Chapter 20 - The Representative Elements

20.1 The Group 5A Elements

• Elements show a wide variety of chemical properties
  • Nitrogen and phosphorus are nonmetals
• All group members except N form molecules with five covalent bonds
• The ability to form p bonds decreases dramatically after N
• Chemistry of nitrogen:
  • Most nitrogen-containing compounds decompose exothermically, forming the very stable N2 molecule, which explains the power of nitrogen-based explosives
  • The nitrogen cycle, which consists of a series of steps, shows how nitrogen is cycled in the natural environment
• Nitrogen fixation changes the N2 in the air into compounds useful to plant
  • The Haber process is a synthetic method of nitrogen fixation
  • In the natural world, nitrogen fixation occurs through nitrogen-fixing bacteria the root nodules of certain plants and through lightning in the atmosphere
• Ammonia is the most important hydride of nitrogen
  • It contains pyramidal NH3 molecules
  • It is widely used as a fertilizer
• Hydrazine (N2H4) is a powerful reducing agent
• Nitrogen forms a series of oxides including N2O, NO, NO2, and N2O5
• Nitric acid is a very important strong acid manufactured by the Ostwald process
• Chemistry of phosphorus:
  • Exists in three elemental forms: white (contains P4 molecules), red, and black
  • Phosphine (PH3) has bond angles close to 90 degrees
  • Phosphorus forms oxides including P4O6 and P4O10

20.2 The Chemistry of Nitrogen

• The decomposition of nitroglycerin is a complex process that occurs in many steps
• Pure nitroglycerin is quite dangerous because it explodes with little provocation
• Most high explosives are organic compounds that, like nitroglycerin, contain nitro groups and produce nitrogen and other gases as products.
• The Haber process is one example of nitrogen fixation.
  • The ammonia produced can be applied to the soil as a fertilizer since plants can readily employ the nitrogen in ammonia to make the nitrogen-containing biomolecules essential for their growth.
  • Nitrogen fixation also results from the high-temperature combustion process in automobile engines
  • Another natural nitrogen fixation process is provided by bacteria that reside in the root nodules of plants such as beans, peas, and alfalfa
  • They produce ammonia at soil temperature and 1 atm of pressure, whereas the Haber process requires severe conditions of 400°C and 250 atm.
• Nitric acid is an important industrial chemical used in the manufacture of many products, such as nitrogen-based explosives and ammonium nitrate for use as fertilizer
• Azeotrope: A solution that distills at a constant temperature without a change in composition

20.3 The Chemistry of Phosphorus

• Although phosphorus lies directly below nitrogen in Group 5A of the periodic table, its chemical properties are significantly different from those of nitrogen.
• The differences arise mainly from four factors: nitrogen’s ability to form much stronger p bonds, the greater electronegativity of nitrogen, the larger size of the phosphorus atom, and the availability of empty valence d orbitals on phosphorus.
• Phosphorus is very reactive and bursts into flames on contact
  • Phosphine is analogous to ammonia, although it is a much weaker base and much less soluble in water
• The terminal oxygens are the nonbridging oxygen atoms
• The mineral hydroxyapatite, the principal component of tooth enamel, can be converted to fluorapatite by reaction with fluoride
  • Fluoride ions added to drinking water and toothpaste help prevent tooth decay because fluorapatite is less soluble in the acids of the mouth than hydroxyapatite
• Soluble phosphate fertilizers are manufactured by treating phosphate rock with sulfuric acid to make superphosphate of lime

20.4 The Group 6A Elements

• Metallic character increases going down the group but no element behaves as a typical metal
• Chemistry of oxygen: • Elemental forms are O2 and O3 • Oxygen forms a wide variety of oxides • O2 and especially O3 are powerful oxidizing agents
• Chemistry of sulfur:
  • The elemental forms are called rhombic and monoclinic sulfur, both of which contain S8 molecules
  • The most important oxides are SO2 and SO3

20.5 The Chemistry of Oxygen

• Oxygen is present in the atmosphere in oxygen gas and ozone
• Most of the energy we need to live and to run our civilization comes from the exothermic reactions of oxygen and carbon-containing molecules.
• The most common elemental form of oxygen (O2) constitutes 21% of the volume of the earth’s atmosphere.
• Ozone can be prepared by passing an electric discharge through the pure oxygen gas
  • Ozone is much less stable than oxygen at 25°C and 1 atm
  • Although ozone effectively kills the bacteria in water, one problem with ozonolysis is that the water supply is not protected against decontamination
  • The oxidizing ability of ozone can be highly detrimental, especially when it is formed in the pollution from automobile exhausts
  • Ozone exists naturally in the upper atmosphere of the earth.
  • The ozone layer is especially important because it absorbs ultraviolet light and thus acts as a screen to prevent radiation

20.6 The Chemistry of Sulfur

• Sulfur is found in nature both in large deposits of the free element and is widely distributed ores
• About 60% of the sulfur produced in the United States comes from the underground deposits of elemental sulfur found in Texas and Louisiana
  • This sulfur is recovered using the Frasch process developed by Herman Frasch
  • In contrast to oxygen, elemental sulfur exists as S2 molecules only in the gas phase at high temperatures
  • Sulfur monoxide, which can be produced in small amounts when gaseous sulfur dioxide is subjected to an electrical discharge, is very unstable
  • Because sulfuric acid has a high affinity for water, it is often used as a dehydrating agent.
• Gases that do not react with sulfuric acids, such as oxygen, nitrogen, and carbon dioxide, are often dried by bubbling them through concentrated solutions of the acid
• Hydrogen sulfide is a strong reducing agent in aqueous solution, producing a milky-looking suspension of finely divided sulfur as one of the products
• The preparation of sulfur trioxide provides an example of the compromise that often must be made between thermodynamics and kinetics

20.7 The Group 7A Elements

• They are all nonmetals
• It forms hydrides of the type HX that behave as strong acids in water except for HF, which is a weak acid
• The oxyacids of the halogens become stronger as more oxygen atoms are present
• The interhalogens contain two or more different halogens
• When dissolved in water, the hydrogen halides behave as acids, and all except hydrogen fluoride are completely dissociated.
• Hydration becomes more exothermic as the charge density of an ion increases. Thus, for ions of a given charge, the smallest is most strongly hydrated
• The deciding factor is entropy
• Fluorine forms only one oxyacid, hypofluorous acid, but at least two oxides
  • The name for OF2 is oxygen difluoride rather than difluorine oxide because fluorine has a higher electronegativity than oxygen and thus is named as if it were an anion
• The halogens react readily with most nonmetals to form a variety of compounds
  • They react with each other to form interhalogen compounds

20.8 The Group 8A Elements

• The Group 8A elements, the noble gases, are characterized by filled s and p valence orbitals
• All elements are monatomic gases and are generally very unreactive
• The heavier elements form compounds with electronegative elements such as fluorine and oxygen
• Helium was identified by its characteristic emission spectrum as a component of the sun before it was found on earth
  • The major sources of helium on earth are natural gas deposits, where helium was formed from a particle decay of radioactive elements
• Of the Group 8A elements, only krypton and xenon have been observed to form chemical compounds

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