Cæsium is a soft, silvery-gold alkali metal with the symbol Cs and atomic number 55.


It has a melting point of 28°C (82°F), which means it will be liquid on a warm summer day, and revert to a solid later that night after the ambient temperature cools. Cæsium is just one of five elemental metals that are liquids at or near room temperature.


Its name comes from the Latin word for sky-blue because when burned, cæsium turns the flame a lovely blue colour.

Since the 1990s, the largest application of the element has been as caesium formate for drilling fluids, but it has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Nonradioactive caesium compounds are only mildly toxic, but the pure metal’s tendency to react explosively with water means that caesium is considered a hazardous material, and the radioisotopes present a significant health and ecological hazard in the environment.

Caesium is also know for its use in atomic clocks and use the electromagnetic transitions in the hyperfine structure of caesium-133 atoms as a reference point. The first accurate caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK.

These clocks measure frequency with an error of 2 to 3 parts in 1014, which corresponding to an accuracy of 2 nanoseconds per day, or one second in 1.4 million years. The latest versions are more accurate than 1 part in 1015, about 1 second in 20 million years.  The Caesium standard is the primary standard for standards-compliant time and frequency measurements. Caesium clocks regulate the timing of cell phone networks and the Internet.

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