Nitrogen Group Elements

Type of physical science: Chemistry

Field of study: Chemistry of the elements

The nitrogen group elements consist of nitrogen, phosphorus, arsenic, antimony, and bismuth. Nitrogen and phosphorus are the most common elements and, together with their compounds, have many important commercial applications. They also play essential roles in biological processes.

Overview

The nitrogen group elements consist of five elements--nitrogen, phosphorus, arsenic, antimony, and bismuth--and compose group V of the periodic table. The chemical symbols for the elements are N, P, As, Sb, and Bi, respectively. The symbol for antimony is derived from stibium, the Latin word for the element. Nitrogen and phosphorus are both nonmetals, while arsenic and antimony are semimetals, or metalloids, which have properties of both nonmetals and metals. Antimony is generally considered more distinctly metallic than arsenic, and bismuth is a metal.

The discovery of the nitrogen group elements spans several thousand years. Ancient civilizations dating back to 4,000 B.C. were familiar with antimony compounds. Reference to a black substance called stibnite (a mineral composed of antimony and sulfur) can be found in the Old Testament and was supposedly used by Queen Jezebel as a cosmetic to darken her eyes.

Pliny, a first-century Greek scholar, wrote of the medicinal uses of this substance. Arsenic compounds were also well known in the ancient world. The minerals orpiment and realgar (both compounds of arsenic and sulfur) were valued because of their bright colors (yellow and red, respectively), and were used by primitive people to decorate their faces and to paint pottery.

Alloys containing elemental antimony were used in the fifteenth century for bells and mirrors.

Bismuth also was probably known in ancient times, since it occurs in nature as a free metal. It is similar in appearance to other metals such as lead and tin, however, and therefore was not clearly recognized as a distinct metal until the eighteenth century, when it was extracted from minerals by a Frenchman, Claude-Francois Geoffroy, in 1753. Arsenic compounds were used in small doses by early physicians. While small doses of arsenic can be tolerated by humans, arsenic is quite lethal to some microorganisms that cause disease. Although ancient physicians knew nothing about microorganisms or their relationship to illness, it is likely that the medicinal value of some arsenic compounds was recognized by trial and error. Since in larger doses it is toxic, arsenic has also been employed by assassins throughout the ages. There has long been debate over the fate of Napoleon, who apparently died from arsenic poisoning, and whether his ingestion of arsenic was intentional or accidental. Accidental arsenic poisoning was common in the eighteenth and nineteenth centuries, since arsenic compounds were used to color paints and wallpaper. At one point, even candies were colored with arsenic compounds. It is generally believed that the element itself was probably isolated around 1250 by Albert Magnus. It is known for certain, however, that Johann Schroeder prepared it in 1649 by reducing the oxide with charcoal. Phosphorus was accidentally discovered in 1669 by a German alchemist, Henning Brand. Brand extracted the element from urine and observed that on exposure to air it produced phosphorescence (glowed in the dark), and as a result, the element remained a curiosity for many years. Later, phosphorus was extracted from bones and, subsequently, minerals such as phosphate rock. Nitrogen was the most recently discovered group V element. Its discovery is generally credited to Daniel Rutherford, who isolated it from air in 1772; however, other noted European chemists of the period, such as Carl Scheel, Henry Cavendish, and Joseph Priestley, were aware of the existence of an inert gas in air.

Collectively, the group V elements constitute less than 0.2 percent, by mass, of the earth's crust (including the atmosphere). Almost half of this (0.11 percent) is phosphorus, and 0.03 percent is nitrogen. Arsenic, antimony, and bismuth all have very low natural abundances.

