p-Block Elements: Nitrogen and Oxygen Family (Group 15 & 16)

Understand periodic trends, industrial preparations, and oxoacid strength across the Nitrogen and Oxygen families for CBSE Boards and NEET/JEE.

Groups 15 and 16 of the p-Block cover the Nitrogen family (N, P, As, Sb, Bi) and the Oxygen family (O, S, Se, Te, Po), forming a high-weightage section of Class 12 Inorganic Chemistry. Both groups are tested heavily in CBSE Boards, NEET and JEE through periodic trends, oxidation states, and the industrial preparation of key compounds like ammonia and sulphuric acid.

In Group 15, oxidation states range from -3 to +5, and hydride bond angles decrease down the group (NH3: 107 degrees, PH3: 93 degrees, AsH3: 91.8 degrees) due to falling electronegativity of the central atom. Ammonia is manufactured industrially by the Haber process (N2 + 3H2 in equilibrium with 2NH3, iron catalyst with molybdenum promoter, about 200 atm and 723 K), while nitric acid is produced by Ostwald's process through catalytic oxidation of ammonia over a platinum-rhodium gauze. Phosphorus exists as white P4 (reactive, poisonous, shows chemiluminescence), red P (polymeric, less reactive) and black P (most thermodynamically stable), and its oxoacids increase in basicity with the number of P-OH bonds: H3PO2 is monobasic, H3PO3 is dibasic, and H3PO4 (orthophosphoric acid) is tribasic.

In Group 16, oxygen shows an interesting exception to the group's usual -2 oxidation state pattern: O2 is paramagnetic due to unpaired electrons in its antibonding molecular orbitals, while ozone (O3) is diamagnetic, bent, and best described as a resonance hybrid. Sulphur exists as rhombic (alpha) sulphur below 369 K, converting to monoclinic (beta) sulphur above that temperature. Sulphuric acid is manufactured by the Contact Process: sulphur is burnt to SO2, catalytically oxidised to SO3 using a vanadium pentoxide (V2O5) catalyst, and absorbed in concentrated H2SO4 to form oleum, which is then diluted to the required strength.

For board and entrance exams, the most frequently tested traps are the oxoacid basicity of phosphorus compounds (counting P-OH bonds, not total hydrogen atoms), the paramagnetic nature of O2 explained through Molecular Orbital Theory, and the exact reagents and conditions of the Haber and Ostwald processes. Dr. Aarzoo Saini emphasises drilling these industrial process conditions and oxoacid structures as fixed reference tables at We-Gyaan Classes Roorkee.