Cesium

Cesium, or caesium, was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff, the first element ever found by spectroscopy, detected through two bright blue lines in the spectrum of a mineral water. They named it from the Latin caesius, sky-blue, after those lines. Cesium is a soft, golden-tinged alkali metal, liquid near room temperature and violently reactive, whose most famous role is in the atomic clock. Besides defining the second, caesium is used in the photocells and drilling fluids of industry and in ion propulsion, and its extreme reactivity and single loosely held electron make it a favourite working material of precision physics.

Caesium's reactivity is so extreme that the pure metal reacts violently even with ice at low temperatures, and it must be shipped and stored in sealed glass ampoules under vacuum or inert gas rather than handled as an ordinary laboratory reagent. The caesium atomic clock's extraordinary precision, accurate to better than a second over tens of millions of years in the best laboratory standards, underlies the exact timing signals that make GPS satellite navigation and international financial and telecommunications networks function, so that a metal almost too dangerous to touch quietly keeps the clocks that keep the modern world synchronised.

Cesium opened the age of spectroscopic discovery: Bunsen and Kirchhoff's new method let them see, in the coloured lines of a flame, the presence of an element too dilute to isolate by ordinary chemistry, and the sky-blue lines that named caesium were the first such revelation, followed within months by rubidium. The element later took on a deeper significance for measurement itself. Since 1967 the international definition of the second has been fixed by the frequency of a particular transition in the cesium-133 atom, so that the fundamental unit of time is now counted out by the vibrations of this blue-lined metal. Cesium has no ancient symbolism; its dignity is that of precision, naming the very beat of modern time.

Caesium also carries a harder modern reputation: the radioactive isotope caesium-137, a common fission product, was the dominant long-lived contaminant released at both the 1986 Chernobyl and 2011 Fukushima nuclear accidents, its roughly thirty-year half-life and chemical similarity to potassium letting it enter soil, crops and the food chain for decades afterward, making caesium monitoring a standard part of nuclear accident response and land remediation ever since. Most of the world's commercial caesium is refined from pollucite, a caesium aluminosilicate mineral, with the single largest known deposit at Bernic Lake in Manitoba, Canada, supplying much of the metal used in drilling fluids and atomic clocks alike.