The beaches of New Zealand'swestern coast are remarkable for their black sands. These sands contain ilmenite (a source of iron and titanium) in commercially exploitable amounts.
These metals—titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), and nickel (Ni)—are all of major industrial importance. They are important in the making of ceramics and ceramic glazes and in various high-performance alloys, which are used to make special steel, such as steel for tools. These elements and their compounds are also used as catalysts (substances that speed up chemical reactions) in the production of certain chemicals and synthetic materials.
Titanium is a lightweight, silver-gray metal.
It is widely distributed in the earth's crust, being the ninth most abundant element. However, it is never found in a pure state, usually occurring in the minerals ilmenite or rutile. Titanium resists corrosion and rust better than stainless steel. It can be drawn into fine wire and is not affected by strong acids. It withstands temperatures up to about 427° C (800° F.). It also has a higher strength-weight ratio than steel. However, it is difficult and expensive to extract titanium from the ores in which it is found. For this reason, the metal is not widely used, being restricted to aircraft and jet engines. It is also used in the manufacture of white paints, plastics, paper, and porcelain enamels.
Vanadium is a silver-white metal that is found in very small quantities throughout the earth's crust. It is used mostly in the manufacture of "ferrovanadium," an alloy of iron and 50 to 70 per cent vanadium. This strong, rust-resistant alloy is used in the production of steel parts for airplanes, cars, and locomotives, and for making high-speed cutting tools. It is also suitable for use in nuclear reactors. Vanadium is an important trace element in the human body.
Chromium is a glossy, gray, fairly soft metal. It is found in nature usually combined with oxygen and iron in a mineral called chromite. Sometimes called chrome, the metal does not easily corrode and becomes very shiny when polished. For these reasons, chromium is often used to plate metals used for car bumpers, door handles, and decorative trim. Stainless steel contains at least 10 per cent chromium. Stainless steel, because it doesn't easily rust, is often used for making eating utensils and kitchen equipment. Chromium-steel alloys are used to make ball bearings, safes, armor plating for ships and military vehicles, and the cutting edges of high-speed machine tools.
Manganese is a brittle, silver-gray metal, plentiful throughout the earth's crust, though not always accessible. The leading producers of the metal are the Soviet Union, South Africa, and Brazil. Manganese is employed mainly in special steels used in making heavy-duty machinery, safes, electromagnets (for radar transmitters and computer storage units), and stainless steel. It is used in the manufacture of dry cell batteries, dyes, paints, fertilizers, and compounds for water purification. Manganese is a trace element required by all plants and animals, including human beings.
Iron, a silvery-white metal in its pure state, is found in nature combined with other elements in iron ores. It is very abundant throughout the earth's crust. Iron is by far the most commonly used structural metal. It is often combined with other metals of this series into steel alloys. All kinds of steel can be grouped as carbon steel (the most common), alloy steel, stainless steel, and tool steel. Iron is also indispensable to the human body. The average adult male body contains one-eighth ounce (3.5 grams). About 65 per cent is found in hemoglobin, which carries oxygen from the lungs to the various parts of the body. Iron is also needed for the proper functioning of cells, muscles, and other tissues.
Cobalt is a hard, silver-white metal that is also magnetic, like iron and nickel. It is relatively rare in the earth's crust. In the making of
Major groups of elements: The transition metals 41
Oxygen Scrap iron Limestone
Molten, impure iron tapped from blast furnace and poured into ladles
Molten, impure iron in ladles taken to basic oxygen furnace for making into steel
Concentrated ore heated with limestone, coke and air in blast furnace, producing molten, impure iron
pulverized and concentrated to increase proportion of iron
Waste gases (CO + C02) out
Outer steel casing
fcaC03 + SiO, 4 i CaSi03 + CO,,
C + C02 -2CO !
2C + 02 -. 2C0 I
Fe203 + 3CO -2Fe + 3C0P
Inner lining of refractory bricks
Iron ore (Fe203 + impurities mainly Si02), coke (C) and limestone (CaC03) in
Hot air in
Moften slag Molten iron
Molten slag (CaSi03) out
Molten, impure^ ->
steel alloys, cobalt's ability to withstand high temperatures makes it ideal for use in gas turbines, jet engines, and other equipment that operates at high temperatures. Cobalt is also alloyed with aluminum and nickel or iron for use as magnets in radios, TV sets, and other devices. An isotope of cobalt—cobalt 60— is used in the treatment of cancer.
