George Charles de Hevesy, Hungarian radiochemist. In 1923, Hevesy made two major contributions to science. The first was the discovery of the element hafnium (symbol Hf), predicted from Mendeleev's periodic table. The second was the development of a technique called radioactive labelling. In this, an element within a system was replaced by one of its chemically-identical radioactive isotopes. This could then be traced through a reaction with ease, identified by its radioactivity. For these discoveries, Hevesy was awarded the 1943 Nobel Prize for chemistry. Photographed in 1959.