Nitrogen is distributed throughout the earth, and the atmosphere is composed of about 78 percent nitrogen by mass (or more than 4,000 trillion tons). By contrast, the atmosphere of Mars is 2.6 percent nitrogen. The earth's atmosphere also contains small amounts of gaseous nitrogen compounds such as nitrogen oxides and nitric acid, which are formed during electrical storms and as a result of atmospheric pollution from automobiles and aircraft. Nitrogen occurs in only a few minerals, saltpeter (potassium nitrate) and Chile saltpeter (sodium nitrate) being the most abundant. The other group V elements all occur in mineral form. About two hundred minerals containing phosphorus are known, the most common being a series called the apatites, which are principally composed of calcium, phosphorus, oxygen, and group VII elements. Common minerals of arsenic include arsenopyrite (composed of arsenic, iron, and sulfur) and realgar and orpiment (referred to above). Antimony occurs in low concentrations in more than a hundred minerals and most commonly is found in stibnite. Bismuth is found chiefly in bismite (bismuth oxide), bismutite (bismuth carbonate), and bismuthinite (bismuth sulfide). Small amounts of metallic bismuth are found free in nature.

Nitrogen is obtained commercially by fractional distillation of liquefied air. This process exploits the different boiling points of the components of air. Liquid nitrogen boils at a slightly lower temperature than liquid oxygen, and so the two elements (which collectively make up about 99 percent of air) can be separated. Nitrogen is stored and transported in the liquid state in insulated containers, or as a compressed gas under high pressure in metal cylinders. The largest source of phosphorus is the apatite minerals, sometimes collectively referred to as phosphate rock. This is heated with sand and carbon to more than 1,000 degrees Celsius to produce phosphorus as a gas, which is then passed through water, where it solidifies. Arsenic is obtained from the mineral arsenopyrite. This is heated in air to form arsenic oxide, which is then reduced with carbon to the element. Antimony is similarly obtained from stibnite, or, alternatively, by reducing stibnite with molten scrap iron. Bismuth can be extracted from bismuthinite by roasting in air, followed by reduction with carbon. Most bismuth, however, is produced as a by-product in the refining of other metals such as lead, tin, silver, and gold.

As is often the case, the origin of an element's name is frequently related to a specific property or characteristic of either the element itself or a common compound in which the element is found. Nitrogen is so named because it was found to be present in the mineral saltpeter. This was known in ancient times as "niter," a term coming from the Greek word nitron. The ability of phosphorus to emit light resulted in its being named from the Greek phosphoros, meaning "light-bearing." The precise origins of the names of the other group V elements are uncertain. Arsenic possibly is derived from the Greek arsenikos, meaning "male," because of the ancient belief that some elements had sexes. This, in turn, may have come from an ancient Persian word, zarnick (zar meaning "gold"), which referred to the yellow color of the arsenic mineral orpiment. Since antimony was commonly found combined with other metals in numerous minerals, it likely derives its name from the Greek anti plus monos, meaning "not alone." "Bismuth" possibly comes from the German weisse Masse, meaning "white mass," since some of its white compounds were commonly employed as early cosmetics. The name was Latinized to "bisemutum."

While the nitrogen group elements share certain chemical similarities, variation in their chemical and physical properties indicates that they are distinct from one another. At room temperature, nitrogen is a colorless, odorless gas, while the other elements are all solids. At low temperature (-196 degrees Celsius), nitrogen condenses to a colorless liquid, and at -210 degrees Celsius it freezes. Nitrogen has a low solubility in water, 0.023 cubic centimeter per cubic centimeter of water at 0 degrees Celsius. Nitrogen consists of diatomic molecules (molecules composed of two atoms) that are held together by a strong chemical bond (a covalent triple bond). Since diatomic molecules generally need to be converted into atoms in order to react chemically, the existence of strong forces holding the atoms together makes nitrogen very unreactive, or inert. This also explains why very few minerals exist that contain nitrogen, since an inert element would not be expected to combine readily with other substances to form compounds in nature. By contrast, phosphorus is an extremely reactive element and must be stored underwater to prevent spontaneous combustion, which occurs when it is exposed to oxygen in the air. In addition, there are a large number of phosphorus-containing minerals, which further indicates the element's high reactivity.

