Chapter Twenty One

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Chapter Twenty One

• The p-Block Elements

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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 Earths 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.

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The p-Block Elements
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Properties And Trends In Group 3A
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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.

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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, Na2B4O710H2O, a hydrated borate, is the
  primary source of boron in nature.

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Structure Of Diborane, B2H6
three center bond
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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.

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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

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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.

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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.

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Bonding In Al2Cl6
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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.

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Images from chapter 11 States of
matter Allotropes of carbon
Diamond
Graphite
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Buckyball
Carbon Nanotubes
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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.

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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.

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Structure of Silica, SiO2
Amorphous
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A Two-Dimensional sheet inthe structure of Mica
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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 NN
  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.

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Fractional Distillation Of Air
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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) º 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)

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Phosphorus

• Phosphorus is the eleventh most abundant element
  in Earths 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.

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Molecular Structures OfWhite And Red Phosphorus
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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.

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Molecular Structures OfP4O6 And P4O10
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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.

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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.

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The Frasch Process For Mining Sulfur
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Structures Of The Sulfate And Thiosulfate Ions
thiosulfate
sulfate
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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

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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.

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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.

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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).

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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.

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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.