Potassium << puh TAS ee uhm >> is a silvery metallic element. It reacts readily with both oxygen and water. In nature, because of this characteristic, potassium always occurs combined with other elements. It is found in the form of minerals, such as carnallite and sylvite. Sir Humphry Davy, an English chemist, first isolated potassium as a pure metal in 1807.
Potassium, with a density of 0.856 gram per cubic centimeter at 20 °C, is the lightest metal except lithium (see Density). It is so soft that it can be cut with a knife. Potassium has an atomic number (number of protons in its nucleus) of 19. Its relative atomic mass is 39.0983. An element’s relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of carbon 12, the most abundant form of carbon. The chemical symbol of potassium is K. Potassium melts at 63.2 °C and boils at 766 °C. One isotope (form) of the element, potassium 40, is radioactive. The age of a substance can often be determined by analyzing the amount of potassium 40 it contains. Chemists classify potassium as an alkali metal. For information on the position of potassium on the periodic table, see the article Periodic table.
Potassium is a relatively abundant element and makes up nearly 21/2 percent of Earth’s crust. Large deposits of its principal compounds, including potassium chloride and potassium sulfate, occur in parts of Canada and Germany. The Dead Sea is another major source of potassium compounds.
Scientists have developed a wide variety of uses for potassium and its compounds. Potassium metal, used chiefly in sodium-potassium alloys, is usually obtained from molten potassium chloride through a special chemical process. These alloys, which are liquids at room temperature, are used in the heat-transfer systems of some types of nuclear reactors called fast breeders. Manufacturers use potassium carbonate, also called potash, in making certain kinds of glass and soaps. They use potassium nitrate, known as saltpeter, in producing matches and explosives. Some potassium compounds are used for medical purposes. For example, potassium bromide is a sedative, and potassium iodide promotes the discharge of mucus from the nose and from the throat.
Plants require potassium for growth. Therefore, soil must contain potassium compounds to produce crops of high quality and yield. Potassium chloride is widely used in commercial fertilizers for most crops. But potassium sulfate is a better fertilizer for tobacco and crops that would be harmed by chloride. See Fertilizer (Inorganic fertilizers).
Potassium also is essential for human beings and other animals. It plays a part in metabolism, the process by which organisms change food into energy and new tissue. For example, potassium helps enzymes speed up some chemical reactions in the liver and the muscles. Such reactions produce an important carbohydrate called glycogen, which regulates the level of sugar in the blood and helps provide energy for the muscles. Potassium, together with sodium, also contributes to the normal flow of water between the body fluids and the cells of the body. A daily diet that includes fruit, vegetables, and meat supplies enough potassium for the normal needs of the human body.
