potassium

Soft, waxlike, silver-white, metallic element, atomic number 19, relative atomic mass 39.0983. Its chemical symbol comes from the German kalium. It is one of the alkali metals (in Group 1 of the periodic table of the elements), and has a very low density – it floats on water, and is the second lightest metal (after lithium). It is one of the most reactive in the reactivity series of metals, oxidizing rapidly when exposed to air and reacting violently with water. Of great abundance in the Earth's crust, it is widely distributed. It is found in salt and mineral deposits in the form of potassium aluminium silicates and potassium nitrate (saltpetre). Potassium has to be extracted from its compounds by electrolysis because of its high reactivity.

Potassium is the main base ion of the fluid in the body's cells. Along with sodium, it is important to the electrical potential of the nervous system and, therefore, for the efficient functioning of nerve and muscle. Shortage, which may occur with excessive fluid loss (prolonged diarrhoea, vomiting), may lead to muscular paralysis; potassium overload may result in cardiac arrest. It is also required by plants for growth. The element was discovered and named in 1807 by English chemist Humphry Davy, who isolated it from potash in the first instance of a metal being isolated by electrolysis.