Oxygen is made in very large quantities by the fractional distillation of liquid air. In this process, air is filtered and dried and carbon dioxide removed. The resulting gas mixture—containing nitrogen, oxygen, and the rare gases (mainly argon)—is compressed and cooled until it liquefies. The liquefied air is then distilled in a column. The liquid becomes richer and richer in oxygen, and the gas distilled off richer and richer in nitrogen. Liquid oxygen, which is pale blue in color, is removed 99.5 percent pure. It is transferred to steel pressure vessels for further use.
Oxygen can also be made by the electrolysis of water. In this process, an electric current passed through water yields oxygen at the anode (the positive electrode at one end of the current) and hydrogen at the cathode (the negative electrode at the other end). Both of these gases can be collected in this manner and stored.
Commercially, oxygen is used in the production of steel, the manufacture of chemicals from natural gas, and the formation of important industrial oxygen compounds, such as ox-irane and sodium peroxide. Oxygen is mixed with fuel in welding torches, producing a very hot flame with a temperature close to 6000° F. (3300° 0. LOX (liquid oxygen) can be used to make rocket fuel and blasting explosives. In both cases, LOX has to be mixed with liquid fuels. For example, to make rocket fuel, either kerosene or liquid hydrogen can be used.
Ozone is a gas like ordinary oxygen, but it has a garliclike odor and very different chemical properties. A molecule of ozone is made up of three atoms of oxygen. A molecule of ordinary oxygen is made up of only two atoms. Ozone is found in its highest concentration at a height of 19 miles (30 kilometers) above the earth's surface. This so-called ozone layer is of vital importance in protecting life on earth from too much exposure to the sun's ultraviolet radiation, which harms living tissue. About 95 to 99 per cent of this radiation is blocked by the ozone layer. Ozone is also found in smaller amounts in the lower atmosphere, where it contributes to air pollution, being a component of smog.
Ozone is produced naturally when radiation from the sun strikes oxygen in the earth's upper atmosphere, converting some of it into ozone. It is also produced during lightning
storms, when electricity might convert oxygen into ozone.
Commercially, an "ozonizer" uses electric discharges to convert oxygen into ozone. The ozone can then be used in water purification processes or as a bleach in making waxes, oils, and fabrics.
Recent testing of the ozone layer has demonstrated that waste gases from industry and from aerosol sprays are damaging it. As a result, steps are being taken by the U.S. government and other governments to control the use of certain pollutants that are causing the gradual breakdown of the ozone layer.
The commercial preparation of oxygeninvolves the liquefaction of air under pressure. Liquid air is then distilled to remove nitrogen and argon and other rare gases, leaving liquid oxygen.
Fishes"breathe" the oxygen in water. Fresh water contains about 3 per cent of dissolved oxygen. This is removed in a fish's gills and passed to the animal's bloodstream.
Oxygenwas discovered in 1774 independently by two chemists, Karl Scheele (1742-1786) in Sweden and Joseph Priestly (1733-1804) in England. They obtained the gas by heating mercuric oxide. But it was the French chemist Antoine Lavoisier (1743-1794) who correctly
explained the role of oxygen in combustion. He also coined the name oxygen from Greek words meaning acid-maker. He wrongly believed that all acids contain oxygen. At. no. 8; at. mass 15.9994; m.p. -118.8° C; b.p. -182.962" C.
Oxidescan be classified into three main types: acidic, basic, and neutral. Acidic oxides react with water to form acids. Examples include carbon dioxide, which forms carbonic acid, and sulfur trioxide, which forms sulfuric acid. Basic oxides form bases with water.
An example is sodium oxide, which forms sodium hydroxide. Neutral oxides, such as nitrous oxide and water, are neither acidic nor basic. Acids and bases are discussed more fully in the article "Key chemical reactions."
Ozonewas discovered in 1840 by the German chemist Christian Friedrich Schonbein. Pure ozone is a pale blue gas. At. mass 47.998.
Sulfur to poloniumfollow oxygen to fill the remainder of Group 6A of the periodic table. Tellurium is unusual in having a higher atomic mass than its right-hand neighbor (iodine).
The Frasch methodis an
ingenious process of mining sulfur from deep under the ground. The sulfur occurs in a layer mixed with calcite (calcium carbonate). It is reached by means of a bore hole. Three concentric pipes are sunk down the hole. Water superheated to 311"F. (155° C) and under pressure is pumped down the outer pipe. Compressed air is forced down the inner one. The hot water melts the sulfur, which accumulates at the end of the pipes. A frothy mixture of sulfur, air, and water then passes up the middle pipe to the surface. There, it is run off to set in large molds. More than 80 per cent of the world's sulfur is obtained this way in Texas and Louisiana.
Sulfur to polonium
The elements sulfur (S), selenium (Se), tellurium (Te), and polonium (Po) make up, with oxygen, Group 6A of the periodic table. From sulfur to polonium, the elements are chemically very similar, even though there is a definite gradation of properties down the group. Oxygen has different properties from the rest—primarily because it combines so readily with other elements. It is also a gas, whereas the others are solids. Oxygen is discussed in the previous article.
