p-Block Elements
This section covers p-Block Elements: group trends, anomalous behaviour of the first member, inert-pair effect, oxides and oxoacids, allotropes of phosphorus and sulphur, halogens, interhalogens and noble gases. Questions often ask for structure-based reasoning, not isolated facts.
Core rules
- Down a group, atomic size increases and ionisation enthalpy generally decreases; the first member shows anomalous properties because of small size and absence of d orbitals.
- Inert-pair effect makes lower oxidation states more stable down groups 13–15, e.g. Pb²⁺ is more stable than Pb⁴⁺.
- Oxoacid strength increases with the number of terminal oxygen atoms and with electronegativity of the central atom.
- Halogens show decreasing oxidising power down the group; interhalogens are formed between different halogens.
Worked example
Explain why H₃PO₃ is dibasic. Its structure has one P=O bond, two P–OH groups and one P–H bond. Only the hydrogen atoms attached to oxygen are ionisable, so H₃PO₃ can release two protons and is dibasic. By contrast, H₃PO₂ has one P–OH group and is monobasic. The P–H hydrogen is not acidic because the P–H bond is not polar enough to release H⁺ in water.
Traps
Do not count all hydrogen atoms as acidic; do not call H₃PO₃ tribasic; remember the inert-pair effect when comparing oxidation states; do not confuse bleaching powder's formula with a simple chloride; check the structure of oxides before predicting acidity or basicity.