Though he published only eight scientific papers in a career spanning just four years, British physicist Harry Moseley profoundly changed our understanding of matter. As a 25-year-old graduate student in Ernest Rutherford’s laboratory in Manchester, Moseley used newly discovered X-rays to redefine the Periodic Table, showing it was organized by atomic number — the number of protons in an atom’s nucleus — rather than by atomic weight, as chemists had long believed. Each element in the table has one more proton than the element before it. For this work he was nominated for the Nobel Prize, but when World War I broke out, Moseley enlisted and was killed by a sniper’s bullet. His shocking death forever changed the way Britain and other countries thought about the role of scientists in war.