Caesium

Caesium (symbol: Cs) is a chemical element with atomic number 55. It belongs to the alkali metals group, which are the most reactive metals when compared to other groups. Caesium is a soft metal with a pale gold colour, and is one of the few elements that melts near room temperature (its melting point is approximately 28 degrees Celsius).

Because of its high reactivity, caesium is not found in nature in pure form. It reacts rapidly with water and air, and is therefore stored in mineral oil or sealed ampules containing an inert gas such as argon. The most common mineral from which it is extracted is pollucite.

History of Discovery

Caesium was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff using spectroscopy—the separation of light from the element to identify it. The name comes from the Latin word 'caesius', meaning sky-blue, referring to the blue lines that appeared in its spectrum. Caesium was among the first elements to be discovered using spectroscopy.

Chemical and Physical Properties

Caesium has one electron in its outer shell, which is easily removed. This gives it a very low ionization energy, making it an element that readily forms the Cs⁺ ion. When placed in water, it releases hydrogen gas and caesium hydroxide, which is a strong base. Its stable natural isotope is caesium-133.

Applications

The most common application of caesium is in atomic clocks. The frequency arising from a specific change in the caesium-133 atom is used to define the second in the SI system of units. Caesium is also used in drilling fluids for oil wells (caesium formate) and in some photoelectric devices that convert light into electric current.

Caesium-137 and Radioactivity

Caesium-137 is a radioactive isotope produced by nuclear fission in nuclear reactors and nuclear weapons. Its half-life is approximately 30 years, making it a long-lived contaminant in the environment following nuclear accidents. It is also used in cancer treatment equipment (radiotherapy) and in industrial measurement, but requires strict safety management.