The six elements of group 2A of the periodic table are called alkaline earth metals because their oxides have long been known as alkaline earths, compounds that form basic alkaline solutions that neutralize acids. In order of in-, creasing chemical reactivity, the six are: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Although they are less reactive than the alkali metals, they all react with water and have similar properties. No member of this group is ever found in pure form in nature, so they must be separated from the mineral compounds in which they occur.
Beryllium and magnesium
Beryllium is a rare, gray metal similar in appearance to aluminum and magnesium, but lighter and stronger and with a much higher melting point. It is used as a “moderator’ in nuclear reactors, where it slows fast neutrons, making it possible to control the rate of fission. It is highly poisonous and occurs sparingly, primarily in the mineral beryl, so its uses are limited to high-tech metallurgical applications. Small amounts are added to copper and other metals to create strong, corrosion-resistant, lightweight alloys. In solid-rocket propellant, it increases the thrust of the rocket, and its high melting point of 1287°C makes it useful in the nose cones for missiles.
Beryllium was discovered in the silicon-containing mineral beryl (from which its name derives) in 1798 by the French chemist Louis Vauquelin (1763-1829). The free metal was first isolated in 1828 by the German chemist Fredrich Wohler (1800-1882) and, independently, by the French chemist Antoine Bussy (1794-1882). Its atomic number is 4, and its atomic mass is 9.0128. Its melting point is 1278±5° C, and its boiling point, 2970° C.
Magnesium, a silvery-white metal weighing only two-thirds as much as aluminum, is the eighth most abundant element in the earth’s crust, making up large percentages of common minerals, such as olivine, and rocks, such as dolomite. Most magnesium metal used in industry is obtained by electrolysis of magnesium chloride from seawater. Alloyed with aluminum and zinc, it is used to make lightweight parts for automobiles, aircraft, and tools. One of its compounds—Epsom salt—is used medicinally as a purgative, and another—milk of magnesia—as a laxative. Magnesium is essential to plant and animal life.
Magnesium was first isolated as the free metal in 1808 by the British scientist Humphry Davy (1778-1827), although its compounds (for example, Epsom salts and magnesia) were known much earlier. The element is named after the ancient Greek region of Magnesia. Its atomic number is 12, and its atomic mass is 24.305. Its melting point is 650° C, and its boiling point, 1110±10° C.
Calcium, strontium, and barium
Calcium, a soft, silvery-white metal, is the fifth most abundant element, making up about 3.5 per cent of the earth’s crust. The commonest and cheapest of the alkaline earth metals, its compounds occur naturally in many forms, including chalk, marble, gypsum, limestone, seashells, and the mineral calcite. Huge quantities of these compounds are used in building and construction. Heating limestone produces quicklime, which is treated with water to make slaked lime, used in mortar and brickwork. Hydroxide and sulfate compounds are the basis of cement and plaster, respectively, and are also used in tanning leather, refining petroleum, and making fertilizer, paint, and many other products. Calcium is essential to life— necessary for the growth and maintenance of teeth ana bones. It helps control muscle action and blood clotting, as well as transmission of impulses by neurons of the nervous system. Calcium deficiency causes rickets, but an oversupply of calcium causes hardening of joints and formation of kidney stones.
Calcium was first isolated as the free metal in 1808 by Humphry Davy. Its name is derived from the Latin calx, meaning lime—a calcium compound known since ancient times. Its atomic number is 20, and its atomic mass is 40.08. Its melting point is 839° C, and its boiling point, 1484° C
Strontium, a soft, silvery metal, is used in fireworks and flares to produce a brilliant red color. Fallout from nuclear weapons tests contains SR-90, a radioactive isotope that can collect in human bone and cause leukemia and bone cancer.
Strontium was first found at Strontian (hence, the element’s name) in Scotland in 1790 by the British scientist Adair Crawford. The metal itself was isolated in 1808 by Humphry Davy. Its atomic number is 38, and its atomic mass is 87.62. Its melting point is 769° C, and its boiling point, 13845 C.
Barium is a soft, heavy silvery metal with a limited range of uses. Barium nitrate gives fireworks a green color, and barium sulfate is ingested prior to X-ray examination of the gastrointestinal tract.
Barium derives its name from the Greek barys— meaning heavy—although the metal is not particularly dense. It was first detected in 1774 by the Swedish chemist Karl Scheele (1742-1786). The free metal was isolated in 1808 by Humphry Davy. Its atomic number is 56, ana its atomic mass is 137.33. Its melting point is 725° C, and its boiling point is about 1140° C.
Radium, a white metal that tarnishes in the air, is highly radioactive, as are its compounds, found mainly in uranium and thorium ores. Radioactive decay of uranium produces radium isotopes, which are also unstable and decay eventually to lead. Once used in treating cancer and making fluorescent paint, radium has been replaced by cheaper, safer substitutes. The radiation it emits destroys body tissue and can cause bone cancer.
Radium was discovered in the radioactive mineral pitchblende (a uranium ore) in 1898 by the French scientist Pierre Curie (1859-1906) and his Polish-born wife Marie (1867-1934).
The metal itself was isolated in 1910 by Marie Curie. Its name is derived from the Latin radius, meaning ray. Its atomic number is 88, and its atomic mass is 226.025. Its melting point is about 700° C, and its boiling point is about 1140° C. The half-life of its stablest isotope is 1,600 years.