Cesium is the fifth member of the alkali family, the elements that make up Group 1 of the periodic table. Its atomic number is 55, its atomic mass is 132.9054, and its chemical symbol is Cs.

Properties

Cesium is a silvery white, shiny metal that is very soft and ductile. Its melting point is so low (83.3°F or 28.5°C) that it melts easily in the heat of a person's hand. Its boiling point is 1,301°F (705°C) and its density is 1.90 grams per cubic centimeter.

Cesium is a very active metal, one of the most active of all elements. It reacts violent with oxygen in the air and with water. So much energy is released in the latter reaction that the hydrogen formed as a product ignites. Cesium is normally stored under kerosene as a safety precaution to prevent it from reacting with oxygen or water vapor in the air. Cesium also reacts vigorously with acids, the halogens, sulfur, and phosphorus.

Occurrence and Extraction

Cesium is moderately abundant in the Earth's crust with an estimated abundance of about 1-3 parts per million. It is found in an ore of lithium called lepidolite but, most abundantly, in the ore known as pollucite (Cs₄Al₄Si₉O₂₆).

Discovery and Naming

Cesium was discovered in 1861 by two German chemists, Robert Bunsen and Gustav Kirchoff. The two had been instrumental in developing the new analytical technique of spectroscopy, and they found spectral lines for a previously unidentified element while studying a sample of spring water. They suggested the name cesium, from the Latin word caesius meaning "sky blue" because blue is the color of the element's spectral lines.

Uses

Cesium and its compounds have relatively few commercial uses. It is used in photovoltaic cells that convert the energy of sunlight into electricity. It is also used in the cesium clock, a device that measures time by means of the wavelength of light given off by one of the elements isotopes, cesium- 133.