For lithium through rubidium this frequency is in the ultraviolet, but for caesium it enters the blueвЂ“violet end of the spectrum in other words, the plasmonic frequency of the alkali metals becomes lower from lithium to caesium. The golden colour of caesium comes from the decreasing frequency of light required to excite electrons of the alkali metals as the group is descended. A few amalgams have been studied: CsHgДў is black with a purple metallic lustre, while CsHg is golden-coloured, also with a metallic lustre.

It mixes with all the other alkali metals (except lithium) the alloy with a molar distribution of 41% caesium, 47% potassium, and 12% sodium has the lowest melting point of any known metal alloy, at в€’78В вЊ (в€’108В вЏ). It forms well-defined intermetallic compounds with antimony, gallium, indium, and thorium, which are photosensitive. At temperatures below 650В вЊ (1,202В вЏ), it does not alloy with cobalt, iron, molybdenum, nickel, platinum, tantalum, or tungsten.

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.Д¬aesium crystals (golden) compared to rubidium crystals (silvery)Д¬aesium forms alloys with the other alkali metals, gold, and mercury ( amalgams). The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. 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. Since then, caesium has been widely used in highly accurate atomic clocks. 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. The first small-scale applications for caesium were as a ' getter' in vacuum tubes and in photoelectric cells. The German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in 1860 by the newly developed method of flame spectroscopy. Caesium-137, a fission product, is extracted from waste produced by nuclear reactors. The element has 40 known isotopes, making it, along with barium and mercury, one of the elements with the most isotopes. It has only one stable isotope, caesium-133. It is the least electronegative element, with a value of 0.79 on the Pauling scale. The most reactive of all metals, it is pyrophoric and reacts with water even at в€’116В вЊ (в€’177В вЏ). Caesium has physical and chemical properties similar to those of rubidium and potassium.

It is a soft, silvery-golden alkali metal with a melting point of 28.5В вЊ (83.3В вЏ), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium ( IUPAC spelling ) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55.

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