Group 5A (or VA) of the periodic table are the pnictogens: the nonmetals nitrogen (N), and phosphorus (P), the metalloids arsenic (As) and antimony (Sb), and the metal bismuth (Bi). The name "pnictogen" is not in common usage; it derived from the Greek work pnigein, "choke" or "stifle," which is a property of breathing pure nitrogen gas.
The Group 5A elements have five valence electrons in their highest-energy orbitals (ns2np3). Nitrogen, phosphorus, and arsenic can form ionic compounds by gaining three electrons, forming the nitride (N3-), phosphide (P3-) and arsenide (As3-) anions, but they more frequently form compounds through covalent bonding. Antimony and bismuth can lose either their outermost p electrons to form 3+ charges, or their outermost s and p electrons to form 5+ charges.
Nitrogen (N, Z=7).
Nitrogen is a colorless, odorless gas; in its elemental form, nitrogen is found as the diatomic molecule N2, in which the two nitrogen atoms are held together by a triple bond. The name of the element is derived from the Latin words nitron and genes, for "nitre (potassium nitrate) forming." Elemental nitrogen comprises about 78% of the Earth's atmosphere; nitrogen is found in the Earth's crust at a concentration of 25 ppm, making it the 30th most abundant element. It is found in the ground primarily in the form of ores containing the nitrate (NO3-) anion, such as nitratine [also known as Chile saltpeter, sodium nitrate, NaNO3], nitrobarite [barium nitrate, Ba(NO3)2], and nitrocalcite [calcium nitrate, Ca(NO3)2].
The nitrogen-nitrogen triple bond is very strong and very difficult to break, so elemental nitrogen is a fairly inert substance. Since nitrogen is fairly inert, it is used in some environments where it is desirable to exclude oxygen (or water vapor). Elemental nitrogen is nonpolar, and has a very low boiling point (77.4K, -195.8ºC, or -320.4ºF). Liquid nitrogen, which is obtained by fractional distillation from liquefied air, is used in the frozen food industry to freeze foods very quickly; it is also used to freeze blood and genetic material.
Nitrogen from the air is converted by some organisms to ammonia, NH3, in a process called nitrogen fixation. (Chemists who study nitrogen obsessively might also be said to have a "nitrogen fixation," but that's not the same thing!) Some species of bacteria form nodules on the roots of legumes (beans), alfalfa, and clover, providing them with nitrogen in exchange for carbohydrates. The nitrogenase enzyme in these bacteria reduces N2 from the atmosphere into ammonia, which can then be converted into nitrate (NO3-), which becomes incorporated into a wide variety of biologically important molecules. Industrially, the conversion of nitrogen to ammonia is accomplished by the Haber synthesis (Fritz Haber, 1905, Nobel Prize in Chemistry, 1918), in which nitrogen gas reacts with hydrogen gas at temperatures of around 400°C at 200 to 300 atmospheres of pressure, using a catalyst of fused MgO, Al2O3, and SiO2. The ammonia that produced is removed by refrigerating it until it liquefies at -33.4°C.
N2(g) + 3H2(g) ® 2NH3(g)
Ammonia is a weak base; in solution, it forms the ammonium ion, NH4+, which is the form that nitrogen is found in most fertilizers, in the salts ammonium nitrate, NH4NO3, and ammonium sulfate, (NH4)2SO4.
The name "ammonia" is derived from the name sal ammoniac, a white powder produced in kilns used for burning camel dung outside the Temple of Ammon in Ancient Egypt. This substance is what we now know as ammonium chloride, NH4Cl.
