A second, the basic unit of time, is defined by caesium. And an isotope of caesium, produced in atmospheric nuclear tests, means it is also useful for dating things.
Caesium has the chemical symbol Cs, the atomic number 55 and its name comes from the Latin word caesius, meaning sky blue.
Professor Allan Blackman from AUT says that caesium (which Americans spell cesium) is an alkali metal and is the most reactive of all metals, exploding when added to water. It is also pyrophoric and ignites spontaneously in air when the temperature is below 55°C.
The Elemental podcast is celebrating 150 years since the periodic table was first published by Russian chemist Dmitri Mendeleev.
Find out more about events during the United Nation’s International Year of the Periodic Table.
Nights with Bryan Crump is also celebrating the chemical elements during their Friday night Sonic Tonic and Element of the Week.
Professor Allan Blackman is at Auckland University of Technology.