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

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

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

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

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

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

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

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