope) and earned him the 1914 Nobel Prize for Chemistry.

Richards graduated from Haverford College, Pa., in 1885 and took advanced degrees at Harvard University, where he became instructor in chemistry in 1891 and full professor in 1901.

Richards greatly improved the technique of gravimetric atomic weight determinations, introducing quartz apparatus, the bottling device, and the nephelometer (an instrument for measuring turbidity). Although the atomic weight values of Jean Servais Stas had been regarded as standard, about 1903 physicochemical measurements showed that some were not accurate. Richards and his students revised these figures, lowering, for instance, Stas's value for silver from 107.93 to 107.88. Richards' investigations of the atomic weight of lead from different sources helped to confirm the existence of isotopes. His later researches were concerned mainly with the physical properties of the solid elements and included much original work on atomic volumes and compressibilities.