Polonium

Polonium was the first element the Curies found, in July 1898, teased out of pitchblende residues months before radium, and Marie named it for Poland — then partitioned out of existence — as a deliberate political statement. Polonium-210 emits alpha particles at a prodigious rate: a single gram generates 140 watts of heat and glows blue from the air it excites, which made it a heat source for early lunar rovers and a neutron initiator in the first fission weapons. Harmless outside the body and catastrophic inside it, it was used to murder Alexander Litvinenko in London in 2006, a case solved because the polonium trail was traceable through hotels and aircraft. Most tables colour it as a metalloid; it is in fact a genuine metallic conductor with a unique simple-cubic structure.