Rubidium, cesium, and francium
Rubidium is widely distributed in the earth's crust, being more abundant overall than lead, copper, or zinc. However, it appears in such small amounts that its production is limited. It is most often used industrially as a catalyst to speed up chemical reactions. It is also used in making photocells and vacuum tubes.
Cesium is widely distributed in low concentrations in brines and mineral waters. It is used as the time-measuring element in atomic clocks. Scientists are also studying the possibilities of using cesium in the generation of power and as a fuel for space vehicles.
Francium has more than 20 isotopes, all of which are short-lived and radioactive. No weighable amount of the element has even been extracted, and at any one time there is only about 25 grams of it throughout the whole of the earth's crust.
Sodium vaporis used in some streetlights because the orange light emitted when an electric current passes through the vapor is very intense. It also penetrates mist and fog better than does white light.
Many types of preserved meats—most sausages, for example—contain potassium nitrate. This compound inhibits the growth of harmful microorganisms such as Salmonella, which causes food poisoning.
Lithiumwas discovered in 1817 by the Swedish chemist lohann Arfvedson (1792-1841). Its name derives from the Creek lithos, meaning stone. At. no. 3; at. mass 6.941; m.p. 180.54° C;b.p. 1347° C.
Sodiumwas discovered in 1807 by the British scientist Humphry Davy (1778-1827). He named it after the alkaline substance soda. At. no.
11; at. mass 22.9898; m.p. 97.8° C; b.p. 881° C. The symbol for sodium, Na, comes from its Latin name natrium.
Potassiumwas isolated by Humphry Davy in 1807. It was originally called kalium, the Latinized form of the Arabic word for alkali. At. no. 19; at. mass 39.0983; m.p. 63.2° C; b.p. 766° C.
in 1861 by the German chemists Robert Bunsen (1811-1899) and Gustav Kirchhoff (1824-1887). It is so named because of the dominant red lines in its spectrum. The Latin rubidus means red. At. no. 37; at. mass 85.4678; m.p. 38.84° C; b.p. 688° C.
Cesiumwas discovered in 1860 by Robert Bunsen and Gustav Kirchhoff. It was
named after the characteristic blue lines in its spectrum. At. no. 55; at. mass 132.905; m.p. 28.4° C; b.p. 678.4° C.
Franciumwas discovered in 1939 by the French chemist Marguerite Perey (1900-1975). She named it after her homeland. At. no. 87. Little else is known about the element because no stable isotopes exist. There is one nat-
ural isotope, which is the longest-lived (half-life 21 minutes).
Date: 2015-12-11; view: 1032