Cesium also spelled Caesium, It is a chemical element with the symbol Cs and atomic number is 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C (83.3 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. It has physical and chemical properties similar to those of rubidium and potassium. The most reactive of all metals, it is pyrophoric and reacts with water even at −116 °C (−177 °F). It is the least electronegative element, with a value of 0.79 on the Pauling scale.
German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in 1860 by the newly developed method of flame spectroscopy. The first small-scale applications for caesium were as a “getter” in vacuum tubes and in photoelectric cells. In 1967, acting on Einstein’s proof that the speed of light is the most constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre.
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.
Physical Properties of Caesium
- Caesium is the softest element. It is a very ductile, pale metal, which darkens in the presence of trace amounts of oxygen.
- When in the presence of mineral oil, it loses its metallic lustre and takes on a duller, grey appearance.
- It has a melting point of 28.5 °C (83.3 °F), making it one of the few elemental metals that are liquid near room temperature. Mercury is the only elemental metal with a known melting point lower than caesium.
- In addition, the metal has a rather low boiling point, 641 °C (1,186 °F), the lowest of all metals other than mercury. Its compounds burn with a blue or violet colour.
- The golden colour of Cs comes from the decreasing frequency of light required to excite electrons of the alkali metals as the group is descended. For lithium through rubidium this frequency is in the ultraviolet, but for caesium it enters the blue–violet end of the spectrum; in other words, the plasmonic frequency of the alkali metals becomes lower from lithium to Cs.