Metals and Minerals
 
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Cs
Caesium

History

In the mid-19th century, most elements had been discovered and confirmed by actually extracting them from their compounds chemically. But, chemists were beginning to develop other techniques besides just reacting one chemical with another. Electrolysis was one such tool; English chemist Humphry Davy purified several elements by use of electricity. Another tool was the spectroscope.

A spectroscope analyzes the colors with which things burn or glow. Copper is green, sodium is yellow, and so on. Around 1850, German chemists Robert Bunsen and Gustav Kirchhoff began analyzing these color spectra systematically. While burning various known elements in the now-familiar Bunsen burner, the two scientists established the flame signatures of such substances as lithium, sodium, and potassium.

Bad Duerkheim, GermanyBunsen and Kirchhoff received water from the mineral springs at Dürkheim, Germany (right). After removing the known calcium, strontium, and other elements, they observed that the mineral residue emitted a pair of unknown blue spectral colors when burned in Bunsen's colorless flame. From 44,000 liters of mineral water, the scientists produced a few grams of the new element. They named it caesium from the Latin caesius for sky-blue. (The spelling cesium is also often seen.)

Mining and Production

Caesium is not very abundant in the earth, but it is a very large atom that tends to separate and concentrate in crystalline mineral formations. The commercially interesting mineral is pollucite, a caesium aluminum silicate rock. It is not mined separately, but is extracted as a byproduct of lithium mining from the the closely-associated mineral lepidolite. (Rubidium is also derived from this mining.) Caesium is mined in Canada, Zimbabwe, and Namibia, though the actual small amounts are not reported.

The processing of caesium minerals is very straightforward because of the element's distinctive chemistry. The usual product is caesium chloride (CsCl), which binds more tightly than any of the related chlorides of lithium, sodium, or potassium. There is not much call commercially for the pure metal, but it can be had by electrolysis or further chemistry.

Properties and Uses

Caesium is a very soft and ductile metal. It is highly reactive, with properties similar to but more extreme than the other metals above it in the periodic table. Caesium burns in air and explodes in water. It forms salts and other compounds with many other elements.
The large atoms of caesium separate easily. Besides being soft, the element has a very low melting point, near room temperature. In this it resembles gallium or even mercury.

A caesium compound, caesium formate, is used as a drilling mud in oil wells. This is caesium's main commercial use. The compound is heavy and inert, which is ideal for preventing blowouts while not otherwise reacting.

master atomic clock array, US Naval ObservatoryThe length of the second is officially defined in terms of an oscillation frequency (hyperfine transitions) within the nucleus of a caesim-133 atom. Therefore, atomic clocks use caesium as their reference standard. Modern atomic clocks are accurate to better than 1 second in 1 million years.

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