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Caesium is a chemical element with symbol Cs and atomic number 55.  It is silvery gold, soft, and ductile. It is the most electropositive and most alkaline element. Caesium, gallium, and mercury are the only three metals that are liquid at or around room temperature. Caesium reacts explosively with cold water and reacts with ice at temperatures above -116°C. Caesium hydroxide is a strong base and attacks glass. Caesium reacts with the halogens to form a fluoride, chloride, bromide, and iodide. Caesium metal oxidises rapidly when exposed to the air and can form the dangerous superoxide on its surface.  Caesium is a naturally-occurring element found in rocks, soil, and dust at low concentrations. Natural caesium is present in the environment in only one stable form, as the isotope 133Cs.