Chapter Twenty One The p-Block Elements Introduction • The p-block includes all the noble gases except helium, all the non-metals except hydrogen, all the metalloids, and even a few metals, including Al, Sn, and Pb. • Three of the p-block elements - O, Si, and Al - are the most abundant elements in Earth’s crust. • Six p-block elements - C, N, O, P, S, and Cl - are among the elements making up the bulk of living matter. • Five others - B, F, Si, Se, and I - are required in trace amounts by most plant and animal life. • C and S can occur in the free state. The p-Block Elements Properties And Trends In Group 3A Properties And Trends In Group 3A • ns2np1 valence electron configuration • Boron (B) holds tightly to these valence electrons because of its small size. Thus, boron typically forms covalent compounds. • Aluminum (Al) readily forms +3 cations. • Gallium (Ga), indium (In), and thallium (Tl) all form +3 cationns, but also +1 cations, which are most stable for Tl. This is due to so - called “inert pair” on Tl. Boron • Most of the chemistry of boron compounds is based on the lack of an octet of electrons about the central boron atom. • These compounds are electron deficient, and this deficiency causes them to exhibit some unusual bonding features. • Boron hydride (BH3) forms a coordinate covalent bond with another atom that has a lone pair of electrons to complete its octet; this is called an adduct. • Diborane, B2H6, has bonding only fully explained by molecular orbital theory. • Borax, Na2B4O7·10H2O, a hydrated borate, is the primary source of boron in nature. Structure Of Diborane, B2H6 three –center bond Aluminum • The most important metal of Group 3A is aluminum. • Pure aluminum is a malleable, ductile, silvery- colored metal with a density of only about one-third that of steel. • The metal is not very strong, but its strength increases when it is alloyed with Cu, Mg, or Si. • Over 5 million tons of the metal are produced per year in the United States • Most of it is used in lightweight alloys. Production Of Aluminum • The process of isolating pure aluminum by electrolysis of bauxite ores is called the Hall- Heroult process. • Al(OH)4¯ Al2O3 Al Properties And Uses Of Aluminum • Aluminum is a good reducing agent. • As an active metal, aluminum readily reacts with acids to produce hydrogen gas. 2 Al (s) + 6 H+ (aq) 2 Al3+ (aq) + 3 H2 (g) • Aluminum also dissolves in basic solutions. 2 Al (s) + 6 H2O (l) + 2 OH- (aq) 2 [Al(OH)4]- (aq) + 3 H2 (g) • Because its combustion is a highly exothermic reaction, powdered aluminum is used as a component in rocket fuels, explosives, and fireworks. • Perhaps most familiar is the use of aluminum in beverage cans, cookware, and as a foil for wrapping foods. Aluminum Compounds • Aluminum oxide (Al2O3), also called alumina, is amphoteric. Al2O3 (s) + 6 H+ (aq) 2 Al3+ (aq) + 3 H2O (l) Al2O3 (s) + 3 H2O (l) + 2 OH- (aq) 2 [Al(OH)4]- (aq) It is used in the manufacture of ceramic materials and is used as an abrasive for grinding wheels and sandpaper. • Among the aluminum halides, AlF3 differs considerably from the others in that it is the only one to have the properties normally associated with ionic substances. • Lithium aluminum hydride, LiAlH4, is used as a reducing agent in organic chemistry. • Aluminum sulfate is the most important industrial aluminum compound and is used in water treatment. Bonding In Al2Cl6 Carbon • Elemental carbon exists in nature mainly as the two allotropes diamond and graphite • Newly discovered form of carbon: Buckeyball, C60 • Graphite: pencils, electrodes, high-temperature devices, and strong graphite fibers. • Diamonds: jewelry, industrial uses as abrasives, drill bits - among hardest substances known and high thermal conductivity • Carbon also exists in amorphous forms, such as charcoal. Images from chapter 11: States of matter: Diamond Allotropes of carbon Graphite Buckyball Carbon Nanotubes Inorganic Carbon Compounds • Carbon monoxide, carbon dioxide, carbonate are familiar oxides of carbon. • Carbon combines with most metals to form compounds called carbides. • Two other binary compounds of carbon are carbon disulfide, CS2, and carbon tetrachloride, CCl4. • Cyanide ion, (CN-) – forms an insoluble silver salt, AgCN, – acid, HCN, quite weak. Cyanide ion is also quite toxic. Silicon • While carbon readily forms strong C-C bonds (rings, chains), silicon does not – but it does form Si-O-Si containing 3-dimensional solids • A silicon atom, like a carbon atom, forms four bonds in almost all cases. • The most common form of naturally occurring silicon is silica (SiO2)n. It is a network covalent compound. Structure of Silica, SiO2 Amorphous A Two-Dimensional sheet in the structure of Mica Nitrogen • Nitrogen is found in greater abundance in the atmosphere than anywhere else. • There are only two important mineral sources of nitrogen: KNO3 and NaNO3. • Nitrogen compounds occur in all living matter. • Nitrogen molecule, N2, has a very strong N=N triple bond. Consequently it is quite unreactive and is used as an inert blanketing atmosphere in industrial operations. • Liquid nitrogen is used in low-temperature applications. • The only important commercial method of producing nitrogen is the fractional distillation of liquid air. Fractional Distillation Of Air Nitrogen Compounds • Nitrogen exists in compounds in all oxidation states -3 to +5. • Ammonia, NH3, is produced using the Haber process. N2 (g) + 3 H2 (g) 2 NH3 (g) Ho = -92.22 kJ What are the optimum conditions for Haber process? • Urea, CO(NH2)2, is used mainly as a fertilizer. • Nitric acid is produced from the oxidation of ammonia and subsequent reaction with water. Pt/Rh 4 NH3 (g) + 5 O2 (g) 4 NO (g) + 6 H2O (g) 2 NO (g) + O2 (g) 2 NO2 (g) 3 NO2 (g) + H2O (l) 2 HNO3 (aq) + NO (g) Nitric acid is also used as an oxidizing agent. • The common oxides of nitrogen have oxidation number for N from –3 to +5. • Sodium azide, NaN3, is used in air-bag safety systems in automobiles. 