Nickel is a malleable white metal that is magnetic, can be polished to a high gloss, and does not tarnish easily or rust. The leading nickel-producers are the Soviet Union, Canada, and Australia. Nickel's resistance to corrosion makes it invaluable in making storage batteries. Nickel-iron alloys are used to make armor plate and machine parts. Invar (an alloy
Molten steel poured from ladles into ingots, or into a rolling mill to make steel bars
Steel ingots or bars remelted for casting into molds, or otherwise fabricated into finished products
Basic Oxygen Furnace
Molten iron, scrap iron and limestone in
Inner lining of refractory bricks
Outer steel casing
Tap-hole for molten steel
of nickel, iron, and other metals) is used for meter scales and pendulum rods, because it expands very little with temperature changes. Nickel-silver is an alloy used in tableware. The U.S. five-cent piece, also called a nickel, is made of copper and nickel.
Second and third transition series: 1
The eight elements that form the first halves of the second and third transition series can be treated as one group. They are zirconium (Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), hafnium (Hf), tantalum (Ta), tungsten (W), and rhenium (Re). These elements vary in abundance from the relatively rare to com-
Molten iron, scrap iron limestone and oxygen heated in oxygen furnace, producing refined, basic steel
Molten steel poured into ladles and I alloying metals added \ (if desired) to produce specific alloy steels
The main stages in the commercial production of iron and its chief alloy, steel,are illustrated in the flow diagram (above). The key stages are the reduction of the iron ore to impure iron in a blast furnace (far left), and the refining of this impure iron in an oxygen furnace to produce basic steel (left). The iron from the blast furnace contains up to 5 per cent of impurities, which make the metal brittle. These are removed by oxidation at a high temperature in the oxygen furnace. This process is so efficient that scrap iron can be added for direct conversion to steel. The basic steel produced consists of iron with less than one per cent carbon. Other metals may then be added to make alloy steels.
Titaniumwas discovered by William Gregor of England in 1791. It was named after the Titans in Greek mythology by Martin Klaproth of Germany in 1795. At. no. 22; at. mass 47.88; m.p. 1667+10" C;b.p. 3287'C.
Vanadiumwas discovered in 1830 by the Swedish chemist Nils Sefstrom (1787-1845). He named it after Vanadis, the Scandinavian goddess of beauty. At. no. 23; at. mass 50.9415; m.p. 1890+10° C;b.p. about 3380' C.
Chromiumwas discovered in 1797 by the French chemist Louis Nicolas Vauquelin (1763-1829). Its name comes from the Greek chromos, meaning color. Many of its compounds are highly colored. At. no. 24; at. mass 51.996; m.p. 1900° C; b.p. 2690° C.
Manganesewas recognized as an element in 1774 by the Swedish chemist Karl Scheele (1744-1786). It was isolated in the same year by the Swedish chemist Johan Gahn (1745-1818). Its name is
probably a corruption of Magnesia, the name of a region of ancient Greece. At. no. 25; at. mass 54.938; m.p. 1244±3° C; b.p. 1962° C.
Ironhas been known since 4000 B.C., although it was not in wide use until about 1000 B.C. Its chemical symbol, Fe, derives from the Latin name for the element, ferrum. At. no. 26; at. mass 55.847; m.p. 1535° C; b.p. 3000° C
Cobaltwas isolated in about 1735 by the Swedish
chemist Georg Brandt (1694-1768). However, its blue-colored compounds had been known centuries earlier. Its name derives from the German kobold, meaning goblin. At. no. 27; at. mass 58.9332; m.p. 1495° C; b.p. about 2870° C.
Nickelwas isolated in 1751 from an ore containing nickel and arsenic. It was isolated by the Swedish chemist Axel Cronstedt (1722-1765). The ore he used was once called (in German) kupfernickel, from which
the element later derived its name. At. no. 28; at. mass 58.69; m.p. 1555° C; b.p. 2837° C
42 Major groups of elements: The transition metals II
The block of transition metalsfrom zirconium to technetium and from hafnium to rhenium exhibit similar properties and can be treated as a single group. All are rare in nature—apart from technetium, which does not occur naturally at all. As a group, they tend to have only a few, highly specialized uses. Zirconium (found in the gemstone zircon), tungsten, and molybdenum (both used in various alloys) are probably the most familiar.
plete nonoccurrence in nature. All are metals, with considerable similarities in their properties, notably in their vertical relationships in the periodic table. Zirconium and hafnium, for example, have generally similar metallic properties and compounds, as do niobium and tantalum, and molybdenum and tungsten. All these elements form many complex compounds.
