caesium(redirected from Element 55)
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Related to Element 55: Element 66
a ductile silvery-white element of the alkali metal group that is the most electropositive metal. It occurs in pollucite and lepidolite and is used in photocells. The radioisotope caesium-137, with a half-life of 30.2 years, is used in radiotherapy. Symbol: Cs; atomic no.: 55; atomic wt.: 132.90543; valency: 1; relative density: 1.873; melting pt.: 28.39±0.01°C; boiling pt.: 671°C