The nitrogen family consists of the five elements that make up Group 15 of the periodic table: nitrogen, phosphorus, arsenic, antimony, and bismuth. These five elements share one important structural property: they all have five electrons in the outermost energy level of their atoms. Nonetheless, they are strikingly different from each other in both physical properties and chemical behavior. Nitrogen is a nonmetallic gas; phosphorus is a solid nonmetal; arsenic and antimony are metalloids; and bismuth is a typical metal.
Nitrogen is a colorless, odorless, tasteless gas with a melting point of −210°C (−346°F) and a boiling point of −196°C (−320°F). It is the most abundant element in the atmosphere, making up about 78 percent by volume of the air that surrounds Earth. The element is much less common in Earth's crust, however, where it ranks thirty-third (along with gallium) in abundance. Scientists estimate that the average concentration of nitrogen in crustal rocks is about 19 parts per million, less than that of elements such as neodymium, lanthanum, yttrium, and scandium, but greater than that of well-known metals such as lithium, uranium, tungsten, silver, mercury, and platinum.
The most important naturally occurring compounds of nitrogen are potassium nitrate (saltpeter), found primarily in India, and sodium nitrate (Chile saltpeter), found primarily in the desert regions of Chile and other parts of South America. Nitrogen is also an essential component of the proteins found in all living organisms.
Credit for the discovery of nitrogen in 1772 is usually given to Scottish physician Daniel Rutherford (1749–1819). Three other scientists, Henry Cavendish, Joseph Priestley, and Carl Scheele, could also claim to have discovered the element at about the same time. Nitrogen was first identified as the product left behind when a substance is burned in a closed sample of air (which removed the oxygen component of air).
Uses. The industrial uses of nitrogen have increased dramatically in the past few decades. It now ranks as the second most widely produced chemical
in the United States with an annual production of about 57 billion pounds (26 billion kilograms).
The element's most important applications depend on its chemical inertness (inactivity). It is widely used as a blanketing atmosphere in metallurgical processes where the presence of oxygen would be harmful. In the processing of iron and steel, for example, a blanket of nitrogen placed above the metals prevents their reacting with oxygen, which would form undesirable oxides in the final products.
The purging (freeing of sediment or trapped air) of tanks, pipes, and other kinds of containers with nitrogen can also prevent the possibility of fires. In the petroleum industry, for example, the processing of organic compounds in the presence of air creates the potential for fires—fires that can be avoided by covering the reactants with pure nitrogen.
Nitrogen is also used in the production of electronic components. Assembly of computer chips and other electronic devices can take place with all materials submerged in a nitrogen atmosphere, preventing oxidation of any of the materials in use. Nitrogen is often used as a protective agent during the processing of foods so that decay (oxidation) does not occur.
Another critical use of nitrogen is in the production of ammonia by the Haber process, named after its inventor, German chemist Fritz Haber (1868–1934). The Haber process involves the direct synthesis of ammonia from its elements—nitrogen and hydrogen. The two gases are combined under specific conditions: (1) the temperature must be 500 to 700°C (900 to 1300°F), (2) the pressure must be several hundred atmospheres, and (3) a catalyst (something that speeds up chemical reactions) such as finely divided nickel must be present. One of the major uses of the ammonia produced by this method is in the production of synthetic fertilizers.
About one-third of all nitrogen produced is used in its liquid form. For example, liquid nitrogen is used for quick-freezing foods and for preserving foods in transit. Additionally, the very low temperatures of liquid nitrogen make some materials easier to handle. For example, most forms of rubber are too soft and pliable for machining at room temperature. They can, however, first be cooled in liquid nitrogen and then handled in a much more rigid form.
Three compounds of nitrogen are also commercially important and traditionally rank among the top 25 chemicals produced in the United States. They are ammonia (number 6 in 1990), nitric acid (number 13 in 1990), and ammonium nitrate (number 14 in 1990). All three of these compounds are used extensively in agriculture as synthetic fertilizers. More than 80 percent of the ammonia produced, for example, goes into the production of synthetic fertilizers.
In addition to its agricultural role, nitric acid is an important raw material in the production of explosives. Trinitrotoluene (TNT), gunpowder, nitroglycerin, dynamite, and smokeless powder are all examples of the kind of explosives made from nitric acid. Slightly more than 5 percent of the nitric acid produced is also used in the synthesis of adipic acid and related compounds used in the manufacture of nylon.
Phosphorus exists in three allotropic forms (physically or chemically different forms of the same substance): white, red, and black. The white form of phosphorus is a highly active, waxy solid that catches fire spontaneously when exposed to air. In contrast, red phosphorus is a reddish powder that is relatively inert (inactive). It does not catch fire unless exposed to an open flame. The melting point of phosphorus is 44°C (111°F), and its boiling point is 280°C (536°F). It is the eleventh most abundant element in Earth's crust.