Zirconium is a gray-white metal that is widely distributed in the earth's crust. It is found in the minerals baddeleyite and zircon, from which it is extracted by a process that uses molten magnesium. If a zircon crystal is large, it may be used for jewelry. Otherwise, zirconium is very valuable for use in atomic reactors. Because it has a high resistance to corrosion and does not readily absorb neutrons, it is used to make the cores of the reactors and to coat the uranium fuel elements. Zirconium is also very resistant to high temperatures and is useful to the ceramics industry. It is used for making laboratory crucibles (melting pots for metals) and for lining furnaces. As a powdered metal, zirconium is used in smokeless flash powders, blasting caps, and fireworks. Because it is a very strong metal, zirconium is used to make tools such as rock drills. Some of the best superconducting metals are
niobium-zirconium alloys, containing 20 to 40 per cent zirconium.
Niobium, also called columbium, is a soft metal that varies in color from silver-white to gray, it is usually found in nature combined with the metal tantalum. Niobium is used in making high-strength steels for use in oil and gas pipelines, drill bits, and turbine blades. It is also used in the cores of certain nuclear reactors for increased strength and resistance to high temperatures.
Molybdenum is a hard, silver-white metal. Most of the world's supply comes from Colorado. Chile and Canada also produce the metal. When combined with certain other metals, molybdenum increases strength, toughness, and resistance to heat and chemicals. It is used in armor plate, tool steels, and high-strength steels for aircraft engines and missile parts. Molybdenum compounds are used in high-temperature lubricants and in the refining of petroleum.
Technetium occurs in nature in very small quantities. It was the first artificially created element and is now obtained as a by-product from atomic fission. All its isotopes are unstable and radioactive. They have no significant use except for one form, which is used in radiotherapy of thyroid disorders.
Hafnium is a silver-colored metal that is always found in combination with zirconium. Hafnium absorbs neutrons better than most metals and is used to control the rate of reactions in the reactors of nuclear submarines. It is also used in cutting tools, aircraft engine parts, and some gas-filled and incandescent lamps.
Tantalum is a rare metal that occurs in na- | ture with the element niobium in the minerals columbite and tantalite. It is also obtained as a by-product in the extraction of certain kinds of tin. Like niobium, tantalum is used in high-strength steels and nuclear reactors. It is also used in the electronics industry, in the production of camera lenses, and in medicine for bone repair and internal stitching.
Tungsten is a moderately hard, silver-white
Molybdenumis used as an alloying agent in armor plating which basically consists of alloy steel. Molybdenum is one of the most effective elements for increasing the hardenability and toughness of steel. In addition, molybdenum is highly corrosion resistant. It is also unusual in retaining its strength and hardness at extremely high temperatures. These properties make the metal itself and its alloys very useful for industrial chemical equipment.
Major groups of elements: The transition metals II 43
metal. It occurs in nature in the minerals scheelite and wolframite. It is also called wolfram. Tungsten has the highest melting point of all metals. It is therefore used in equipment that must withstand very high temperatures. It is made into heating filaments for vacuum tubes used in radios, TV's, and other electronic equipment. It is also used to make filaments for electric lights and contact points for the ignition systems of cars and trucks. Like molybdenum, tungsten is also used for high-strength steels that remain strong at high temperatures. It is used in the tips of high-speed cutting tools and in mining and petroleum drills. Compounds of tungsten are used in fluorescent lamps.
Rhenium is a rare, costly, silvery-white metal that can withstand high temperatures. To make electrical equipment that is heat-resistant, it is mixed with platinum or tungsten. It is also used in making fine wires called filaments for use in light instruments, such as mass spectrographs.
Second and third transition series: II
Six of the transition metals are known as the precious metals. They are ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt). They occupy positions in the second halves of the second and third transition series. Although differing in atomic weight, the physical and chemical properties of each element are similar to those of the vertical member directly below or above. All the dements in this group are rare, but tend to occur together, being obtained and separated only with difficulty. Most of these metals come from Canada, South Africa, and the Soviet Union, often as minor components of copper ores.
