II.

Properties and Occurrence

Potassium was discovered and named in 1807 by the British chemist Sir Humphry Davy. The element’s name comes from potash, a potassium compound originally obtained by soaking wood ash in a pot of water and allowing the water to evaporate. The metal is silvery white and can be cut with a knife. It has a hardness of 0.5. Potassium exists in three natural isotopic forms, with mass numbers 39, 40, and 41. Potassium-40 is radioactive and has a half-life of 1.26 billion years. The most abundant isotope is potassium-39. Several radioactive isotopes have been artificially prepared. Potassium melts at about 63°C (about 145°F), boils at about 760°C (about 1400°F), and has a specific gravity of 0.86; the atomic weight of potassium is 39.098.

Potassium metal is prepared by the electrolysis of fused potassium hydroxide or of a mixture of potassium chloride and potassium fluoride. The metal oxidizes as soon as it is exposed to air and reacts violently with water, yielding potassium hydroxide and hydrogen gas. Because hydrogen gas produced in the reaction with water burns spontaneously, potassium is always stored under a liquid such as kerosene, with which it does not react.

Potassium is found in nature in large quantities, ranking eighth in order of abundance of the elements in Earth’s crust, in various minerals such as carnallite, feldspar, saltpeter, greensand, and sylvite. Potassium is a constituent of all plant and animal tissue as well as a vital constituent of fertile soil.