Many elements (for example, carbon, sulfur, and oxygen) exist in several forms having different structures that are called allotropes. Under normal conditions, nitrogen exists in only one form, but most of the other group V elements have several allotropes. Phosphorus has at least four allotropes, the two most common being a red and a white form (the latter is sometimes yellow as a result of impurities). White phosphorus is a waxy solid, a highly reactive form of the element that takes fire spontaneously in air. It is extremely poisonous (a lethal dose is about 0.05 grams) and may cause severe burns if it contacts the skin. It melts at 44 degrees Celsius. When heated to more than 200 degrees Celsius, white phosphorus changes into the red form that is less toxic and much less reactive. Molecules of white phosphorus consist of four atoms arranged in a tetrahedral shape, while red phosphorus is composed of many of these molecules joined together.

Only white phosphorus emits light. Arsenic and antimony are both brittle solids and are poor conductors of heat and electricity. Both exist in allotropic forms. The common allotrope for both has a metallic appearance that has a steel-gray color for arsenic and a bluish-white luster for antimony. Their melting points are 817 degrees Celsius and 631 degrees Celsius, respectively.

Bismuth is a brittle metal and has a silvery metallic luster with a pinkish tinge. Unlike most substances, it expands (by more than 3 percent) when it solidifies from the molten state. With the exception of mercury, bismuth has the poorest heat conductivity of any metal and is also a poor conductor of electricity. The metal melts at 271 degrees Celsius.

All the group V elements react with oxygen under the appropriate conditions. Although very unreactive at ordinary temperatures, nitrogen reacts with oxygen when heated or subjected to electric sparks to produce a series of nitrogen oxide compounds. More than half a dozen nitrogen oxides are known, and they differ in their nitrogen to oxygen ratio. For example, molecules of nitrous oxide, or laughing gas, contain two atoms of nitrogen to one atom of oxygen. Phosphorus readily combines with oxygen when it ignites in air. Arsenic, antimony, and bismuth react with oxygen when heated and form the corresponding oxides. Generally, the group V elements react with oxygen in combining ratios of either 2:3 or 2:5 to form compounds in which the elements have oxidation states of plus three or plus five, respectively. Oxidation states are charges assigned to an atom that result when it gains or loses electrons through electron transfer or sharing during reactions with other atoms. Since oxygen has a much greater electronegativity than the group V elements (that is, a greater affinity for electrons), it generally shares three or five of the group V elements' valence electrons (electrons that are the farthest away from the nucleus). The acidity of the group V oxides decreases down the group. Thus, oxides of nitrogen and phosphorus are acidic, antimony oxides are amphoteric (can behave as either acids or bases), and bismuth oxide is basic. This is a common trend observed for oxides of elements in groups III through VII.

Nitrogen and phosphorus also combine with other elements including metals and nonmetals. When two elements combine to form a compound, electrons are attracted more strongly to the element having the highest electronegativity. Consequently, the reaction of nitrogen or phosphorus with elements of lower electronegativities produces compounds in which nitrogen or phosphorus generally have negative oxidation numbers, commonly -3 for nitrogen and -3 or -5 for phosphorus. Reactions of the group V elements with nonmetals having higher electronegativities (for example, oxygen or fluorine) produce positive oxidation numbers. Most of the group V elements react with hydrogen, sulfur, and the group VII elements (for example, chlorine and fluorine). The reaction between nitrogen and hydrogen, which occurs at high temperature, under pressure, and in the presence of a catalyst, produces ammonia and is an important industrial procedure known as the Haber process.

Applications

Nitrogen is the only group V element that has considerable commercial application in its elemental form. Because of the low temperature of liquid nitrogen, it is used in the food industry for rapid freezing of food and in refrigeration systems. Similarly, in medicine, it is used to freeze quickly biological samples such as blood, bone marrow, tissue, and semen. Scientists use liquid nitrogen for experiments that require very low temperatures. In the 1980's, materials were discovered that became superconductors at the temperature of liquid nitrogen.

