Caesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 Â°C (82 Â°F), which makes it one of only five elemental metals that are liquid at or near room temperature.aesium is an alkali metal and has physical and chemical properties similar to those of rubidium and potassium. The metal is extremely reactive and pyrophoric, reacting with water even at â116 Â°C (â177 Â°F).