The isotope of the element hydrogen with atomic weight 2.0144 and symbols ²H or D. Considerations of nuclear stability and a discrepancy between the chemical and physical atomic weights of hydrogen led to the prediction of a stable isotope of hydrogen of mass 2. A successful search for this isotope, deuterium, was made by H. C. Urey, F. G. Brickwedde, and G. M. Murphy in 1931. The terrestrial natural abundance of deuterium is 1 in 6700 parts of ordinary hydrogen (protium), of atomic weight 1.0078. Small variations in natural sources are found as a result of fractionation by geological processes. Industrial hydrogen, particularly that generated by electrolysis of water, may contain significantly less deuterium.