Superconductors carry electricity without the energy loss that normally accompanies electricity transmission through metallic wires. Liquid nitrogen is also used by the oil industry to build up large pressures in wells, to force crude oil upward. Uses for nitrogen gas result from the inert nature of nitrogen. It provides an inert atmosphere for packaging foods and wine, thereby preventing spoilage through oxidation or mold formation. Other applications are pressurizing electric cables and telephone wires, use in the metals industry (for example, welding, soldering and brazing), and as a propellant gas in aerosol cans.

Because of its easy combustion in air, white phosphorus was first used in the seventeenth century to make matches, but workers in these early match factories became ill as a result of the element's toxicity, which led to phosphorus being replaced by less hazardous materials. The heads of strike-anywhere matches were manufactured from a compound of phosphorus and sulfur (phosphorus sulfide). Later, in 1855, safety matches were invented, and these used the less hazardous red phosphorus, which was a component, together with powdered glass and glue, in the striking surface used to ignite the match. The heads of these matches contained a compound of another group V element, antimony sulfide. Contemporary uses for the element include fireworks, incendiaries, smoke bombs, tracer bullets, and rodent poisons. Most phosphorus is converted into compounds.

Arsenic, antimony, and bismuth are all used in alloys. Arsenic is alloyed with lead to make lead shot, since it improves the roundness of the molten drops during the manufacture of the shot. It also improves the corrosion and thermal properties of copper and brass. Both arsenic and antimony are added to lead that is used in storage batteries, tanks, pipes, and pumps, and they also enhance the corrosion resistance of these materials. Pewter contains about 7 percent antimony. Very pure arsenic and antimony are used in semiconductors. Bismuth is used in alloys as a result of its unique property of expanding when the molten metal cools. This makes it important for alloys used in casting, which must expand to fill all the space in a mold. When mixed with other metals, such as lead, tin, and cadmium, bismuth forms fusible alloys that melt at very low temperatures, even in hot water. For example, Rose's metal (50 percent bismuth, 25 percent lead, 25 percent tin) melts at 94 degrees Celsius, and Wood's metal (50 percent bismuth, 25 percent lead, 12.5 percent tin, 12.5 percent cadium) melts at about 65 degrees Celsius. These low-melting alloys are useful for making electric fuses and safety plugs for boilers, and automatic sprinkler systems used in fire detection. In the case of the latter, heat from a fire melts the safety plugs, turning on the water flow to extinguish the fire. An alloy of bismuth and manganese called bismanol has been found to be an effective permanent magnet.

Compounds of the group V elements, particularly nitrogen and phosphorus, have many important applications. Since they are essential elements for plant growth, nitrogen and phosphorus compounds are major components of fertilizers. Compounds used in agriculture as sources of nitrogen include ammonia and ammonium sulfate. Ammonia is applied to soil either as pure ammonia or dissolved in water, and ammonium sulfate is a solid fertilizer. Sources of phosphorus include phosphoric acid and calcium phosphates. Together with potassium, nitrogen and phosphorus constitute the three primary fertilizer nutrients. Commercial fertilizers are rated by their available nitrogen, phosphorus, and potassium percentage ratios. For example, a 5-10-5 fertilizer contains 5 percent, 10 percent, and 5 percent of nitrogen, phosphorus, and potassium compounds, respectively.

More than 25 percent of all ammonia produced is used in agriculture, and the remainder is used to produce other compounds involved in nylon production, in making detergents, in water purification, and in the production of pharmaceuticals. Ammonia, which is a gas at room temperature, is produced by the Haber process. This industrial process was developed early in the twentieth century by the German chemist Fritz Haber, who received the 1918 Nobel prize in Chemistry for developing this process. This created a considerable amount of controversy, however, since Haber also worked for the German chemical-warfare service at the start of World War I, where he supervised the first use of chlorine gas in battle as a chemical-warfare agent.

