Caesium is a soft metal which is often liquid at room temperature due to its relatively low melting point (28.5C). Cesium is a member of the alkali group of metals. It is one of three metals that occur as a liquid at room temperature, the others being mercury and gallium. It is an extremely reactive metal, reacting violently in contact with water and being rapidly attacked in air.As with other alkaline group metals, caesium can be prepared by electrolysis of the fused halides but, in addition, it can also be prepared by heating the chloride with calcium and distilling out the molten metal. It is not a common element, its abundancy on Earth being 7 ppm. As a photosensitor, caesium has a peak response at 800nm in the infrared, both thermal- and photo-emission being high.  Alloys of caesium with antimony, gallium, indium and thorium are generally photosensitive. Caesium is also used in atomic clocks, a standard measure of time, based on the caesium ion resonance of 9,192,631,770 Hz. Elemental CesiumCesium's main commercial source is pollucite ore; however, it is also found in beryl, avogadrite, pezzottaite, and londonite. In its elemental form, cesium has a silvery gold appearance.