p-Block Elements: Group 16
Classification of oxides, ozone and sulphur – allotropic forms
An oxide is defined as a binary compound of oxygen which combines with another element in an ionic, covalent or dative bond. Oxygen is highly electronegative, and it reacts with most of the elements to form oxides. Oxides can be defined as simple (e.g., MgO, Al2O3) or mixed (Pb3O4, Fe3O4).Simple oxides can be either acidic, basic oramphoteric in character. An acidic oxide reacts with water to form an acid (e.g., SO2, Cl2O7, CO2, N2O5 ). SO2 is highly reactive in water that givesH2SO3 which is an acid. Generally, only non-metal oxides are acidic but oxides ofsome metals in high oxidation state also have acidic character.
Fig1: Structure of the ozone molecule as a resonance hybrid of four canonical forms.
Ozone is an allotrope of oxygen which contains three atoms. It is highly reactive and is found at a height of about 20 kilometres above sea level. It is produced from atmospheric oxygen in the presence of sunlight that works as a catalyst. The ozone layer shields the earth’s surface from excess ultraviolet (UV) radiations.When a slow dry stream of oxygen is passed through a silent electrical discharge, conversion of oxygen to ozone (10%) occurs. The product is known as ozonised oxygen.
3O2 → 2O3; ΔHV (298 K) = +142 kJ mol–1
Pure ozone can be found as a paleblue gas, dark blue liquid or a violet-black solid.It has a characteristic smell and it is harmless in small quantity.At higher concentration, breathing becomes uncomfortable resulting in headache and nausea.It is also is thermodynamically unstable with respect to oxygen since its decomposition into oxygen results in the liberation of heat (ΔH is-ive) and an increase in entropy (ΔS is +ve).
Sulphur is found in numerous allotropes. The yellow rhombic(α-sulphur) and monoclinic (β -sulphur) forms are the most important out of all the allotropes. The nomenclature is as follows:
- α – sulphuror octahedral sulphuror rhombic sulphur.
- β – sulphur or monoclinic or prismatic sulphur
- γ – sulphur or monoclinic sulphur
- χ – sulphur or plastic Sulphur
At room temperature is rhombic sulphur is stable. It transforms into monoclinic sulphur when heated above 369 K.Rhombic sulphur (α-sulphur) is yellow in colour, m.p. 385.8 K and specific gravity2.06. It is insoluble in water. It dissolves to someextent in benzene, alcohol, ether etc. and it is readily soluble in CS2. Monoclinic sulphur (β-sulphur) hasm.p. is 393 K and specific gravity 1.98. It is soluble in CS2. It is prepared by melting rhombic sulphur in a dishand cooling, till crust is formed. At 95.5° both rhombic and monoclinic sulphuris at equilibrium and this temperature is known as Transition temperature of sulphur. When liquid sulphur is poured in water plastic sulphur of χ – sulphur will be formed. The α, β, γ – sulphurshave S8 rings which are puckered rings or crown shape. Several other modifications of sulphur containing 6-20sulphur atoms per ring havebeen synthesised in the last two decades.
Fig 2: Allotropes of sulphur and their symmetry.