JEE Chemistry The P Block Elements Notes Set 19

Group 16: Oxygen Family

1. Group 16 elements O, S, Se, Te and Po have ns²np⁴. Oxygen is a diatomic paramagnetic gas (two unpaired electrons in antibonding pi orbitals) and the most abundant element in the crust. Oxygen can be obtained by heating HgO, KClO₃ or KMnO₄, and industrially by fractional distillation of liquid air (O₂ boils at 90 K, N₂ at 77 K).
2. Ozone O₃ is an allotrope, bent (117 degrees) and diamagnetic. It is made by passing a silent electric discharge through O₂. It is a powerful oxidising agent: PbS + 4O₃ gives PbSO₄ + 4O₂, 2KI + O₃ + H₂O gives 2KOH + I₂ + O₂, and it turns tetramethylbase paper violet. Ozone in the stratosphere absorbs harmful UV radiation. O₃ is absorbed by oil of turpentine or oil of cinnamon, and O₂ by alkaline pyrogallol.
3. Hydrides H₂E: thermal stability and acidic strength change as H₂O > H₂S > H₂Se > H₂Te in stability. Bond angle decreases from H₂O (104.5 degrees) to H₂Te (about 90 degrees). Reducing power increases down the group.
4. Sulphur exists as S₈ puckered rings (rhombic and monoclinic forms, showing allotropy and polymorphism). SO₂ is formed by burning S, by heating sulphides in air, or by Cu + conc. H₂SO₄. It is a reducing agent (decolourises acidified KMnO₄ and turns acidified K₂Cr₂O₇ green, forming Cr³⁺) and an oxidising agent only with strong reductants such as H₂S (SO₂ + 2H₂S gives 3S + 2H₂O).
5. SO₂ bleaches by reduction (temporary bleaching), while Cl₂ bleaches by oxidation (permanent). Sulphuric acid is made by the contact process (V₂O₅ catalyst). Caro's acid is H₂SO₅ (peroxomonosulphuric acid) and Marshall's acid is H₂S₂O₈.
6. Oxoacid and oxide notes: SO₃ is a strong acid anhydride. H₂SO₄ is a dehydrating agent, and sodium thiosulphate Na₂S₂O₃ (hypo) is used in photography.

Group 17: Halogens

1. Halogens F, Cl, Br, I and At have ns²np⁵. All are diatomic, form uninegative ions, and show oxidising power decreasing from F₂ to I₂, and each halogen displaces heavier halogens from their salts (Cl₂ + 2KBr gives 2KCl + Br₂). Fluorine shows only the -1 oxidation state because it is the most electronegative and has no d-orbitals. F is the smallest atom in the group.
2. Fluorine is the strongest oxidising agent because of a low enthalpy of dissociation of F-F, a very high hydration enthalpy of F^-, and a high electrode potential (+2.87 V), even though its electron affinity is less than chlorine's. The order of electron affinity is Cl > F > Br > I. Fluorine is made by electrolysis of KHF₂ in anhydrous HF (Moissan's method).
3. Chlorine: made by electrolysis of brine, or from HCl with MnO₂ (MnO₂ is the oxidising agent: MnO₂ + 4HCl gives MnCl₂ + Cl₂ + 2H₂O). It is dried over conc. H₂SO₄, and bleaches only in the presence of moisture (oxidation by nascent oxygen from HOCl). Cold dilute NaOH gives NaOCl and NaCl; hot concentrated NaOH gives NaClO₃ and NaCl. Euchlorine is a mixture of Cl₂ and ClO₂. Chlorine water in sunlight gives HCl and O₂.
4. Oxides and oxoacids: F₂O (oxygen is +2, so it is oxygen difluoride), Cl₂O, ClO₂ and Cl₂O₇. Acid strength of oxoacids: HOCl < HOClO < HOClO₂ < HOClO₃ (HClO₄ is the strongest acid). Interhalogen compounds (ClF₃, IF₇) are more reactive than the halogens.
5. HF is a weak acid with strong hydrogen bonding (and the bifluoride ion HF₂^- forms). Acid strength: HF < HCl < HBr < HI. HF attacks glass, forming SiF₄.

Group 18: Noble Gases

1. Noble gases He, Ne, Ar, Kr, Xe and Rn have ns²np⁶ (He is 1s²) and zero valency. They are monoatomic, chemically inert, and have the highest ionisation energies in their periods. Argon is the most abundant (0.93% of air). Helium is the second most abundant element in the universe, is found in natural gas and monazite sand, and has the lowest boiling point (4.2 K).
2. Inter-atomic forces are only weak van der Waals (dispersion) forces, so boiling points rise with atomic size from He to Rn. Ease of polarisation, adsorption on charcoal (at low temperature) and solubility in water also increase down the group; He is the least polarisable.
3. Xenon forms the most compounds. XeF₂ (sp³d, linear), XeF₄ (sp³d², square planar) and XeF₆ (distorted octahedral) are made by reacting Xe with F₂ at different Xe:F₂ ratios and conditions. XeF₆ + H₂O gives XeOF₄ and finally XeO₃. XeO₃ is a pyramidal explosive solid. Xe forms only compounds with even oxidation states (+2, +4, +6, +8).
4. Uses: He in balloons and, mixed with oxygen, for deep-sea divers (it is less soluble in blood than nitrogen, avoiding the bends); Ne in advertising discharge tubes (red-orange glow); Ar for inert atmospheres and filling electric bulbs; liquid He as cryogenic coolant; Rn in radiotherapy.
5. Radon is radioactive with a half-life of 3.8 days. Rn is absent from the normal atmosphere because of this, though traces arise from radioactive decay of radium.

Anomalous Behaviour and Comparisons of p-Block Elements

1. The first element of each group (B, C, N, O, F) differs from the rest because of its small size, high electronegativity and absence of d-orbitals, which limits its covalency to 4 (so N forms NCl₃ only, and F shows only -1).
2. The inert pair effect makes the lower oxidation state (group number minus 2) more stable for heavier elements: Tl⁺, Pb²⁺, Bi³⁺ (and Sn²⁺ is a reducing agent while Sn⁴⁺ is stable).
3. Acidic strength of oxides increases from left to right in a period and decreases down a group, as in P₂O₃ > As₂O₃ > Sb₂O₃.
4. Catenation is highest for carbon, then Si, and S forms chains and rings. Multiple bond formation (p-pi p-pi) is common for the second-period elements C, N and O.
5. Electron deficient, electron precise and electron rich hydrides: B₂H₆ (deficient), CH₄ (precise), and NH₃, H₂O and HF (rich, with lone pairs and hydrogen bonding).