Polonium

Polonium sits in group 16 below sulfur, selenium and tellurium, and is usually classed as a metal or metalloid. Like its group-mates it forms compounds in the +2 and +4 states, such as polonium dioxide, but its chemistry is hard to study because every isotope is radioactive. The common isotope, polonium-210, has a half-life of 138 days and releases so much alpha energy that a gram sample heats itself to around 500 °C and glows blue from ionised air around it.

Polonium is the only element known to crystallise in a simple cubic structure, with an atom at each corner of a cube and nothing else. Alpha particles from polonium travel only a few centimetres in air and are stopped by paper or skin, so the metal is relatively safe outside the body. Inside the body it is among the most toxic substances known, because every decay deposits concentrated energy into nearby cells. Microgram amounts can be lethal.

Discovery

Marie and Pierre Curie discovered polonium in July 1898 while processing tonnes of pitchblende from Joachimsthal in Bohemia. They noticed that a bismuth-containing fraction was far more radioactive than uranium could explain. Marie named the new element after her homeland, Poland, then partitioned between Russia, Prussia and Austria, hoping to draw attention to its cause. It was the first element discovered through its radioactivity.