Other important compounds of nitrogen and phosphorus are used in the production of fertilizers, synthetic fabrics, drugs, and explosives such as TNT and nitroglycerin (nitric acid); in metallurgy processes, electroplating, and insecticides (sodium cyanide); in explosives, herbicides, insecticides, and rocket propellants (ammonium nitrate); in food preservatives (sodium nitrite); in safety matches (phosphorus sulfide); and in soaps, detergents, soft drinks, and rustproofing (phosphoric acid). Nerve gases, which are of potential use in chemical warfare, are organic derivatives of phosphorus and are some of the most toxic substances known. Compounds of arsenic and antimony are used in pesticides (sodium arsenite and arsenic oxide), in paint pigments (antimony trioxide and antimony sulfide), and in ceramic enamels and as flameproofing agents (antimony trioxide). Compounds of bismuth are used in cosmetics and to make artificial pearls (bismuth oxychloride and bismuth subnitrate).

All organisms need nitrogen and phosphorus to survive. Both are essential elements in DNA and RNA, the complex molecules responsible for transferring genetic information in cells.

Nitrogen is also an essential element in proteins, and phosphorus compounds are found in bone, nerve, and muscle tissue and in teeth. Although arsenic is toxic, trace amounts are actually needed for the growth of red blood cells in bone marrow, and the average healthy human body contains about 0.007 gram of arsenic. A process known as nitrogen fixation is an important source of nutrients for plants. Microorganisms such as bacteria convert nitrogen from the atmosphere directly into water-soluble nitrogen compounds, which plants can absorb through their roots. These bacteria inhabit the roots of certain plants such as alfalfa, clover, beans, and peas. When harvested, these plants leave nitrogen-rich roots in the ground, which can be plowed into the soil to provide nitrogen for the next season's crops.

Context

Nitrogen and phosphorus are the most important group V elements. Together with their compounds, they are of enormous economic significance to the chemical industry and in general to countries that manufacture them. In particular, the widespread use of nitrogen and phosphorus compounds in agriculture has been of enormous value in maintaining the world's food supply.

Compounds of nitrogen are used in explosives and therefore are essential for military purposes as well as for nonmilitary operations such as mining, tunneling, demolition, and quarrying. These peaceful uses of explosives have made possible the construction of roads and bridges, where large amounts of rock and earth required moving. Liquid nitrogen has been extremely important in the areas of medicine and food preparation and in the development of scientific research and technology such as superconductivity. Since it is inexpensive and easily made, it is readily available for use in hospitals, universities, and industry where low temperatures are needed.

Principal terms

COMPOUND: a pure substance that is composed of at least two kinds of atoms

ELEMENT: a pure substance that contains only one kind of atom

GROUP: a vertical column of elements in the periodic table, the members of which have similar chemical and physical properties

OXIDES: compounds composed of oxygen and another element; for example, bismuth oxide is composed of bismuth and oxygen

REDUCTION: the process that occurs when an atom, ion, or molecule gains electrons; substances that donate electrons, such as carbon and hydrogen, are called reducing agents and are used to extract elements from their compounds

Bibliography

Andrew, S. P. S. "The Fixation of Nitrogen." EDUCATION IN CHEMISTRY 14 (July, 1978): 114. An interesting and easy-to-read account of nature's ability to convert nitrogen from the atmosphere into soluble nitrogen compounds.

Maugh, T. H. "It Isn't Easy Being King." SCIENCE 203 (1979): 637. An interesting article that deals with arsenic poisoning in the Middle Ages.

McQuarrie, Donald A., and Peter A. Rock. DESCRIPTIVE CHEMISTRY. New York: W. H. Freeman, 1985. Chapter 7 contains a description of the physical properties, manufacture, and uses of the group V elements. The section contains a number of color photographs of the elements, minerals, and compounds.

Rochow, Eugene G. THE METALLOIDS. Boston: D. C. Heath, 1966. This small book has a section (chapter 6) that deals with arsenic and antimony. It contains some interesting historical information and some basic chemical facts about these elements and their compounds.

Weast, Robert C., ed. HANDBOOK OF CHEMISTRY AND PHYSICS. 66th ed. Boca Raton, Fla.: CRC Press, 1986. Section B5 of this edition contains a half-page or so description of every element. Information on the discovery, occurrence, physical properties, manufacture, uses, and important compounds of the group V elements can be found here. Revised and updated annually.

Acids and Bases