JEE Chemistry The P Block Elements Notes Set 04

Group 13 and 14 Elements

1. Boron is a covalent, high melting non-metal with a strong tendency to form electron-deficient compounds. BCl3 is hydrolysed to H3BO3 and HCl and acts as a Lewis acid. Diborane has two terminal B-H bonds on each boron as 2c-2e bonds and two bridging 3c-2e bonds, with sp3 boron.
2. Boric acid is a weak monobasic Lewis acid forming [B(OH)4]-; it is layered by hydrogen bonding. Borax contains the [B4O5(OH)4]2- ion and gives coloured metaborate beads. The inert pair effect makes the +1 state stable only for thallium. Aluminium is protected by its oxide film.

Carbon Family and Silicates

1. Graphite has delocalised pi electrons and conducts; diamond is hard and an insulator; silicon carbide is an abrasive. Silicones are made from R2SiCl2 (chains), RSiCl3 (cross-links) and R3SiCl (chain stoppers).
2. Silicates are built from SiO4 tetrahedra: isolated, pyro (one shared corner), cyclic and single chain (two shared corners), double chain and sheet (three shared corners) and framework (all four shared).

Nitrogen Family

1. Nitrogen is inert because of its triple bond. NH3 is dried with CaO, reacts with excess Cl2 to give NCl3 and forms the brown precipitate with Nessler's reagent. PH3 is weakly basic, poisonous and burns because of P2H4.
2. White phosphorus is the reactive P4 form kept under water, red phosphorus is polymeric and safe. The oxyacids H3PO2, H3PO3 and H3PO4 are mono-, di- and tribasic. Nitric acid is a strong oxidant, giving NO2 with concentrated and NO with dilute solutions.

Oxygen Family

1. Ozone is a diamagnetic bent molecule and a strong oxidiser. SO2 is a reducing and bleaching gas; H2S turns lead acetate paper black. Conc. H2SO4 oxidises HBr and HI, so those acids cannot be prepared with it.
2. Oxyacids of sulphur differ in their S-S, S-O-S or O-O links: thiosulphuric, pyrosulphuric and Caro's acids.

Halogens and Noble Gases

1. Fluorine is the strongest oxidant and shows only -1. Acidity of the oxyacids of chlorine rises with the oxidation state: HClO < HClO2 < HClO3 < HClO4. The chromyl chloride test confirms chloride.
2. Xenon forms XeF2 (linear), XeF4 (square planar), XeF6 (distorted octahedral), XeO3 (pyramidal) and XeO4. Boiling points of the noble gases rise down the group with polarisability. Helium is used in diving mixtures and cryogenics.