Caesium (also spelled cesium, IPA: /ˈsiːziəm/) is a chemical element in the periodic table that has 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), francium (27 °C), mercury (-39 °C), and gallium (30 °C). This element is most notably used in atomic clocks.
The variant spelling cesium is sometimes used, especially in North American English, but caesium is the spelling used by the IUPAC, although since 1993 it has recognized cesium as a variant as well.