NITROGEN GROUP ELEMENT

any of the chemical elements that constitute Group Va of the periodic table. This group consists of nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). All of the elements of the nitrogen group occur in the Earth's crust. Moreover, nitrogen itself is the most abundant gas in the atmosphere and on the average constitutes about 16 percent by weight of the complex compounds called proteins that are present in all living organisms. Phosphorus is the 12th most abundant element in the terrestrial crust and one of the most widely distributed. Because of its high chemical reactivity, however, it does not occur uncombined. The other three members of the group are found in substantially smaller amounts. They exist both in the native state and in the form of compounds. The nitrogen elements probably have the widest range in physical state of any group in the periodic table. Nitrogen, for example, is a gas that liquefies at roughly -210 C, whereas bismuth is a solid that melts at 271 C. Nitrogen and phosphorus are nonmetals; arsenic and antimony are metalloids; and bismuth is a typical metal. The elements of this group also exhibit considerable diversity in appearance. Nitrogen is colourless both as a gas and as a liquid. Phosphorus occurs in a variety of allotropic forms, including the familiar highly reactive white form, a much less reactive red form, and a black modification that appears to be the most stable of all. Arsenic exists ordinarily as a dull gray metallic solid. Antimony is a silver, metallic-appearing but somewhat brittle solid. Bismuth is a silver-white metal with a trace of pink in its lustre. The nitrogen group elements are clearly differentiated from each other chemically, but they do manifest certain general similarities in chemical behaviour. These similarities reflect common features of the electronic structure of their atoms. Each element of the group has an outermost shell of five electrons. In every case, these electrons occupy an s orbital completely with two electrons and contribute one electron to each of three p orbitals. The arrangement of outer electrons in the atoms of nitrogen elements therefore provides three half-filled orbitals that can form three covalent bonds by interaction with half-filled orbitals of the atoms of other elements; an unshared, or lone, pair of electrons, remains after the three covalent bonds have been formed. With the exception of nitrogen, all other elements in this group can also form five covalent bonds. Figure 1: Modern version of the periodic table of the elements. To see more information about an any of the chemical elements that constitute Group Va of the periodic table (see Figure). The group consists of nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). The elements share certain general similarities in chemical behaviour, though they are clearly differentiated from one another chemically, and these similarities reflect common features of the electronic structures of their atoms. Probably no other group of the elements is more familiar to the layman than this group. Although the five elements together make up less than 0.2 percent by weight of the Earth's crust, they assume an importance far out of proportion to their abundance. This is especially true of the elements nitrogen and phosphorus, which comprise 2.4 and 0.9 percent, respectively, of the total weight of the human body. The nitrogen elements have, perhaps, the widest range in physical state of any group in the periodic table. Nitrogen, for example, is a gas that liquefies at about -200 C, whereas bismuth is a solid melting at 271 C and boiling at about 1,560 C. Chemically, too, the range in properties is wide, nitrogen and phosphorus being typical nonmetals; arsenic and antimony, metalloids; and bismuth, a metal. Even in appearance these elements exhibit great variety. Nitrogen is colourless both as a gas and as a liquid. Phosphorus exists in a variety of physical modifications, or allotropic forms, including the familiar white, highly reactive form that must be stored under water to prevent it from igniting in the air; a much less reactive red or violet form; and a black modification that, although least known, appears to be the most stable of all. Arsenic exists mainly as a dull gray metallic solid, but a more reactive yellow, solid form is also known, and there are indications that other forms exist under certain conditions. Antimony is a silver, metallic appearing, but somewhat brittle solid; and bismuth is a silver-white metal with a trace of pink in its lustre. Together with carbon, hydrogen, oxygen, and sulfur, the first two members of this group, nitrogen and phosphorus, are the principal chemical elements incorporated into living systems. Nitrogen and phosphorus are readily removed from the soil by plant growth, and therefore they are immensely important components of plant foods. Such designations as 5105 on commercial fertilizers represent the respective weight percentage composition of the material in terms of nitrogen, phosphoric oxide, and potassium oxide (potassium being the third principal element needed for healthy plant growth). Nitrogen in fertilizers may be in the form of sodium or potassium nitrates, ammonia, ammonium salts, or various organic combinations. Phosphorus is supplied chiefly as inorganic phosphate. These same elements, nitrogen and phosphorus, can also be used in ways less helpful to man. The explosives in conventional warfare are heavily dependent on their content of nitrogen compounds, and the deadly nerve gases are composed of organic compounds of phosphorus. On the other hand, arsenic, which is notorious for its toxicity, is most useful in agriculture, where its compounds are an aid in controlling harmful insect pests. Antimony and bismuth are used chiefly in metal alloys, because they impart unique and desirable properties to these alloys. Additional reading A concise summary of nitrogen chemistry is contained in William L. Jolly, The Inorganic Chemistry of Nitrogen (1964). More detailed is the comprehensive compendium by C.A. Streuli and P.R. Averell (eds.), The Analytical Chemistry of Nitrogen and Its Compounds, 2 vol. (1970), which includes especially useful tables. A popularized, readable, introductory account is Isaac Asimov, The World of Nitrogen, rev. ed. (1962). Information about nitrogen and its compounds and about the other elements of the nitrogen group and their compounds is found in such standard works on inorganic chemistry as M. Cannon Sneed, J. Lewis Maynard, and Robert C. Brasted (eds.), Comprehensive Inorganic Chemistry, 8 vol. (195361); John C. Bailar, Jr., et al. (eds.), Comprehensive Inorganic Chemistry, 5 vol. (1973); and F. Albert Cotton and Geoffrey Wilkinson, Advanced Inorganic Chemistry, 5th ed. (1988); in R.T. Sanderson, Chemical Bonds and Bond Energy, 2nd ed. (1976), a brief book giving new insights into the nature of chemical bonds; and in Eugene G. Rochow, The Metalloids (1966), a concise, readable survey of the elements that border between metallic and nonmetallic. Detailed discussions on fixation are provided in W.J. Broughton (ed.), Nitrogen Fixation (1981 ). More detailed information on phosphorus may be found in D.E.C. Corbridge, The Structural Chemistry of Phosphorus (1974), and Phosphorus: An Outline of Its Chemistry, Biochemistry, and Technology, 4th ed. (1990); and Harold Goldwhite, Introduction to Phosphorus Chemistry (1981). R. Thomas Sanderson The Editors of the Encyclopdia Britannica