The platinum group elements are all high-density metals with generally similar properties. They resist forming compounds with oxygen. They have high melting points. They can form extremely hard alloys, for example, for electrical contacts and the tips of pen nibs. In
Zirconium, hafnium, and niobiumare used in nuclear reactors, such as the one shown. Zirconium and niobium, or sometimes an alloy of the two, are used to clad the fuel elements. They allow neutrons to pass through them easily. In contrast, hafnium is used in some nuclear reactors for its high neutron absorption. This property makes it valuable as a control-rod material.
Tungsten carbide(WC) is
one of the most important compounds of tungsten because of its extreme hardness. This makes it useful for the cutting edges of machine tools.
Zirconiumwas identified in 1789 by the German chemist Martin Klaproth (1743-1817). It was not isolated until 1824, by the Swedish chemist Jons Ber-zelius (1779-1848). Its name derives from that of its main source, the semiprecious mineral zircon. At. no. 40; at. mass 91.224; m.p. 1857° C; b.p. 4200° C.
Niobiumwas first discovered in 1801 by the British chemist Charles Hatchett (1765-1847). He named it co-lumbium, a name still used occasionally with the chemical symbol Cb. The element was rediscovered in 1844 by the German chemist Hein-rich Rose (1795-1864). He distinguished it from tanta-
lum and named it niobium, after Niobe, the daughter of Tantalus in Greek mythology. At. no. 41; at. mass 92.9064; m.p. 2468+10° C; b.p. 4742° C.
Molybdenumwas identified in 1778 by the Swedish chemist Karl Scheele (1744-1786). It was isolated in 1782 by another Swedish chemist, Peter Hjelm. Its name derives from the Greek molyb-dos, meaning lead, because it was once thought to be a lead ore. At. no. 42; at. mass 95.94; m.p. 2617° C; b.p. about 4612° C.
Technetiumwas the first element to be produced artificially. Its name comes from the Greek technetos,
meaning artificial. It was discovered in 1937 by the Italian scientists Carlo Perrier and Emilio Segre. At. no. 43; at. mass (of longest-lived isotope) 98; m.p. about 2200° C; b.p. about 4850° C.
Hafniumwas discovered in 1923 by the Dutch physicist Dirk Coster (1889-1950) and the Hungarian chemist Georg von Hevesy (1885-1966). Hafnium is derived from Hafnia, the Latin name for Copenhagen (Denmark), where the element was discovered. At. no. 72; at. mass 178.49; m.p. 2227° C; b.p. 4602° C.
Tantalumwas discovered in 1802 by the Swedish chemist Anders Ekeberg
(1767-1813). He named it after Tantalus in Greek mythology because its isolation had proved to be such a tantalizing task. At. no. 73; at. mass 180.948; m.p. 2996° C; b.p. 5425+100° C
Tungstenderives its name from the Swedish words tung, meaning heavy, and sten, meaning stone. It was first isolated in 1783 by the Spanish scientist Fausto de Elhuyar and his brother Juan. They obtained the metal from the mineral wolframite (iron and manganese tungstate). Hence, the element's alternative name of wolfram and its chemical symbol W. At. no. 74; at. mass 183.85; m.p. about 3400° C; b.p. about 5600° C.
Rheniumwas predicted to exist by the Russian chemist Dmitri Mendeleev (1834-1907) in 1869. It was not discovered until 1925, by the German chemists Walter Noddack (1896-1960), Ida Tacke (1896-1978), and Otto Berg, who named it after the River Rhine. At. no. 75; at. mass 186.207; m.p. 3180° C; b.p. about 5627° C.
44 Major groups of elements: The transition metals III
The block of transition metals from ruthenium to palladium and from osmium to platinumare
commonly called the platinum group metals. They usually occur together in nature, although they are rare. They are usually difficult to extract and separate from each other.
the periodic table, elements combining with second row metals form less stable compounds than those of the same element combined with third row metals. Compounds of second row metals are, therefore, more chemically reactive.
Ruthenium is a hard and brittle metal. It is sometimes combined with platinum and palladium in order to increase hardness. An alloy of platinum and 10 per cent ruthenium is used for making electrical contacts in certain types of small generators employed in aircraft.
