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.
Cesium has 55 electrons, one for every proton, so the atom carries no charge. Below are the same 55 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 6s¹
Short form: [Xe] 6s¹, where [Xe] stands for the 54 electrons xenon already has.
The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 3 here is 3s² 3p⁶ 3d¹⁰, and 2 + 6 + 10 is 18.
Cesium sits in period 6 and has 6 shells. The period number counts the shells.
The outer shell holds 1 electron. Those are the ones cesium can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 1 means 1.
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.