Cesium (Cs)

A second is 9,192,631,770 oscillations of the radiation emitted when a cesium-133 atom flips between the two hyperfine levels of its ground state. Not approximately — exactly, by definition, since 1967. Every clock, every timestamp, every satellite navigation fix and every financial trade timestamp in the world is ultimately traceable to that number and to this element.

Why the loosest electron in the table ended up defining time

Cesium's single outer electron sits further from its nucleus than the outer electron of any other stable element, and is held more weakly: cesium has the lowest first ionisation energy ever measured, and the largest atomic radius anyone has ever actually measured.

That last qualifier is doing real work, because the card for francium one row below carries 348 pm against cesium's 343. Nobody has ever assembled a weighable speck of francium, so its radius is a figure read off the trend rather than off a diffraction pattern. Among elements whose size has been determined by experiment, cesium is the largest, and it is likely to stay that way. Francium is also expected to be slightly less eager to give up its electron than cesium, not more, because relativity draws its outermost orbital inward — so the group trend that makes cesium extreme quietly stops working immediately below it.

The property that mattered for metrology is a consequence of that loose electron. The magnetic interaction between it and the nucleus splits the ground state into two levels separated by a microwave-frequency gap, and that gap is well isolated, unusually insensitive to stray fields, and falls at a frequency that 1950s electronics could generate and count. Cesium also has exactly one natural isotope, so there is no mixture to average over, and it is easy to produce as an atomic beam.

The first cesium standard was built at the National Physical Laboratory in 1955. Within twelve years it had displaced the rotation of the Earth as the basis of the second, which is why leap seconds exist: the planet is a worse clock than the atom, and the two have to be reconciled occasionally.

Modern cesium fountain clocks toss a cloud of laser-cooled atoms upward and interrogate them twice, on the way up and on the way down, extending the measurement time to about a second and reaching uncertainties around one part in 10^16 — a drift of less than a second over the age of the universe. Optical clocks based on other elements are now more accurate still, and a redefinition is under discussion, but for the moment cesium remains the anchor of the SI second and, through it, of the metre, the ampere, the kelvin and the candela.

The first element nobody had to see

Robert Bunsen and Gustav Kirchhoff's spectroscope was new in 1859, and its first triumph was Kirchhoff's demonstration that the dark lines in the solar spectrum matched the bright lines of terrestrial elements — chemistry, performed on the Sun.

In 1860 they turned the same instrument on the residues from Bad Dürkheim mineral water and saw two brilliant blue lines that belonged to no known substance. That was the entire evidence. They had no sample of the element, no weight, no compound, no reaction; they had two lines, and they announced a new element on the strength of them. It was the first element ever discovered spectroscopically, and it changed what counted as evidence in chemistry — within a few years the same method had produced rubidium, thallium, indium and helium, the last of these found in the Sun before it was found on Earth.

The name is Latin caesius, meaning a bluish grey, used by classical authors of the sky and of pale eyes. As with several elements of this period, it describes the light, not the metal, which is a soft, pale gold.

Caesium or cesium

The element is spelled two ways and both are correct in their own jurisdiction. IUPAC's official spelling is caesium, following the Latin diphthong, and that is standard in Britain, Europe and most of the Commonwealth. American usage dropped the a, as it did in encyclopaedia and mediaeval, giving cesium, which is what the American Chemical Society and US standards bodies use. Aluminum and sulfur are the other two elements where the two systems genuinely diverge, and in each case IUPAC ruled once and neither side entirely complied.

The heaviest liquid that is not a slurry

Cesium's commercial tonnage goes almost entirely down oil and gas wells. A deep well needs a drilling fluid dense enough to balance the pressure of the formation it is cutting through, and the conventional route to that density is to suspend barite particles in the mud — solids that scour equipment, plug the pores of the reservoir rock and settle out whenever circulation stops.

Cesium formate solution is a true solution with no solids at all, and it reaches a specific gravity of about 2.3 — denser than most rock. It is also thermally stable, non-corrosive and biodegradable. The catch is the price, which is high enough that the fluid is treated as an asset rather than a consumable: it is pumped out at the end of the job, cleaned and returned for reuse, under contracts written around its recovery.

The other established uses turn on the low work function of the metal's surface. A cesium coating makes it easy for light to eject electrons, which is why cesium-antimonide and multi-alkali photocathodes sit inside photomultiplier tubes, image intensifiers and night vision devices. Cesium was also the propellant in the first generation of experimental ion thrusters, before it was abandoned in favour of inert gases that do not condense on and corrode the spacecraft.

Goiânia, 1987

Cesium-137 comes out of fission with a half-life of just over 30 years, and it is the isotope that determines how long land stays restricted after a nuclear release. It dominated the long-term contamination from both Chernobyl and Fukushima, because its chemistry resembles potassium closely enough that plants and animals take it up, and its half-life is long enough to persist and short enough to be intensely radioactive.

The worst civil accident involving it happened without a reactor. In September 1987 two scavengers entered a derelict radiotherapy clinic in Goiânia, in central Brazil, and removed the source assembly from an abandoned teletherapy machine. They sold it to a scrapyard, where it was broken open. Inside was cesium chloride as a fine powder that glowed blue in the dark.

It was distributed among family, neighbours and friends as a curiosity. Four people died, 249 were found to be contaminated, more than a hundred thousand were monitored, and several houses were demolished and topsoil removed. The IAEA classifies it as one of the most serious radiological accidents on record, and it is the reason for much of the international framework governing orphan sources and scrap metal monitoring.

Eighty per cent of the world's pollucite

Cesium's mineral is pollucite, a cesium aluminum silicate found in a small number of complex granitic pegmatites. One deposit dominates everything: the Tanco mine at Bernic Lake in Manitoba, which is reported to hold on the order of 80% of known world reserves, alongside the tantalum, lithium and rubidium in the same body.

That concentration is extreme even by the standards of the minor metals. World production runs to a few tens of tonnes annually, and effectively the whole of it comes out of that single orebody, now under Chinese ownership, in a jurisdiction where the operator has sought permission to drain part of a lake in order to reach more of it. For an element on which the definition of the second depends, the supply chain is remarkably slender — though the quantity metrology actually consumes would fit in a suitcase.