Rhodium is a silver-white metal that is very resistant to corrosion and can be polished to a high gloss. It is used as a finish for mirrors, searchlights, and jewelry. Usually alloyed with platinum, rhodium is used in high-temperature electrical connections and in aircraft turbine engines. The same alloy is used as a catalyst (to speed up chemical reactions) in the manufacture of nitric acid for fertilizers and explosives. A platinum-rhodium alloy is also
used in the production of high-octane gasoline and aromatic compounds.
Palladium is a silver-white metal, soft and shiny. Because it resembles platinum, palladium is often used in its place. The metal is also harder, lighter, and less expensive than platinum. Palladium is very malleable. It can be flattened into sheets or drawn into wire. Mixed with gold, it is used to make "white gold" jewelry. It is also used in making surgical instruments. A certain type of palladium, called palladium black, is important in a process called hydrogenation. This is a process that improves the quality of oils and is used in making gasoline. Palladium is also combined with platinum into devices that reduce the amount of pollution given off by car engines.
Osmium, in its pure form, is a fine, black powder or a hard, blue-gray metal. It is twice as heavy as lead. Of all the metals in the platinum group, osmium reacts most readily with the air. Especially when heated over 93° C, osmium forms a poisonous vapor that can damage the skin or even the eyes and the lungs. Osmium is used in the making of electric light filaments, tips for pen points, and phonograph needles.
Iridium is a hard, brittle, white metal. It is more resistant to corrosion than any known metal. It is also one of the hardest metals. Being too brittle by itself, however, it is most often used to increase the hardness of platinum. Platinum-iridium alloys are used in making jewelry, fountain pen tips, bearings for navigational compasses, and small crucibles (containers for heating chemicals). The alloys are also used for certain other equipment that operates at high temperatures or around corrosive substances.
Platinum is a silver-white metal that is one
The most recent technique for extracting the platinum group metals
(PGMs) is based on solvent extraction. Part of a refinery in which this process is carried out is shown in the photograph. The details of the solvent extraction of PGMs are closely-guarded secrets. Basically, however, a mixture of the metals in aqueous solution is mixed with an organic reagent. This reagent does not mix with water. Further, it reacts with only one of the metals, thereby removing it from the aqueous phase and into the organic phase. The metal-reagent solution is then processed to extract the metal in pure form. This process is repeated to obtain each of the metals in turn.
Major groups of elements: The transition metals III
The sealin the photograph (left) is made of platinum, a precious metal used extensively in jewelry and other such ornamental objects. Platinum has a brilliant silver-white appearance and is very malleable. It is resistant to corrosion by acids, being attacked only by caustic alkalis.
The tips of pen nibsIright) are often made of osmium-iridium alloy, a substance that is very hard and has excellent corrosion resistance. Almost all osmium produced is used in alloys because the pure metal is brittle, even at high temperatures.
of the heaviest substances known. A specific quantity of platinum weighs 21 times as much as an equal amount of water. Platinum is more valuable than gold. Its strength, hardness, color, and freedom from tarnish make it ideal for gem settings. Yet it is soft enough to be etched with delicate designs. Platinum is more malleable than other metals; only gold and silver are easier to shape. As already discussed, platinum has many uses in alloys with other metals of the platinum group. The International Standard kilogram of mass is made from an alloy of 90 per cent platinum and 10 per cent iridium, because the metal is so stable. In medicine, compounds of platinum are used in chemotherapy to treat cancerous tumors.
Rutheniumwas isolated in 1844 by the Russian chemist Karl Klaus (1796-1864). Its name comes from the medieval Latin Ruthenia for a region in central Europe now part of the Soviet Union. At. no. 44; at. mass 101.07; m.p. about 2250° C; b.p. about 3900" C.
Rhodiumwas isolated in 1803 by the British chemist William Wollaston (1766-1828). He derived its name from the Creek rhodon, meaning rose, because of
the red color of many of the element's compounds in solution. At. no. 45; at. mass 102.906; m.p. 1966+3° C; b.p. 3727±100°C.
Palladiumwas isolated in
1803 by William Wollaston. It was named after Pallas, a recently discovered aster oid. At. no. 46; at. mass
106.42; m.p. 1552° C; b.p. 2940° C.
Osmium was discovered in
1804 by the British chemist Smithson Tennant (1761 -
1815). He named it for the unpleasant smell of some of its compounds. The Greek osme means odor. At. no. 76; at. mass 190.2; m.p. 2700° C; b.p. about 5300° C.
