The five elements of Group 4A of the periodic table range from the nonmetallic carbon (C) and silicon (Si) through semimetallic germanium (Ge) to the metals tin (Sn) and lead (Pb). Silicon and germanium are important in the new semiconductor technology, while tin and lead have long been major ingredients in a range of low-melting alloys.
Carbon is unique among the chemical elements. Without it, life could not exist.
Carbon has been known since prehistoric times. Its name is derived from the Latin for charcoal, carbo. Its atomic number is 6, and its atomic mass is 12.011. It sublimes (changes from a solid directly to a gas) at 3500° C.
Carbon compounds form the basis of living matter. In nature, pure carbon occurs in three forms, called allotropes. (Allotropy is the existence of an element in two or more distinct physical forms.) Two of carbon’s forms are crystalline and one is amorphous. The crystalline forms are diamond—the hardest natural substance—and graphite—among the softest. Except for diamond and graphite, most carbon exists in combination with other elements. In the atmosphere, it occurs in oxidized form as the gas carbon dioxide. In the earth’s crust, it occurs combined with calcium and oxygen in limestone rock and with hydrogen and oxygen in the fossil fuels—coal, petroleum, and natural gas.
Diamond, because of its hardness, is used in tools designed to cut metal, stone, and concrete. It has been produced industrially since 1955. Graphite, which conducts heat and electricity without burning or melting, is used for battery electrodes and crucibles for melting metals. Its slipperiness makes it an excellent solid lubricant for clocks and door locks, it is also used in paint and pencils and as raw material for synthetic diamonds.
The combustion of fossil fuels generates carbon dioxide, which in turn decreases the amount of solar radiation that escapes from the atmosphere. The gas may thus play a leading role in the “greenhouse” effect, a possible cause of global warming.
Carbon in living matter is a mixture of three isotopes, including 14C, which is radioactive. When an organism dies, its exchange of carbon with the atmosphere stops and the amount of 14C in its body tissues starts to decrease. The rate of decay is known and measurable, so the proportion of 14C to total carbon in a specimen of once-living material provides an indication of the age of the sample. Used with wood, 14C dating methods can measure ages of up to 10,000 years.
The carbon in coal can be used to make synthesis gas, or syngas, a mixture of carbon monoxide and hydrogen. The gas can then be further refined into chemicals and fuels. This process is more expensive than direct combustion of fossil fuels, but deposits of natural gas and petroleum are limited and nonrenewable, so carbon from coal may become the basis of a chemical industry producing synthetic liquid fuels.
Silicon and germanium
Silicon, a hard, lustrous, dark-gray element, is second only to oxygen in abundance, making up about 28 per cent of the earth’s crust. It never occurs in a free, pure state but is always combined with other elements, especially with oxygen, with which it forms over 800 varieties of minerals called silicates. Most sand and all quartz are made of silica (silicon dioxide). Pure silicon is a semiconductor, used in the manufacture of integrated circuits for computer chips, solar cells, and a variety of devices that transmit electric signals. Silicon compounds are widely used in industry in glass, abrasives, synthetic rubber, lubricants, insulators, and water-repellent sealing materials.
Silicon was first isolated in 1823 by the Swedish chemist Jons Berzelius (1779-1848). Its name is derived from the Latin silex, meaning flint. Its atomic number is 14, and its atomic mass is 28.0855. Its melting point is 1410° C, and it boiling point is 2355° C.
Germanium, a hard, brittle, grayish-white metal, is widely used as a semiconductor, especially in diodes and solar batteries. It is also used in certain kinds of telephone lines, data-transmission lines, and computer cables, as well as some wet-cell storage batteries, and in medicine.
Germanium was predicted in 1871 by the Russian chemist Dmitri Mendeleev (1834-1907). It was not discovered until 1886 by the German chemist Clemens Winkler (1838-1894). He named it after his homeland. Its atomic number is 32, and its atomic mass is 72.59. Its melting point is 937.4° C, and its boiling point is about 2830° C.
This malleable white metal played a major role in the history of industrial development Foodcanning and preservation developed with the advent of tin-plated steel, and fast machines with tin-based bearings. The metal type formerly used in printing contained tin, and pioneering electrical communications devices depended on tin-based solder. Tin can easily be formed into intricate shapes. It adheres to steel and keeps it from rusting and is also applied to staples, paper clips, and food-storage containers. Tin plate is a very thin film of tin used to coat steel. Tin is a major ingredient of solder, with lead; bronze, with copper; and pewter, with antimony and copper. Tin or tin compounds are also used in bearings, dental fillings, toothpaste, and cast iron.
Tin, in the form of bronze (an alloy with copper), has been known since at least 3500 b.g Its chemical symbol, Sn, comes from the Latin word for the element, stannum. One naturally occurring form of tin—gray tin—is powdery at low temperatures. Its atomic number is 50, and its atomic mass is 118.71. Its melting point is 231.9° C, and its boiling point is 2270° C.
This soft, heavy, bluish-gray metal is soft and malleable. Lead has been known as a metal since at least 4000 b.c Its chemical symbol, Pb, is derived from the Latin word for the element, plumbum. Its atomic number is 82, and its atomic mass is 207.19. Its melting point is 327.5° C, and its boiling point is 1740° C.
Lead is easily cast, joined, and converted into pipes or sheets. Its chief use is in the electrodes of storage batteries, providing starting power for a wide range of vehicles. Impervious to radiation, lead is used as shielding material for X-ray equipment and nuclear reactors. Its resistance to corrosion makes it useful as a covering for power cables and in making pipes and storage tanks designed to carry and hold corrosive chemicals. Until recently, lead was added to gasoline to improve engine performance. Lead compounds impart corrosion resistance to paint applied to bridges and other steel structures.
Lead is very toxic, entering the body through the lungs as fine particles in the air. It may damage the kidneys, brain, or liver and • disrupt production of red blood cells. This has led to restrictions and controls on use of lead in paint and gasoline and on emission of leadbearing fumes by factories.