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Gallium, indium, and thallium

Gallium is a soft, silvery-white metal that is found throughout the earth's crust. It is similar to aluminum in many ways. One exception is that it remains a liquid over a wider tempera­ture range than any other element. Thus, it was first used to fill high-temperature thermome­ters. It is used in dental fillings (together with tin and silver), in conductors for transistors, and in memory devices for high-speed com­puters.

Indium is a soft, lustrous, silvery-white metal, rare in the earth's crust It is usually found in certain zinc ores. Its limited industrial use includes coating the heavy-duty bearings of high-speed diesel engines, making them run more smoothly. Indium is also used in making transistors for pocket radios and other electronic devices.

Thallium is a soft blue-gray metal that re­sembles lead. It is very rare, being sparsely distributed in only a handful of minerals. Be­cause it is very poisonous, its use is limited, al­though it has varied uses. A compound of thal­lium is widely used in rat and ant poisons. An isotope (a particular form of an element) of thallium helps diagnose certain types of heart disease. Other compounds are used in devices that measure infrared radiation.

The flowers of hydran­geasmay be pink or blue. Blue ones are so colored because they contain a par­ticular aluminum com­pound. In some cases, pink hydrangea flowers can be made to turn blue by the ad­dition of alum to the soil.

Galliumhas few uses, one of the main ones being in electronic components such as light-emitting diodes.


Fact entries

Boronwas isolated in June 1808 by the French chemists Joseph Cay-Lussac (1778-1850) and Louis-Jacques Thenard (1777-1857). Nine days later, it was independ­ently isolated by the Eng­lishman Humphry Davy (1778-1827). It is named after borax, its main compound. At. no. 5; at. mass 10.81; m.p. about 2300° C; b.p. about 2550- C.

Aluminumwas first iso­lated in crude form in 1825 by the Danish scientist Hans Oersted (1777-1851). Its name is derived from the Latin alumen, meaning alum. At. no. 13; at. mass 26.9815; m.p. 660.37° C; b.p. 2467° C.

Galliumwas discovered in 1875 by the French chemist Paul Emile Lecoq de Bois-baudran(1838-1912). He named it after the Latin word (Gallium) for his homeland. At. no. 31; at. mass 69.72; m.p. 29.78° C; b.p. about 2403° C.

Indiumwas discovered in 1863 by the German scien­tists Ferdinand Reich (1799-1882) and H.T. Richter (1824-1898). Its name derives from the Latin indicum, meaning indigo, the color of the ele­ment's dominant spectral lines. At. no. 49; at. mass 114.82; m.p. 156.61° C; b.p. 2080° C.

Thalliumwas first identi­fied in 1861 by the British scientist William Crookes (1832-1919). Characterized by a bright green line in its spectrum, the element's name comes from the Greek thallos, meaning green twig. At. no. 81; at. mass 204.383; m.p. 303.5° C; b.p. 1457+10°C.


3A 4A 5A
B c N
10.81 12.011 14.0067
Al Si P
26.9815 28.0855 30.9738
Ga Ge As
69.72 72.59 74.9216
In Sn Sb
114.82 118.71 121.75
Tl Pb Bi
204.383 207.2 208.98

Carbonis the first member of Group 4A of the periodic table. Apart from its unique­ness in the number of com­pounds it can form, carbon resembles silicon just below it in many respects. The degree of similarity with other Group 4A ele­ments decreases down the group.

Inorganic carbon

Carbon (C) is unique among the chemical ele­ments in the number and variety of the com­pounds it can form. Without carbon, life would be impossible. Certain compounds of carbon form the basis of living matter, making up the living tissues of all animals and plants. These compounds are the subject of organic chemistry, which has its own section later in the book. This article deals mainly with some of the other compounds of carbon, so-called "inorganic" carbon.

Carbon makes up less than .03 per cent of the earth's crust. It can be found pure in nature in three forms: as diamonds, as graphite, and as amorphous carbon. Diamond is the hardest natural element and one of the hardest of known solids. Graphite is one of the softest. Both these forms are pure carbon, but they have different crystal structures. Amorphous carbon does not have a crystal structure. It consists of shapeless particles that resemble graphite. They are too tiny to be seen with a microscope.

Most carbon exists in combination with other elements. Carbon dioxide in the air is a compound composed of carbon and oxygen. Carbon occurs in natural organic matter, con­verted with time into coal, petroleum, or gas. Carbon is also found in carbonate rock such as limestone.

Diamonds are extremely rare. The principal economic sources are in central and southern Africa and the Soviet Union. About 20 per cent of the world's diamonds are used as gem stones in jewelry. The rest, being imperfectly formed or having poor color, are suitable only for industrial use. Diamonds can cut, grind, and bore into very hard metal quickly and ac-

curately. They are also used to make phono­graph needles for record players. Since 1955, synthetic diamonds have been made industri­ally.

Graphite conducts electricity and heat with­out burning or melting. It is therefore used in making electrodes (electric contact points) for dry batteries and crucibles for melting metals. Graphite is not easily dissolved, so it is used in building tanks for holding strong acids. Graph­ite is very slippery and makes an excellent lu­bricant for machines with small parts, like clocks and door locks. Graphite is also used in making paints, "lead" pencils, and synthetic diamonds.