Sulfur, air and water
Up to 1,000 feet (305 meters)
Selenium and tellurium are usually found as impurities in deposits of sulfur ores. All three elements are solid at room temperature and also very brittle. Polonium, however, is a product of radioactive decay in certain minerals, and therefore unstable. The metallic character of the elements also increases from sulfur to polonium. Sulfur is a yellow nonmetal. Selenium and tellurium show some metallic properties (tellurium has a silvery luster). Polonium is similar to lead, being dense and soft with a low melting point.
Sulfur is a solid yellow nonmetal. It is tasteless and odorless. It often occurs in the form of pale crystals. It is also found in coal, crude oil, natural gas, and oil shales. Many minerals contain sulfur, the most abundant mineral being pyrite, a compound of sulfur and iron. The United States (especially Louisiana and Texas) is the world's largest producer of sulfur. Other producers include Poland, the Soviet Union, Canada, and Japan.
Sulfur compounds have been found in meteorites and in interstellar clouds. The atmosphere of Venus contains sulfur. Some scientists believe that the core of Mars also contains sulfur. The human body, as well as plants and animals, all have small amounts of sulfur.
Two methods are widely used to produce sulfur commercially. The Frasch method extracts sulfur out of layers of calcite (a type of mineral containing calcium and carbon) from deep underground. A process called Claus conversion extracts sulfur from natural deposits of crude oil and raw natural gas.
Almost all sulfur is used in the production of sulfuric acid. Sulfuric acid is dense, oily, colorless, and very corrosive—one of the strongest acids known. A molecule of sulfuric acid consists of two atoms of hydrogen, one atom of sulfur, and four atoms of oxygen. Sulfuric acid is probably the world's most important commercial chemical.
Sulfuric acid is made on a large scale for use in the production of fertilizers from phosphate rock and ammonia. It is used in making dyes, paints, certain medicines, automobile batteries, explosives, paper pulp, and various textiles. Sulfuric acid is used in the refining of petroleum, the making of iron and other metals, and the production of various industrial chemicals. It is used to make strong detergents. It is an excellent dehydrating agent, being able to remove hydrogen and oxygen-the two components of water—from many substances. Sulfuric acid dissolves many metals to form compounds that have a wide variety of industrial uses.
Sulfur is also used by itself in the manufacturing of various products. These include fungicides, insecticides, some types of explosives, shampoos, photographic chemicals, storage batteries, and rubber. It has wide use in fields as divergent as the making of medicines and the laying of highway surfaces.
When sulfur is burned, it combines with oxygen in the air to form sulfur dioxide, a colorless, poisonous gas with a sharp odor. It is found especially in densely populated areas that may also have oil refineries and factories that burn sulfur-containing coal or oil. If dissolved in water droplets, sulfur dioxide can form acid rain, which is harmful to vegetation and can even kill wildlife and harm buildings. For these reasons, the emission of sulfur dioxide into the air by industry is regulated by the U.S. government.
Major groups of elements: Sulfur to polonium 53
Ring structure of 8 sulfur atoms
Selenium is a nonmetal that occurs as a dark-red powder or crystal or as a dark-gray shiny rod. It is extracted from metal sulfide ores— particularly those of nickel, lead, and copper. Although classified as a nonmetal, selenium conducts electricity when light shines upon it. This quality makes it useful in devices that use changes in light to produce changes in electric current. These devices include television cameras, certain kinds of copying equipment, and electric eyes. It is also used to harden rubber and to give a red color to glass and ceramics.
Tellurium is a semimetal that is usually found in combination with other metals in nature. It is usually obtained as a by-product of copper refining. Tellurium is used in the refining of petroleum, the curing of rubber, and the making of certain alloys (combinations of metals). It is sometimes used to color glass and ceramics. Some tellurium also goes into the making of television screens.
Polonium is a radioactive metal produced by the radioactive decay of minerals containing uranium and thorium. It occurs naturally in pitchblende, a black mineral often found near silver deposits. Polonium can also be produced artificially. It gradually decays into an isotope (form) of lead. An isotope of polonium is used as a power source in satellites. It is also used to eliminate static electricity in some industries.
Molten sulfur burned in air to form sulfur dioxide: S ■ 02 » S07
i SO:i dissolved m
i sulfuric acid to
I form oleum:
1 H2S04 * S03 • H2S20
Hot S02 and air passed over catalyst to form sulfur trioxide 2SO, • CV-2SO,
Concentrated sulfuric acid H2S04
y Water added to oleum ■to form sulfuric acid: H2S,07 - H20 >2H2SCv
Solid sulfuroccurs in two crystalline forms. Ortho-rhombic sulfur is found at temperatures below 205° F. (96° C). Monoclinic, or needle-shaped sulfur, exists between the above temperature and the melting point of sulfur, about 246° F. (119° C). Both forms have a molecular structure consisting of a puckered ring of eight sulfur atoms. Molecules of plastic sulfur consist of long zigzag chains.
Sulfuric acidis probably the most important bulk chemical manufactured in industrial countries. Most is made in stainless steel plants (far left) by the contact process. Sulfur dioxide is first obtained by burning sulfur in dry air. It is then oxidized to sulfur trioxide using air and a vanadium oxide catalyst. The sulfur trioxide is absorbed in concentrated sulfuric acid to give oleum. This is then diluted with water.
