Caesium was used as a propellant in early ion engines.

This metal is also used in photoelectric cells due to its ready emission of electrons.

Like other elements of group 1, caesium has a great affinity for oxygen and is used as a 'getter' in vacuum tubes.

135Cs has also been used in this function. This isotope is used because, while it is less prevalent than either 133Cs or 137Cs, 134Cs can be produced solely by nuclear reactions. 134Cs has been used in hydrology as a measure of caesium output by the nuclear power industry.SI defines the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two hyperfine energy levels of the ground state of the 133Cs atom. Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium. Ĭaesium is also notably used in atomic clocks, which are accurate to seconds in many thousands of years. The high density of the caesium formate brine (up to 2.3 sg), coupled with the relative benignity of 133Cs, reduces the requirement for toxic high-density suspended solids in the drilling fluid, which is a significant technological, engineering and environmental advantage. Probably the most widespread use of caesium today is in caesium formate-based drilling fluids for the oil industry.

It is possible that, after the salt Cs +F − has formed, the Cs + ion, which has the same electronic structure as elemental xenon, can, like xenon, be oxidised further by fluorine and form traces of a higher fluoride such as CsF 3, analogous to XeF 2. There is an account that caesium, reacting with fluorine, takes up more fluorine than it stoichiometrically should. CsOH is often stated to be the 'strongest base', but in fact many compounds such as n-butyllithium and sodium amide are stronger. Caesium reacts explosively in cold water and also reacts with ice at temperatures above −116 ☌ (−177 ☏, 157 K).Ĭaesium hydroxide (CsOH) is a very strong base and will rapidly etch the surface of glass. (Technically, francium is the least common alkali metal, but since it is highly radioactive with an estimated 30 grams in the entire Earth's crust at one time, its abundance can be considered zero in practical terms.)Īlong with gallium, francium, and mercury, caesium is among the only metals that are liquid at or near room temperature. Caesium is the least abundant of the five non-radioactive alkali metals. Caesium is the second most electropositive and alkaline of the chemical elements and has the second lowest ionization potential (after francium). This metal is silvery gold in color and is both soft and ductile. The emission spectrum of caesium has two bright lines in the blue part of the spectrum along with several other lines in the red, yellow, and green. The variant spelling cesium is used especially in North American English, and the IUPAC has recognized it as a variant spelling since 1993, but caesium is the spelling used by the IUPAC. This element is most notably used in atomic clocks. It is a soft silvery-gold alkali metal with a melting point of 28 ☌ (83 ☏), which makes it one of the metals that are liquid at or near room temperature, along with rubidium ( 39 ☌ ), francium ( 27 ☌ ), mercury ( −39 ☌ ), and gallium ( 30 ☌ ). Caesium or cesium ( pronounced /ˈsiːziəm/) is the chemical element with the symbol Cs and atomic number 55.