Amorphous carbon occurs in natural fuels, such as coal, oil, or natural gas. Besides their use for heating and cooking, these fuels can be burned without sufficient oxygen to burn completely. This process produces carbon black, which is used in motor tires and black printing inks. The same burning process ap­plied to wood (or soft coal) produces charcoal, which is used to filter out impurities and odors from the air or from water.

The burning of these natural fuels gives off carbon dioxide. It has been suggested that this carbon dioxide traps heat that would other­wise escape into space (the "greenhouse ef­fect"). During the last 100 years, the amount of carbon dioxide in the atmosphere has in­creased by about 15 per cent. This has caused an increase in the global average temperature, which is expected to continue rising. By the middle of the next century, the average tem­perature is predicted to be higher by about 2 per cent. Opinions differ about the possible ef­fects of this climate change; it will undoubt­edly alter plant growth patterns, although many crops respond favorably (in green­houses) to higher carbon dioxide levels.

Carbonhas two crystalline forms. In diamond(above and left), each atom is bonded to four others in a tetrahedral arrangement. Graphite(below and right) is formed of six-membered rings.

Major groups of elements: Inorganic carbon 37


Dissolves in water

Carbon dioxide in water JfL Jk Respiration .Respiration

more expensive than the use of natural fuels like coal and oil. However, when these natural fuels become scarce, carbon may then be­come the basis of a chemical industry produc­ing synthetic liquid fuels.

Carbon dating

Carbon in living matter consists of a mixture of three isotopes (forms of an atom) more or less in equilibrium. One of these isotopes is car­bon 14. Exchange of carbon with the atmos­phere ceases when an organism dies, and car­bon 14 starts to decay. The rate at which it decays is known and can be measured. Thus, the proportion of carbon 14 in the total carbon of an archeological specimen of material that was once living gives an estimate of the age of the sample.

Used with wood, for example, the carbon 14 dating method can measure dates up to about 10,000 years ago. The portion of material used for testing is destroyed during the proc­ess. However, as little as one thirty-thousandth of an ounce (one thousandth of a gram) can be tested—a single thread from a garment, for ex­ample. Factors affecting the accuracy of car­bon 14 dating methods include contamination of samples by other carbon-containing matter, such as plant roots, carbon dioxide in the at­mosphere from burned fossil fuels, and radio­activity from atomic explosions.

The carbon cycleis a

complex chain of processes in which carbon com­pounds circulate among the air, water, living organisms, and minerals. The illustra­tion above shows the princi­pal stages.

Carbon fibersare utilized extensively to reinforce lightweight, high-strength plastics. These are used to make articles such as tennis rackets and other sports equipment.

Manmade fuels

Carbon can be converted into "synthesis gas," a mixture of carbon monoxide and hydrogen. This gas can then be further refined into chemicals and fuels. So far, this method is

Fact entries

Carbonhas been known since prehistoric times. Its name is derived from the Latin for charcoal, carbo. At. no. 6; at. mass 12.01 ^sub­limes (changes from a solid directly to a gas) at 3500° C.

Allotropyis the existence of an element in two or more distinct physical forms. Allotropy is exempli-

fied by carbon, which exists naturally in three main forms: graphite, diamond, and amorphous carbon. Other elements that exhibit allotropy include oxygen, phosphorus, sulfur, and tin. Allotropes may be mono-tropic or enantiotropic. In monotropy, one allotrope is the most stable under all conditions. Carbon, oxygen,

and phosphorus are mono-tropic. Graphite is the sta­blest form of carbon. Di­atomic oxygen (02) is more stable than the triatomic form (ozone, 03). Red phos­phorus is stabler than white phosphorus. In enanti-otropy, different allotropes are stable under different conditions. Sulfur and tin are enantiotropic. Sulfur

forms two types of crystals, rhombic and monoclinic. Rhombic crystals are the most stable form below about 95° C Monoclinic crystals are the most stable between 95° C and 120° C. Similarly, the gray form of tin is the most stable below about 13° C. The white allo­trope is the stablest form at higher temperatures.


3A 4A 5A
B c N
10.81 12.011 14.0067
Al Si P
26.9815 28.0855 30.9738
Ga Ge As
69.72 72.59 74.9216
In Sn Sb
114.82 118.71 121.75
Tl Pb Bi
204.383 2072 208.98

The elements silicon to leadcomprise most of Group 4A of the periodic table. All have practical ap­plications. Silicon (used in glass) and tin (used in can­ning) are the most common.

Tinis widely used for coat­ing steel cans. It is particu­larly suitable for this be­cause it does not corrode easily, is not toxic, and ad­heres firmly to the underly­ing steel.

Silicon to lead

Silicon and the elements below it make up most of Group 4A of the periodic table. The other element in the group, carbon, is dealt with in the previous article and in the section on organic chemistry. The elements in Group 4A vary from the nonmetallic silicon (Si), through semimetallic germanium (Ge), to the metals tin (Sn) and lead (Pb). The first two ele­ments have recently found use in semiconduc­tors, whereas the other two have long been employed as the principal ingredients in a range of low-melting alloys—two or more met­als or other substances that can be mixed to­gether at low temperatures.

Date: 2015-12-11; view: 692

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