Cesium

Cesium

Caesium or cesium[note 1] is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at (or near) room temperature.[note 2] Caesium is an alkali metal and 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 (−177 °F). It is the least electronegative element having a stable isotope, caesium-133. Caesium is mined mostly from pollucite, while the radioisotopes, especially caesium-137, a fission product, are extracted from waste produced by nuclear reactors.

Two German chemists, Robert Bunsen and 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, a specific frequency from the emission spectrum of caesium-133 was chosen to be used in the definition of the second by the International System of Units. Since then, caesium has been widely used in atomic clocks.

Since the 1990s, the largest application of the element has been as caesium formate for drilling fluids. 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. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk if released into the environment.

Occurrence


Caesium is a relatively rare element as it is estimated to average approximately 3 parts per million in the Earth's crust. This makes it the 45th most abundant of all elements and the 36th of all the metals. Nevertheless, it is more abundant than such elements as antimony, cadmium, tin and tungsten, and two orders of magnitude more abundant than mercury or silver, but 3.3% as abundant as rubidium—with which it is so closely chemically associated.

Due to its large ionic radius, caesium is one of the "incompatible elements". During magma crystallization, caesium is concentrated in the liquid phase and crystallizes last. Therefore, the largest deposits of caesium are zone pegmatite ore bodies formed by this enrichment process. Because caesium does not substitute for potassium as readily as does rubidium, the alkali evaporite minerals sylvite (KCl) and carnallite (KMgCl3·6H2O) may contain only 0.002% caesium. Consequently, Cs is found in few minerals. Percentage amounts of caesium may be found in beryl (Be3Al2(SiO3)6) and avogadrite ((K,Cs)BF4), up to 15 wt% Cs2O in the closely related mineral pezzottaite (Cs(Be2Li)Al2Si6O18), up to 8.4 wt% Cs2O in the rare mineral londonite ((Cs,K)Al4Be4(B,Be)12O28), and less in the more widespread rhodizite. The only economically important source mineral for caesium is pollucite Cs(AlSi2O6), which is found in a few places around the world in zoned pegmatites, and is associated with the more commercially important lithium minerals lepidolite and petalite. Within the pegmatites, the large grain size and the strong separation of the minerals create high-grade ore for mining.

One of the world's most significant and richest sources of the metal is the Tanco mine at Bernic Lake in Manitoba, Canada. The deposits there are estimated to contain 350,000 metric tons of pollucite ore, which represent more than two-thirds of the world's reserve base. Although the stoichiometric content of caesium in pollucite is 42.6%, pure pollucite samples from this deposit contain only about 34% caesium, while the average content is 24 wt%. Commercial pollucite contains over 19% caesium. The Bikita pegmatite deposit in Zimbabwe is mined for its petalite, but it also contains a significant amount of pollucite. Notable amounts of pollucite are also mined in the Karibib Desert, Namibia. At the present rate of world mine production of 5 to 10 metric tons per year, reserves will last for thousands of years.

Symbol		Cs
Atomic Number		55
Atomic Weight		132.90543
Oxidation States		+1
Electronegativity, Pauling		0.79
State at RT		solid
Melting Point, K		301.6
Boiling Point, K		951.6

Appearance and Characteristics

Harmful effects:

Cesium must be kept under an inert liquid/gas or in a vacuum to protect it from air and water. Cesium compounds are considered to be mildly toxic.

Characteristics:

• Cesium is silvery-gold, soft, ductile alkali metal. It is liquid in a warm room, melting at 28.4 oC (83.1 oF).
• Cesium is one of the few metals that is liquid near room temperature. The others are gallium, francium and mercury.
• Cesium is an extremely reactive metal and the most alkaline of the elements. It reacts explosively upon contact with water producing cesium hydroxide (CsOH), an extremely strong base that can rapidly corrode glass.

Uses of Cesium

• Cesium is used in atomic clocks, which are incredibly accurate. NIST-F1, America’s primary time and frequency standard, is a cesium fountain atomic clock developed at the NIST laboratories in Boulder, Colorado. NIST-F1 contributes to the international group of atomic clocks that define Coordinated Universal Time (UTC), the official world time. As scientists continue to improve its technology, uncertainty in NIST-F1′s measurement of time is continually improving. Currently it neither gains nor loses as much a second in more than 60 million years – but see strontium.
• Cesium is also used in photoelectric cells and as a catalyst in the hydrogenation of organic compounds. The metal is used as a ‘getter’ in vacuum tubes.
• Cesium hydroxide is used to etch silicon.