Cesium

Cesium is a soft, pale gold metal that melts at about 28 °C and explodes on contact with water. Robert Bunsen and Gustav Kirchhoff discovered it in 1860, the first element found with a spectroscope; its name comes from the Latin for sky blue, after the blue lines in its light. It is mined mainly from the mineral pollucite.

Cesium's best-known job is keeping time. Since 1967, the second has been defined by the natural frequency at which cesium-133 atoms flip between two energy states: exactly 9,192,631,770 cycles. Cesium salts are also used in heavy drilling fluids for oil and gas wells.