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Cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (83 °F), which makes it one of only five metals that are liquid at or near room temperature. Cesium has physical and chemical properties similar to those of rubidium and potassium. The metal is extremely reactive and pyrophoric, reacting with water even at −116 °C. It is the least electronegative element that has stable isotopes, of which it has only one, cesium-133. This is mined mostly from pollucite, while the radioisotopes, especially cesium-137, are extracted from waste produced by nuclear reactors.

The two German chemists Robert Bunsen and Gustav Kirchhoff discovered it in 1860 by the newly developed method of flame spectroscopy. The first small-scale applications for cesium have been as "getter" in vacuum tubes and in photoelectric cells. In 1967, a frequency of cesium-133 was used to define the second by the International System of Units. Since then it has been widely used in atomic clocks. Since the 1990s, the largest application of the element has been as cesium formate for drilling fluids. It has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope cesium-137, with a half-life of about 30 years, is used in medical applications, industrial gauges, and hydrology. While the element has a mild chemical toxicity, the radioisotopes present a high health risk in case of radiation leaks and has been named a hazardous material.


Cesium forms alloys with the other alkali metals as well as with gold, and amalgams with mercury. At temperatures below 650 °C, it alloys with cobalt, iron, molybdenum, nickel, platinum, tantalum or tungsten. On the other hand, it is known to form intermetallic compounds with antimony, gallium, indium and thorium, which are known to be photosensitive.

Isolated cesium is extremely reactive and very pyrophoric. In addition to igniting spontaneously in air, it reacts explosively with water (even cold), even more so than the other members of the first group of the periodic table. The reaction with solid water occurs even at temperatures as low as −116 °C. Because of its high reactivity, the metal is classified as a hazardous material. It is stored and shipped in dry mineral oil or in other dry saturated hydrocarbons or in an inert atmosphere [such as argon or nitrogen] or vacuum in sealed borosilicate glass ampoules. In quantities of more than about 100 grams, cesium is shipped in hermetically sealed stainless steel containers. When glass ampoules are used, they are shipped wrapped in foil and packed in an inert cushioning material, such as vermiculite, each in a metal can.

The chemistry of cesium is very similar to that of other alkali metals, and is particularly closely associated to that of rubidium, the element above cesium in the periodic table. Some small differences arise from the fact that it has a higher atomic mass and is more electropositive than other (non-radioactive) alkali metals. Cesium is the most electropositive stable chemical element.


The base value of each unit of ranges between 2 and 15Ð per unit, with up to 5 units being found at any one time.

Presence on Mars: Rare

Martian Minerals
Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6
Group 3 |Antimony | Astatine | Barium | Bismuth | Cesium | Francium | Hafnium | Indium | Iodine | Iridium | Lanthanum | Lead | Mercury | |Osmium | Platinum | Polonium | Radium | Radon | Rhenium | Tantalum | Tellurium | Thallium | Tin | Tungsten | Xenon|