Iridiumwas discovered in 1804 by Smithson Tennant. He derived its name from the Latin iris, meaning rainbow. Its compounds exhibit a variety of colors. At. no. 77; at. mass 192.22; m.p. 2410° C; b.p. 4130° C.
Platinumderives its name from the Spanish platina, meaning "little silver," because of its resemblance to the latter metal. It was originally discovered by the Italian Julius Scaliger in 1557. It was reliably reported in South America in 1735 by the Spanish mathematician Antonio de Ulloa. It was then brought to Europe in 1741 by the British metallurgist Charles Wood. At. no. 78; at. mass 195.08; m.p. 1772° C; b.p. about 3827° C.
Nitrogenis the first element in Group 5A of the periodic table. It is by far the most abundant gaseous element on earth, constituting more than three-quarters of the volume of the atmosphere.
The nitrogen cycleis a
complex series of transformations. Atmospheric nitrogen is converted by living organisms and lightning into inorganic and organic nitrogen compounds. They are then converted back to gaseous nitrogen. The principal stages in the cycle are illustrated in the diagram below.
Nitrogen (N), the first element in Group 5A of the periodic table, is a colorless, odorless, and tasteless gas. It is essential to all living matter. Nitrogen makes up approximately 78 per cent by volume of the earth's atmosphere. It is also found in plant and animal proteins. There are few inorganic or mineral deposits containing nitrogen because most of its compounds dissolve in water. Deposits of sodium nitrate (Chile saltpeter) are found in Chile and in some other areas with a dry climate. These have been mined for use as a fertilizer and to make explosives.
Nitrogen can also be made artificially. Air is cooled until it liquefies and then cooled even further to the boiling point of nitrogen, — 195.8° C. The nitrogen is then removed.
Most nitrogen is used for the production of fertilizers from ammonia (a compound of nitrogen and hydrogen) or nitric acid (a compound of nitrogen, hydrogen, and oxygen). The gas itself is also used in the chemical, electrical, and metals industries. In the food industry, it prevents spoilage by mold. Liquid nitrogen is easily prepared and is used as a refrigerant
The nitrogen cycle
On the average, nitrogen makes up about 16 per cent of animal and vegetable proteins. The other 84 per cent consists of carbon, hydrogen, oxygen, and sulfur. Only a few simple organisms are able to use nitrogen directly from the air for the manufacture of the proteins they require for growth and tissue maintenance. Plants, in general, take nitrogen from the soil in the form of nitrates, nitrites, and ammonium salts—all compounds containing nitrogen. Animals and humans absorb most of the nitrogen they require from eating plants or other animals. There is, therefore, a vital relationship between plants, animals, the soil, and the nitrogen in the air. This interdependence
is known as the nitrogen cycle.
Complex environmental, biological, and chemical processes cycle nitrogen out of the air and into soil and water. From there, nitrogen is absorbed by the living tissue in plants, animals, and humans. When living tissue dies, bacteria release the nitrogen back into the earth and into the air.
Nitrogen finds its way to the soil in rain water as dilute nitric acid and nitrous acid, two different combinations of hydrogen, nitrogen, and oxygen. These two compounds are formed in the atmosphere when lightning causes nitrogen and oxygen to react and combine with rain water. Falling to the earth, nitric acid and nitrous acid then react with certain chemicals in the soil called bases to form nitrates and nitrites. Plants can use these nitrogen-containing compounds to form proteins. Certain types of bacteria and algae (called nitrogen-fixing bacteria and algae) also take nitrogen directly from the air and convert it into ammonia (a compound of nitrogen and hydrogen), which can then be used by plants.
Bacteria also play an important role in releasing the nitrogen from dead organic matter. Decaying plant tissue releases ammonia to the soil, which builds up there as ammonium salts. More bacteria (called nitrifying bacteria) convert these salts into nitrates and nitrites, which can be used once more by plants for the production of protein. Much of the nitrogen in animal protein is also returned to the soil when animals die and decay. Plant protein ingested by animals is excreted mainly as urea. This substance, when returned to the soil, can react with water to give carbon dioxide and ammonia.
Not all this combined, or "fixed," nitrogen is held in the soil. Some of it is broken down back to nitrogen by bacteria (called denitrifying bacteria) and returned as gaseous nitrogen to the atmosphere.