Element · Cs

Caesium

The element that defines the second, and the softest metal on the periodic table.

Caesium is a soft golden metal that melts just above room temperature and reacts explosively with water. It is the most electropositive stable element, giving up its outer electron more readily than any other, which is what makes it useful and what makes it dangerous to handle.

Its defining role is metrological. Since 1967 the second has been defined by the frequency of a specific transition in caesium-133, and every atomic clock that keeps international time is calibrated against it. Time itself is measured in caesium.

Uses

Atomic clocks are the use that matters most, underpinning satellite navigation, telecommunications synchronisation and the timestamps that financial and computing systems depend on.

The bulk industrial use is quite different: caesium formate brine is a dense, non-damaging drilling fluid for high-pressure oil and gas wells. Caesium's low ionisation energy also makes it a photocathode material in photomultiplier tubes and night vision devices, and caesium-137 — a fission product — is used in radiotherapy and industrial gauges.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is used as

Electrical conduction — application · as a photocathode, exploiting the lowest ionisation energy of any stable element
Timekeeping — application · the second has been defined by a caesium-133 transition since 1967

is extracted from

Pollucite — mineral · essentially the only commercial source Wikidata

is produced by

Metallothermic reduction — process · calcium or barium reducing caesium chloride under vacuum — caesium is too reactive to survive most alternatives

is found in

Pollucite — mineral · substitutes within a solid solution — an individual specimen may hold little of it Wikidata

Sources

Material World
Our own writing
Wikidata
Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
PubChem
US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
NIST — National Institute of Standards and Technology
United States Department of Commerce · US Government work — public information, credit requested

Questions this page answers

Where it comes from, and what it becomes

Follow Caesium back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

Caesium → is produced by (calcium or barium reducing caesium chloride under vacuum — caesium is too reactive to survive most alternatives) → Metallothermic reduction → takes as input (the cheapest reducing metal and the commonest, and the aluminium in thermite) → Aluminium → is extracted from (via alumina — bauxite is refined by the Bayer process before smelting) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is composed of (an early-crystallising constituent of basaltic magma) → Olivine
Caesium → is extracted from (essentially the only commercial source) → Pollucite
Caesium → is produced by (calcium or barium reducing caesium chloride under vacuum — caesium is too reactive to survive most alternatives) → Metallothermic reduction → takes as input (reserved for halides and the most stubborn oxides, where the cheaper metals will not do the job) → Calcium → is produced by (from molten calcium chloride; there is no smelting route) → Molten salt electrolysis → takes as input (after concentration, as one of the two routes to lithium chloride) → Lepidolite
Caesium → is produced by (calcium or barium reducing caesium chloride under vacuum — caesium is too reactive to survive most alternatives) → Metallothermic reduction → takes as input (the cheapest reducing metal and the commonest, and the aluminium in thermite) → Aluminium → is produced by (collects at the cathode while the carbon anodes are consumed) → Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite
Caesium → is produced by (calcium or barium reducing caesium chloride under vacuum — caesium is too reactive to survive most alternatives) → Metallothermic reduction → takes as input (the cheapest reducing metal and the commonest, and the aluminium in thermite) → Aluminium → is extracted from (via alumina — bauxite is refined by the Bayer process before smelting) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is composed of (calcium-rich plagioclase is a defining constituent of basalt) → Plagioclase
Caesium → is produced by (calcium or barium reducing caesium chloride under vacuum — caesium is too reactive to survive most alternatives) → Metallothermic reduction → takes as input (the cheapest reducing metal and the commonest, and the aluminium in thermite) → Aluminium → is extracted from (via alumina — bauxite is refined by the Bayer process before smelting) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is sourced from (partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt) → Peridotite

These are the most distinct paths back. Caesium can be traced through others besides.

Downstream — what it becomes

Caesium → is used as (as a photocathode, exploiting the lowest ionisation energy of any stable element) → Electrical conduction
Caesium → is used as (the second has been defined by a caesium-133 transition since 1967) → Timekeeping