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21.1 Properties and Trends in Group 3A?The elements of group 3A exhibit a range of physical and chemical properties. Many of the trends observed in this group can be understood from considerations of their electron configurations and their respective positions in the periodic table. For example, the inert pair of valence-shell electrons retained in the

ion is also found in several other cations following a transition series.

21.2 Boron?Because they have only three valence electrons, boron atoms tend to form electron-deficient compounds, leading to some unusual bonding patterns. Compounds resulting from the ?addition? of one structure to another, called adducts, are common when boron is present. Also, boron atoms form three-center bonds in the boron?hydrogen compounds called boranes. Other common boron compounds include borax, boric oxide, boric acid, and borates.

21.3 Aluminum?Aluminum is the most industrially important element of group 3A. It is an active metal that is protected against corrosion by a film of

Both

and Al react with acids and strong bases. The production of aluminum is based on the amphoterism of

and the electrolysis of

in molten cryolite. The electron deficiency of aluminum chloride is a useful property in organic syntheses.

21.4 Carbon?Carbon is the key element of organic chemistry, but the free element also has uses. Diamond is prized for hardness and thermal conductivity; while graphite?s electrical conductivity and refractory properties have found extensive use. Metal carbides, carbon oxides, and nitrogen-containing carbon compounds are of importance in a wide number of applications.

21.5 Silicon?Silicon is the key element of the mineral world, occurring as silica,

and as various minerals based on the silicate anion,

Silica and silicates are common constituents of ceramics?solids that are characterized by hardness and brittleness and are chemically and structurally stable at high temperatures. Some synthetic silicon-containing organic compounds are of commercial importance, including the silicon-containing polymers called silicones.

21.6 Tin and Lead?Tin and lead metals are slightly more active than hydrogen and are widely used in alloys. They can have an oxidation number of either

or

with Sn(II) being a good reducing agent and Pb(IV) being a good oxidizing agent.

21.7 Nitrogen?Nitrogen is the major constituent of the atmosphere, and essentially all nitrogen compounds?natural and synthetic?are derived from the atmosphere. Some industrially important nitrogen compounds are ammonia, urea, nitric acid, ammonium salts, hydrazine, hydrazoic acid, and azides.

21.8 Phosphorus?The structure of phosphorus is based on the pyramidal molecule,

in both the white and red modifications. The structures of the oxides

and

are related to that of the

molecule. The principal compounds of phosphorus are the phosphates and polyphosphates.

21.9 Oxygen?Oxygen forms compounds with all elements except the lighter noble gases. Most oxygen is obtained, together with nitrogen and argon, by the fractional distillation of liquid air. Oxygen forms three types of anions when combined with active metals: oxide

peroxide

and superoxide

Ozone,

an allotrope of oxygen, is useful as an oxidizing agent, both in the laboratory and in the chemical industry.

21.10 Sulfur?Sulfur differs from oxygen in important ways, such as in its variety of allotropic forms and the changes they undergo. Its important compounds are the oxides, oxoacids, sulfites, sulfates, and thiosulfates, and many of the reactions of these compounds are oxidation?reduction reactions.

21.11 Selenium, Tellurium, and Polonium?Selenium and tellurium are found as minor but important components of technologies ranging from the coloring of glass to the detection of light. Polonium is highly radioactive and has few common uses.

21.12 Sources and Uses of the Halogens?The halogens (group 7A) are nonmetals; fluorine is the most nonmetallic of all elements. Fluorine and chlorine are prepared by electrolysis, and bromine and iodine by displacement reactions. Two halogens can react to form an interhalogen compound. Halogen atoms can substitute for H atoms in hydrocarbons and other organic compounds.

21.13 Hydrogen Halides?Hydrogen halides form by the direct combination of the elements or by the reaction of a halide salt with a nonvolatile acid. In aqueous solution, the hydrogen halides act as acids.

21.14 Oxoacids and Oxoanions of the Halogens?Important classes of halogen compounds include the oxoacids and their salts. The chemical reactions of these compounds are mostly oxidation?reduction reactions.

21.15 Occurrence of the Noble Gases?Most of the noble gases are found in Earth?s atmosphere. Some, like He and Ar, are produced in quantity through the decay of radioactive isotopes of other elements. Radon is radioactive.

21.16 Properties and Uses of the Noble Gases?Interest in the noble gases centers on their physical properties and inertness. In contrast, the ability of the heavier noble gases to form some chemical compounds provides important insights into bonding theory.

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