A second, the basic unit of time, is defined by caesium, which is also useful for dating things. Find out more about caesium's role as a timekeeper, in ep 16 of Elemental with Allan Blackman from AUT.

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 degrees celcius.

Go to this episode on rnz.co.nz for more details