Element 55 · Alkali Metal
Caesium
Caesium powers the world's most accurate atomic clocks — defining the international standard second.
Element Facts
SYMBOL
Cs
ATOMIC NO.
55
ATOMIC WEIGHT
132.91
CATEGORY
Alkali Metal
PERIOD
Period 6
GROUP
Group 1
Origins
Caesium was discovered spectroscopically in 1860 by Robert Bunsen and Gustav Kirchhoff — the first element to be discovered using the newly invented spectroscope. Named for its brilliant blue spectral lines (Latin “caesius” = sky blue). The most important milestone in caesium technology came in 1955 when Louis Essen and Jack Parry built the first caesium atomic clock at the National Physical Laboratory in Teddington — effectively redefining the second in terms of caesium-133 microwave transitions, which remains the international standard today.
Key Properties
Caesium is element 55, the most reactive alkali metal and one of only five metals liquid at near-room temperature (melts at 28.4°C). Named from Latin "caesius" (sky-blue) for its spectral lines.
Modern Applications
Caesium atomic clocks define the SI second internationally. Specialist drilling fluids in oil exploration (caesium formate brines). Photoelectric cells. Ion propulsion in some satellite engines.
At the Yard
Why QuickStop Metals doesn’t buy Caesium:
Pure caesium is extraordinarily reactive — explodes on contact with water and ignites in air. Handled only by specialist chemical and research operations. No scrap market.
Market Value
Caesium compounds trade at £40,000–80,000/kg for high-purity material. Produced mainly from pollucite ore in Manitoba, Canada. Not a standard scrap metal.