The six elements of group 1A of the periodic table are called alkali metals. The five that are stable enough for study—lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs), in order of increasing chemical reactivity—are all highly reactive. They burn in air and react violently with water to form strong bases called alkalis in which the positive ion of the metal is combined with the negative hydroxide (OH) ion. The sixth alkali metal—francium (Fr)—is radioactive and so unstable that its longest-lived isotope has a half-life of only 22 minutes.
These elements are too reactive to be found free and uncombined in nature. When extracted from minerals and concentrated in their free, metallic form, they must be stored in an inert oil to prevent any contact with air-otherwise they tarnish instantaneously and rapidly corrode. As free metals, they are light, silvery white, and soft enough to cut with a knife. Sodium and potassium occur widely in nature as salts and are essential for many forms of life. They have by far the most industrial importance of all the alkali metals.
The lightest metal known, lithium is only half as heavy as the same volume of water. Like sodium, it reacts with water to release hydrogen, but without generating enough heat to ignite the gas. Lithium hydroxide draws carbon dioxide out of the air, so it serves as a filter in ventilating systems for spaceships and submarines. The metal is added to copper to create a strong, lightweight alloy. Lithium salts are used in batteries, enamels, glass, and ceramic products, and lithium carbonate is used in treating manic-depressive illness.
Lithium was discovered in 1817 by the Swedish chemist Johann Arfvedson (1792-1841). Its name derives from the Greek lithos, meaning stone. Its atomic number is 3, and its atomic mass is 6.941. Its melting point is 180.54° C, and its boiling point, 1347° C.
Sodium and potassium
Sodium is the sixth most abundant element, making up almost 3 per cent of the earth’s crust. In nature, it occurs mainly as sodium chloride—table salt—a major component of seawater. Sodium chloride is obtained by evaporation of seawater and by underground mining of halite, a mineral. Other natural sources of sodium include soda ash (a carbonate salt) and borax. Much of the sodium chloride produced is converted to sodium hydroxide (caustic soda) and chlorine, both of which have many industrial applications. Sodium compounds are used in making glass and ceramics, in household detergents, weedkillers, photographic chemicals, and in tanning leather. Some nuclear reactors use metallic sodium as a coolant. In animals, sodium as well as potassium are needed to maintain a normal flow of water between body fluids and cells and to help prevent excessive water loss. The two elements are also necessary for tissue formation, muscle contraction, and metabolism.
Sodium was discovered in 1807 by the British scientist Humphry Davy (1778-1827). He named it after the alkaline substance soda. Its atomic number is 11, and its atomic mass is 22.9898. Its melting point is 97.8° C, and its boiling point, 881° C. The symbol for sodium, Na, comes from its latin name natrium.
Potassium is the seventh most abundant element, making up 2.5 per cent of the earth’s crust. It occurs naturally in three isotopic forms—39K, 40K, and4lK—one of which (40K) is radioactive, with a half-life of 1.28 billion years. The rate of decay of 40K can be measured and used to determine the age of certain rocks. Unlike sodium, most potassium occurs in the mineral grains of rocks and clays, and is costly to extract. Most of the potassium produced commercially is obtained from beds of salt deposited by former inland seas. Potassium is essential for plant growth. Potassium chloride is used in fertilizer, and potassium nitrate, or saltpeter, is used in explosives. Potassium salts are more expensive than sodium salts but are not hygroscopic (water-attracting), so they are used in compounds that must stay dry, such as potassium nitrate (gunpowder) and potassium chlorate (match heads). Potassium carbonate is used in making glass; other potassium salts, such as iodide and bromide, are used in photography and medicine.
Potassium was isolated by Humphry Davy in 1807. It was originally called kalium, the Latinized form of the Arabic word for alkali. Its atomic number is 19, and its atomic mass is 39.0983. Its melting point is 63.2° C, and its boiling point, 766° C.
Rubidium, cesium, and francium
Although rubidium is widely distributed in the earth’s crust and even more abundant than lead, copper, or zinc, it is never found in concentrated form and is thus very costly to extract. Industrially, it is used in catalysts and photoelectric cells.
Rubidium was discovered 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. Its atomic number is 37, and its atomic mass is 85.4678. Its melting point is 38.84° C, and its boiling point, 688° C.
Cesium is also widely distributed in small amounts, mainly in brines and mineral waters. It is used as the time-measuring element in atomic clocks.
Cesium was discovered in 1860 by Robert Bunsen and Gustav Kirchhoff. It was named after the characteristic blue lines in its spectrum. Its atomic number is 55, and its atomic mass is 132.905. Its melting point is 28.4° C, and its boiling point, 678.4° C.
Francium, the heaviest and most reactive of the alkali metals, has some 20 isotopes, all short-lived and radioactive. It forms in uranium ores by radioactive decay of actinium.
At any given moment, there is only about 25 grams of francium in all of the earth’s crust.
Francium was discovered in 1939 by the French chemist Marguerite Perey (1900-1975). She named it after her homeland. Its atomic number is 87. Little else is known about the element because no stable isotopes exist. There is one natural isotope, which is the longest-lived (half-life 21 minutes).