Phosphorus always occurs in the form of a phosphate, a compound consisting of phosphorus, oxygen, and at least one more element. By far the most abundant source of phosphorus on Earth is a family of minerals known as the apatites. Apatites contain phosphorus, oxygen, calcium, and a halogen (chlorine, fluorine, bromine, or iodine). The state of Florida is the world's largest producer of phosphorus and is responsible for about a third of all the element produced in the world.
Phosphorus also occurs in all living organisms, most abundantly in bones, teeth, horn, and similar materials. It is found in all cells, however, in the form of compounds essential to the survival of all life. Like carbon and nitrogen, phosphorus is cycled through the environment. But since it has no common gaseous compounds, the phosphorus cycle occurs entirely within the solid and liquid (water) portions of Earth's crust.
Uses. About 95 percent of all the phosphorus used in industry goes to the production of phosphorus compounds. By far the most important of these is phosphoric acid, which accounts for about 83 percent of all phosphorus use in industry. A minor use is in the manufacture of safety matches.
Phosphoric acid. Phosphoric acid (H 3 PO 4 ) typically ranks about number seven among the chemicals most widely produced in the United States. It is converted to a variety of forms, all of which are then used in the manufacture of synthetic fertilizer, accounting for about 85 percent of all the acid produced. Other applications of phosphoric acid include the production of soaps and detergents, water treatment, the cleaning and rustproofing of metals, the manufacture of gasoline additives, and the production of animal feeds.
At one time, large amounts of phosphoric acid were converted to a compound known as sodium tripolyphosphate (STPP). STPP, in turn, was used in the manufacture of synthetic detergents. When STPP is released to the environment, however, it serves as a primary nutrient for algae in bodies of water such as ponds and lakes. The growth of huge algal blooms in the 1970s and 1980s as a result of phosphate discharges eventually led to bans on the use of this compound in detergents. As a consequence, the compound is no longer commercially important.
Arsenic and antimony
Arsenic and antimony are both metalloids. That is, they behave at times like metals and at times like nonmetals. Arsenic is a silver-gray brittle metal that tarnishes when exposed to air. It exists in two allotropic forms: black and yellow. Its melting point is 817°C (1502°F) at 28 atmospheres of pressure, and its boiling point is 613°C (1135°F), at which temperature it sublimes (passes directly from the solid to the vapor state).
Antimony also occurs in two allotropic forms: black and yellow. It is a silver-white solid with a melting point of 630°C (1170°F) and a boiling point of 1635°C (2980°F). Both arsenic and antimony were identified before the birth of modern chemistry—at least as early as the fifteenth century.
Arsenic is a relatively uncommon element in Earth's crust, ranking number 51 in order of abundance. It is actually produced commercially from the flue dust obtained from copper and lead smelters (metals separated by melting) since it generally occurs in combination with these two elements.
Antimony is much less common in Earth's crust than is arsenic, ranking number 62 among the elements. It occurs most often as the mineral stibnite (antimony sulfide), from which it is obtained in a reaction with iron metal.
Uses. Arsenic is widely employed in the production of alloys (a mixture of two or more metals or a metal and a nonmetal) used in shot, batteries, cable covering, boiler tubes, and special kinds of solder (a melted metallic alloy used to join together other metallic surfaces). In a very pure form, it is an essential component of many electronic devices. Traditionally, compounds of arsenic have been used to kill rats and other pests, although it has largely been replaced for that purpose by other products.
Antimony is also a popular alloying element. Its alloys can be found in ball bearings, batteries, ammunition, solder, type metal, sheet pipe, and other applications. Its application in type metal reflects an especially interesting property: unlike most materials, antimony expands as it cools and solidifies from a liquid. Because of that fact, type metal poured into dies in the shape of letters expands as it cools to fill all parts of the die. Letters formed in this process have clear, sharp edges.
Bismuth is a typical silvery metal with an interesting reddish tinge to it. It has a melting point of 271°C (520°F) and a boiling point of 1560°C (2840°F). It is one of the rarest elements in Earth's crust, ranking 69 out of 75 elements for which estimates have been made. It occurs most commonly as the mineral bismite (bismuth oxide), bismuthinite (bismuth sulfide) and bismutite (bismuth oxycarbonate). Like arsenic and antimony, bismuth was identified as early as the fifteenth century by the pre-chemists known as the alchemists.
Nearly all of the bismuth produced commercially is used for one of two applications: in the production of alloys or other metallic products and in pharmaceuticals. Some of its most interesting alloys are those that melt at low temperatures and that can be used, for example, in automatic sprinkler systems. Compounds of bismuth are used to treat upset stomach, eczema (a skin disorder), and ulcers, and in the manufacture of face powders.