Nitrogen is found in a wide variety of organic compounds. Compounds which contain carbon-nitrogen single bonds are called amines, and may be thought of as organic derivatives of ammonia (with carbon groups replacing the hydrogen atoms). Amines are weakly basic, and are present in many pharmaceutical compounds, often referred to as alkaloids because of their basicity. If the nitrogen is connected to a carbon which is double-bonded to an oxygen atom, the functional group is called an amide; amino acids are connected together to form proteins by amide linkages. Carbon-nitrogen double bonds, called imines, are also very common. Nitrogen is incorporated into a large number of organic compounds of tremendous biological importance, such as the amino acids, nucleic acids, ATP, proteins, DNA, RNA, etc. Green plants contain a molecule called chlorophyll, which consists of a flat ring of carbon and nitrogen atoms with a large open space in the middle, in which a magnesium ion is bound, held in place by the nitrogen atoms. The hemoglobin molecule contains a similar ring structure, which an iron ion bound in the cavity; this molecule is incorporated into a protein called hemoglobin, which is responsible for transporting oxygen in the bloodstream.
Nitrogen forms many compounds with oxygen. One of the most important of these is nitric oxide, NO, also known as nitrogen monoxide, which is produced in the body from the amino acid arginine; it acts as a vasodilator, causing blood vessels to relax and increasing blood flow. Some heart medications, such as nitroglycerin and amyl nitrate, increase the amount of nitric oxide in the blood, allowing the delivery of more blood to the heart during an episode of heart pain (angina) or heart attack. (The drug Viagra works by a similar mechanism, except that the blood is delivered elsewhere.) Nitric oxide also acts as a neurotransmitter. In the atmosphere, nitric oxide is a pollutant, produced in automobile exhaust and power plants; it can be converted into nitrogen dioxide and nitric acid, HNO3, which leads to increased acidity levels in rain. Nitrous oxide, N2O, also known as dinitrogen monoxide or laughing gas, is a mild anesthetic used in dentistry and surgery; it is also used as an aerosol propellant in cans of whipped cream. Nitrogen dioxide, NO2, is produced in the exhaust from motor vehicles, and reacts with hydrocarbons in the air to produce photochemical smog.
Nitrogen is also found in a number of explosives. Gunpowder contains potassium nitrate, KNO3, also known as saltpeter, which is a powerful oxidizing agent; the other components of gunpowder are sulfur, and coal. Nitroglycerin is made from the molecule glycerin (or glycerol), which is a chain of three carbon atoms, each of which has an OH group on it; in nitroglycerine, the H on the OH is replaced by the nitro group, NO2. Nitroglycerin is dangerously unstable, but when mixed with kieselguhr (diatomaceous earth), which acts as an adsorbent, it is safer to handle, and does not explode until set off by a smaller explosion from a detonator such as a blasting cap. Dynamite was invented by the Swedish chemist Alfred Nobel in 1867; on his death, his fortune was used to establish the Nobel Prizes, which are awarded for outstanding achievements in the sciences, literature, and peace. TNT (2,4,6-trinitrotoluene), is a benzene ring with a CH3 group attached (forming the toluene molecule) to which three nitro groups are attached (which is accomplished by the use of a mixture of concentrated nitric and sulfuric acids). TNT is stable for long periods of time, and is not shock sensitive as nitroglycerin is. HMX, also known as octogen or cyclotetramethylene-tetranitramine, consists of a ring four carbon atoms and four nitrogen atoms, with each nitrogen atom having one nitro group bonded to it; this substance is used in plastic explosives, rocket propellants, and the detonator in nuclear warheads. RDX, also known as cyclotrimethylenetrinitramine, cyclonite, hexogen, and T4, contains a ring of three carbon atoms and three nitrogen atoms, with each nitrogen bonded to a nitro group; it is used in mixtures with TNT, in plastic explosives, and in nuclear weapons. The ammonium nitrate and sulfate found in fertilizers can also produce powerful explosions under the "right" conditions: a ship in the harbor in Texas City which was carrying 5000 tons of ammonium nitrate exploded on April 15, 1947, killing over 500 people; the Alfred P. Murrah Federal Building in Oklahoma City was destroyed by a bomb made from ammonium nitrate and nitromethane (a motor fuel), killing 168 people.
Nitrogen gas is also what inflates the ubiquitous "air" bags that are used as safety devices in automobiles. An airbag canister contains about 200 grams of sodium azide, NaN3; when a collision is detected, a sensor sends an electric current through the sample, causing it to decompose into sodium metal (which is converted into sodium salts by other reagents) and nitrogen gas, which can inflate the cushioning bag in 15 to 25 milliseconds.
