All practice pages/Interatomic theory/Elements
Element 55, an alkali metal
A soft, pale gold metal that melts on a hot day, explodes in water, and defines the length of a second.
Period 6, group 1. The other alkali metals are tinted, and every cell opens its own article.
One valence electron, which cesium gives up easily to form the Cs⁺ ion. Like the other alkali metals, it reacts with water.
Rare, found mostly in the mineral pollucite. The largest known deposit is at Bernic Lake in Manitoba, Canada, with others in Zimbabwe and Australia.
Robert Bunsen and Gustav Kirchhoff found it in 1860 in mineral water from Dürkheim, Germany, as two bright blue lines in their spectroscope, making it the first element discovered by spectroscopy. They named it from the Latin caesius, sky blue.
The second is defined by cesium-133: exactly 9,192,631,770 cycles of the radiation that flips a cesium atom between two energy states, counted by cesium atomic clocks. Cesium formate brines are dense drilling fluids for deep oil and gas wells, cesium goes into photoelectric cells, and radioactive cesium-137 treats cancer and gauges thickness in industry.
Explodes on contact with water and catches fire in air. Radioactive cesium-137, released by the Chernobyl and Fukushima accidents, is a long-lasting contaminant.