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Twenty One

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					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.

				
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posted:3/14/2011
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