Nitrogen is also found in acids, such as nitric acid, HNO3, which is used organic synthesis, in the manufacture of explosives (usually in combination with sulfuric acid to generate the nitronium ion, NO2+), and in the refining of some metals. When mixed with hydrochloric acid, it forms a solution called aqua regia ("royal water"), which is capable of dissolving even extremely unreactive metals such as gold and platinum.
Deep-sea divers can suffer from a condition called "the bends" because of the increased amount of nitrogen dissolved in their blood at high pressure. Increasing the pressure on a gas increases the solubility of a gas in a liquid (Henry's Law); one application of this is seen when a can of soda is opened — at high pressure (in an unopened can, about 4 atmospheres of pressure), carbon dioxide is dissolved in the water in the soda, but when the can is opened, the pressure is released, and the dissolved gas comes bubbling out of the solution. Divers who have been underwater for a long time, especially at great depths, where the pressure is even higher, have increased amounts of nitrogen dissolved in their bloodstream; if they surface too quickly, the nitrogen can come bubbling out of the bloodstream, causing pain in the joints, dizziness, seizures, paralysis, or death. Coming back to the surface slowly allows time for the nitrogen to come out of solution without forming bubbles; alternatively, a breathing mixture of oxygen and helium can be used, since helium is less soluble in the bloodstream than nitrogen.
Phosphorus (P, Z=15).
Phosphorus is a nonmetallic element that exists in several allotropic forms (see below). It is found in the Earth's crust at a concentration of 1000 ppm, making it the 11th most abundant element. It is found in ores called phosphate rocks, which consist primarily of various apatite minerals, such as hydroxylapatite [Ca5(PO4)3(OH)], fluorapatite [Ca5(PO4)3F], and chlorapatite [Ca5(PO4)3Cl].
Phosphorus is found in three main forms:
- White phosphorus is a soft, waxy, flammable substance, consisting of tetrahedral P4 molecules; it is often slightly yellowish because of the presence of impurities (hence, it is sometimes imaginatively known as yellow phosphorus). White phosphorus is highly reactive, and spontaneously ignites at about 30°C in moist air. It is usually stored under water, to prevent exposure to the air. It is also extremely toxic, even in very small quantities.
- Red phosphorus consists of cross-linked molecules of phosphorus, and is stable at room temperature, but can be converted to the more reactive white phosphorus by heat, sunlight, or friction. Red phosphorus is used on the strike surface of the box that safety matches are stored in; the friction caused by dragging the match head across the rough surface converts some of the red phosphorus to white phosphorus, which spontaneously ignites, causing the match head to ignite. Red phosphorus is also used in fireworks, and other explosives.
- Black phosphorus consists of hexagonal sheets of phosphorus atoms (similar in structure to graphite), and is the least reactive form. It has little commercial value, but can be converted to white phosphorus by heating it under pressure.
Phosphorus was the first element to have been discovered since ancient times, although compounds of phosphorus had been known for millennia. It was first isolated by Hennig Brandt, an alchemist from Hamburg, in 1669, from the residue obtained from concentrated, putrefied urine. The name comes from the Greek words phos ("light") and phoros ("bringing"), and refers to the fact that elemental phosphorus glows in the dark, and spontaneously bursts into flame in air. (See this page for a demonstration of the burning of phosphorus.)
Phosphorus is found in the body in the form of phosphate anion, PO43-; usually one or more of the oxygen atoms are connected to carbon groups to form organophosphates. Phosphate groups are used to store chemical energy in the adenosine triphosphate molecule (ATP); phosphate groups hold together the sugar molecules that contain the nucleic acids in DNA and RNA; and phospholipids (fats which contain phosphate groups) are a major component of cell membranes. Phosphorus is stored in the skeletons of animals in the form of hydroxyapatite crystals, [Ca3(PO4)2]3·Ca(OH)2). Phosphate salts are also widely used in fertilizers.
