Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft silvery-gold alkali metal with a melting point of 28 °C (83 °F), which makes it one of the metals that are liquid at or near room temperature, along with rubidium (39 °C [102 °F]), francium (27 °C [81 °F]), mercury (−39 °C [−38 °F]), and gallium (30 °C [86 °F]). This element is most notably used in atomic clocks.
The variant spelling cesium is used especially in North American English, but caesium is the spelling used by the IUPAC.
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