2 NaN3 (s) 2 Na (l) + 3 N2 (g) Phosphorus • Phosphorus is the eleventh most abundant element in Earth’s crust – occurs exclusively in nature as phosphate: PO43 • Elemental forms: – White phosphorus, P4, can be cut with a knife, melts at 44.1 oC, is a non-conductor of electricity, and ignites spontaneously in air (it is stored under water). – Red phosphorus can be obtained by heating white phosphorus to about 300 oC in the absence of air. This allotrope of phosphorus forms long chains of phosphorus atoms joined together. Molecular Structures Of White And Red Phosphorus Compounds Of Phosphorus Phosphorus forms two oxides, P4O6 and P4O10. • In limited O2: P4 (s) + 3 O2 (g) P4O6 (s) P4O6 (s) + 6 H2O (l) 4 H3PO3 (aq) phosphorous acid (diprotic) • In excess O2: P4 (s) + 5 O2 (g) P4O10 (s) P4O10 (s) + 6 H2O (l) 4 H3PO4 (aq) phosphoric acid (triprotic) The oxides P4O6 and P4O10 are the acid anhydrides of phosphorous acid and phosphoric acid, respectively. Molecular Structures Of P4O6 And P4O10 Oxygen • Oxygen is one of the most active non-metals and one of the most important. • The chief reactions of elemental, atmospheric oxygen are oxidation processes. • Uses of oxygen include: – manufacture of iron, other metals, welding, manufacture of chemicals, water treatment, oxidizer, and respiration therapy. • Ozone, O3, is a powerful oxidizing agent, especially in acidic solution. It is also found in the upper atmosphere. Sulfur • Sulfur forms many compounds similar to those of oxygen. However They are differ in important way: – Hydrogen bonding in O compounds, but not in S compounds. – S can employ an expanded valence shell, but O cannot. • Elemental sulfur exists as several molecular species: solid - S8 , vapor - S2 • Elemental sulfur is mined using the Frasch process. • A small amount of sulfur is used directly in vulcanizing rubber and as a pesticide. • Sulfuric acid, H2SO4, is a strong acid and the sulfates, SO42-, have many important uses. Concentrated H2SO4 is an oxidizing agent and dehydrating agent. • Sulfur dioxide and sulfites, SO32-, are widely used in the food industry as decolorizing agents and preservatives. The Frasch Process For Mining Sulfur Structures Of The Sulfate And Thiosulfate Ions sulfate thiosulfate Halogens • Halogens are all non-metals with ns2np5 valence shell electron configuration. • Halogen elements exist as diatomic molecules, X2. • Fluorine is the most reactive, iodine is the least. The oxidizing power decreases from F2 to I2. • Halogens occur naturally only as the halides (X¯) and are converted to halogens usually by electrolysis. • Fluorine is the strongest oxidizing agent of the elements, and is used to make UF6 and SF6. • Chlorine is used as an oxidizer and is important in combination with carbon compounds. • Bromine is a liquid at RT • Iodine is a sublimable solid at RT Hydrogen Halides • All of the hydrogen halides are acids. All except HF are strong acid. • The hydrogen halides can be prepared by direct combination of the elements. H2 (g) + X2 (g) 2 HX (g) • In addition, they all are produced by reaction of an acid with a halide. Oxoacids And Oxoanions Of Halogens • In its compounds, fluorine always has the oxidation number –1. • The other halogens, however, can have positive oxidation numbers: +1, +3, +5, +7. • These oxidation numbers are found in the oxoacids of Cl, Br, and I. Occurrence Of The Noble Gases • Except for helium and radon, the noble gases are found only in the atmosphere. • Helium is found in some natural gas deposits, particularly those underlying the Great Plains of the United States. • Most of the noble gases, except Ar, have escaped from the atmosphere since Earth was formed. Argon is a product of the radioactive decay of potassium-40, a fairly abundant naturally occurring isotope (0.012%). Summary • Because they only have three valence electrons, boron atoms tend to form electron-deficient compounds and this leads to some unusual bonding patterns. • Aluminum, an active metal, reacts with acids and strong bases. • Carbon is the key element in organic chemistry, but the free element also has uses. • Silicon is the key element of the mineral world. • Tin and lead are slightly more active than hydrogen, with tin (II) being a good reducer and lead (IV) a good oxidizer. Summary • Some of the nitrogen compounds described in the chapter are ammonia, urea, nitric acid, ammonium salts, hydrazine, and hydrazoic acids and azides. • Oxygen forms compounds with all elements except the lighter noble gases. Ozone is an allotropic form of oxygen. • The halogens are non-metals and occur naturally only as the halides. • Interest in the noble gases centers on their physical properties and inertness.