Caesium is a chemical element, having the chemical symbol Cs. Its atomic number (the number of protons) is 55. It has a standard atomic weight of 132.90545 g•mol−1. At the normal atmospheric pressure of 1 atm, it is a liquid at temperatures above 28.5 °C.
Caesium is considered a member of the "Alkali metal" class of element. At a pressure of 101.325 kPa, it has a boiling point of 671 °C, and a melting point of 28.5 °C. In its elemental form, Rubidium is a solid at the normal atmospheric pressure of 1 atm and typical room temperatures of 15 − 25 °C.
137Cs is used widely in industry and medicine, but is also a common contaminant from nuclear reactor accidents.