Unknown author · Public domain

Co-discovered hafnium in 1923, closing the last gap Mendeleev had left in the periodic table, and pioneered the use of radioactive tracers in chemistry and biology — a technique that underlies most of modern nuclear medicine. Born into an ennobled Hungarian-Jewish family in Budapest, he fled Germany in 1934 as the Nazis rose and fled Denmark in 1943 ahead of the German occupation, famously dissolving two colleagues' Nobel medals in acid rather than let them be seized, then recovering the gold after the war. Nobel Prize in Chemistry, 1943.