55Cs Caesium

Year Discovered


Discovered By

Robert Bunsen and Gustov Kirchhoff of Germany

Biological Rating

Not necessary for life.


Named from the Latin word meaning sky blue (caesium’s spectra contains two prominent blue lines), caesium is a shiny silvery metal with a gold tint. Outside of the USA, it is spelled caesium. Caesium is the softest of all metals, about as soft as wax and easily cut with a knife. It is also the most active metal, reacting strongly with air and explosively with water. Caesium will even react with ice, and caesium hydroxide is the strongest base known. It has a very low melting point, and would melt if held in one’s hand. Caesium and caesium compounds are used as catalysts and as scavenger metals. It is also used in photoelectric cells, infrared lamps, special glass, and radiation monitoring equipment. Caesium is used in atomic clocks, because of the vibration frequency of the caesium atom. Caesium clocks are accurate to five seconds every 300 years. Caesium, similar to rubidium, has potential for use as a fuel in ion engines. Just burning the metal is a very effective fuel in space (it cannot be used in the atmosphere) being 140 times more efficient than any other known fuel. Caesium replaces potassium in the body, but it has none of potassium’s beneficial characteristics. It causes a poisonous reaction in relatively small doses.

Biological Benefits

Caesium has no known biological use.

Role in Life Processes

No known benefits for life processes in plants and animals.

Percentage Amount in the Human Body: 0.000009%


Caesium is present in trace amounts in a few minerals. It is obtained chiefly from pollucite, although it also is found in lepidolite as an impurity. It is mined primarily in Canada as well as Zimbabwe.