White phosphorus was used in some of the earliest matches, but there were a number of safety problems associated with them. They could ignite far too easily, and were responsible for many (unplanned) fires, and the workers who made the matches suffered from a high incidence of phosphorus poisoning. Chronic exposure to phosphorus can cause a condition called "phossy jaw," in which the jaw bone slowly erodes away. Modern strike-anywhere matches use a non-toxic phosphorus compound called phosphorus sesquisulfide, P4S3, while safety matches use red phosphorus, which is converted to white phosphorus by the friction of dragging the match head across the rough surface of the matchbox.
Phosphorus has also found its way into warfare, and is used in tracer bullets, incendiary bombs, and some of the nerve gases (including sarin and VX).
Phosphoric acid, H3PO4, is used in removing rust from iron, and is also used to acidify some foods (such as cola drinks).
Arsenic (As, Z=33).
Arsenic is a poisonous metalloid that is found in two main allotropic forms: a metallic gray form, and a nonmetallic yellow form. The name of the element is derived from the Greek word arsenikon, the name for the arsenic-containing mineral orpiment. It is found in the Earth's crust at a concentration of 1.5 ppm, making it the 53rd most abundant element. It is found in the ores orpiment [As2S3], realgar [As4S4], aresnolite [As2O3], and others.
Arsenic is often used in insecticides, and is also used as a dopant in solid-state devices such as transistors and lasers.
Antimony (Sb, Z=51).
Antimony is found in several allotropic forms, including a blue-white metal and a powdery gray nonmetal. The name of the element is derived from the Greek words anti and monos, meaning "not alone," apparently because it was never found uncombined with another element (although the etymology of this term is still the subject of much debate); the chemical symbol "Sb" derives from the Latin word for antimony sulfide, stibium. It is found in the Earth's crust at a concentration of 0.2 ppm, making it the 63rd most abundant element. It is found primarily in the ore stibnite [antimony sulfide, Sb2S3].
Antimony is a toxic, causing liver damage in large amounts. Antimony is used in alloys with other metals to make them harder; it is also used in semiconductors. In ancient times, powdered antimony sulfide, Sb2S3, was used to make a mascara called khol. It was also used to make "Greek fire," a weapon used by the Byzantine Greeks from the mid-670s until 1453, when Constantinople fell to the Ottoman Empire. The exact composition of Greek fire is not known, because the secret of making it was lost when Constantinople was captured, but it was apparently a mixture of petroleum, stibnite, and potassium nitrate (saltpeter); it was hurled onto opposing ships by catapults, or sprayed as a stream of burning liquid, and its flames were difficult to extinguish. Pills made of metallic antimony were used during the Middle Ages as laxatives; the antimony would irritate the bowels, causing their contents to be expelled — which included the undigested pill, which could then be re-used (!!).
Bismuth (Bi, Z=83).
Bismuth is a heavy, brittle metal. The name of the element is derived from the German word for the ore bismuthinite, Bisemutum, which was in turn derived from Wismuth, a corrupted form of the phrase Weisse Masse, "white mass." It is found in the Earth's crust at a concentration of 48 ppb, making it the 70th most abundant element. It is found in the ore bismuthinite [bismuth sulfide, Bi2S3], as a trace component of ores of other metals, and as the uncombined metal.
Alloys of bismuth have low melting points, and are used in electrical fuses, solders, and automatic fire sprinklers. It is also used in paints and cosmetics (primarily in the form of bismuth oxide, Bi2O3, a yellow pigment, and bismuth oxychloride, BiOCl, a white pigment). Bismuth salts are also used in treating peptic ulcers. The active ingredient in Pepto-Bismol, a common remedy for upset stomachs, diarrhea, and other stomach and intestinal problems, is bismuth subsalicylate, C7H5BiO4, a derivative of salicylic acid (which is where aspirin is also derived).
John Emsley, The Elements, 3rd edition. Oxford: Clarendon Press, 1998.
John Emsley, Nature's Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2001.
David L. Heiserman, Exploring Chemical Elements and their Compounds. New York: TAB Books, 1992.