Sulfurhas been known since ancient times. It was first classified as an element by the French chemist An-toine Lavoisier (1743-1794) in 1777. The name comes from the Latin sulphurium, meaning brimstone. At. no. 16; at. mass 32.06; m.p. 113-120° C; b.p. 444.6° C.
Seleniumwas discovered by the Swedish chemist Jons Berzelius (1779-1848). He named it after Selene, the Creek word for the moon. At. no. 34; at. mass 78.96; m.p. (gray form) 217° C; b.p. 684.9+1° C.
Telluriumwas extracted from Transylvanian ores in 1782 by the Austrian chemist Franz Miiller von Reic-henstein (1740-1825). He named it after the Latin tel-lus, meaning earth. At. no. 52; at. mass 127.6; m.p. 449.8° C; b.p. 989.9° C.
Poloniumwas discovered in pitchblende in 1898 by the French scientists Pierre Curie (1859-1906) and Marie Curie (1867-1934). It was named after Poland, Marie Curie's homeland. At. no. 84; at. mass (the most stable isotope) 209.
The halogens,which make up Group 7A of the periodic table, show a graduation of chemical properties from the highly reactive fluorine to the comparatively un-reactive iodine.
Fluorine (F), chlorine (CD, bromine (Br), iodine (1), and astatine (At) are five reactive nonmetals commonly called the halogens. They make up Group 7A of the periodic table. They all have a strong, unpleasant odor. They dissolve poorly in water. Care is necessary in handling them, because they burn skin easily. Fluorine and chlorine are gases. Bromine is a liquid at ordinary temperatures. Iodine and astatine are solids.
Halogen means 'salt producer.' Halogens react (combine) with metals to form the salts in the sea. Common table salt, the best example, is sodium chloride, a combination of the metal sodium and the halogen chlorine. In general, the halogens react with most metals and also many nonmetals, producing many useful compounds. These range from antiseptics and medicines to antiknock gasoline and hydrochloric acid.
Fluorine, chlorine, bromine, and iodine are so reactive they do not occur alone in nature. They are always found combined with other elements. Astatine is radioactive. Although small quantities of it exist in uranium ores, almost every one of its 30 isotopes (forms) are created artificially. They are extremely unstable and are of no practical use.
The halogens are electronegative. They tend to take up extra electrons from other elements. The addition of an extra electron to a halogen atom charges it with negative electricity, making the atom into an ion. It is these ions that combine with metals to form salts known as halides. The halogens can even form compounds with themselves. These compounds are known as interhalogens.
Fluorine is a greenish-yellow gas under normal conditions. In nature, however, it is always combined with other elements, being found usually in the minerals fluorite (a combination of calcium and fluorine) and cryolite (an aluminum ore). Fluorine is the most reactive of all elements. It combines with other elements more readily than any other chemical element.
Because it is so reactive, fluorine itself is of little use. Not only do most elements react with it, but some organic compounds catch fire in the gas. Fluorine compounds, known as fluorides, are, however, widely used. Some of them are useful in the chemical and petroleum industries. Others can be used to etch glass, particularly in the manufacture of "frosted" electric light bulbs. Fluorides are used in making an inert plastic that can withstand large differences in temperature. Fluorides are also used in making coolants for household refrigerators and propellants for aerosol sprays. Fluorides are added to toothpastes and drinking water to help reduce tooth decay.
Chlorine is a yellow-green gas that is very strong-smelling and poisonous. It irritates the eyes and throat. In nature, it is most often combined with sodium in sodium chloride—common salt. This compound can be found in sea-water, inland salt lakes, and beds of salt rock.
Large quantities of chlorine are used for bleaching textiles and paper and in the manufacture of bleaching powder and household bleach. Chlorine is used to kill bacteria in the domestic water supply and in swimming pools. It is also used to make weed killers and insecticides, as well as various drugs, dyes, metals, and plastics.
Bromine and chlorinecan |
both be extracted from sea-water. The process for producing bromine (leftand below) relies on the fact that chlorine displaces bromine from its salts. Several uses of chlorine employ its germicidal properties. It is used in the production of drinking water and in the treatment of water for swimming pools (right).
Bromine is a dense, dark-red liquid that easily vaporizes into a reddish-brown gas that has a strongly irritating odor. In nature, it is usually found in seawater and in brine, combined with either sodium or magnesium. Bromine is used in both agriculture and in the petroleum industry. It is also used to manufacture dyes, sedatives, anesthetics, photographic materials, and fire-retardant chemicals.
Iodine is a solid that has a slight metallic luster. It takes the form of grayish-black flakes. In nature, it occurs in brines in a similar state to bromine. It is also highly concentrated in some sea plants. Iodine is used in chemical analysis. It goes into the preparation of photographic film, mild antiseptics, germicides, drugs, and dyestuffs. Iodine is also an important trace element in the human body. It is necessary in the control of the body's rate of physical and mental development. Without it, the thyroid gland in the neck swells in a condition known as goiter.