p-Block Elements
p-Block Elements questions test periodic trends, the anomalous behaviour of the first member of each group, oxoacid strength, and the key industrial compounds.
Core rules
- Inert-pair effect: the stability of the (n − 2) oxidation state grows down groups 13–15, giving Tl⁺, Pb²⁺ and Bi³⁺.
- N₂ is inert because of its triple bond and small size; white phosphorus is reactive because of strained ring bonds.
- NCl₅ does not exist because nitrogen lacks d orbitals; PCl₅ exists as a trigonal bipyramid.
- H₃PO₃ is dibasic (the P–H hydrogen is not ionisable); H₃PO₄ is tribasic.
- Oxoacid strength of chlorine: HClO₄ > HClO₃ > HClO₂ > HClO, as the conjugate base gains resonance stabilisation with more terminal oxygens.
- H₂O is a liquid but H₂S is a gas, because of strong hydrogen bonding in water.
- Bond angles: H₂O (104.5°) < NH₃ (107°) < CH₄ (109.5°), following the number of lone pairs.
- Xenon fluorides: XeF₂ linear, XeF₄ square planar, XeF₆ distorted octahedral; XeF₆ hydrolyses to XeO₃.
- Concentrated H₂SO₄ is both an oxidising agent and a dehydrating agent; the dilute acid is neither.
Worked example
Arrange HClO, HClO₂, HClO₃ and HClO₄ in increasing acid strength. Write the conjugate bases ClO⁻, ClO₂⁻, ClO₃⁻, ClO₄⁻. As terminal oxygens increase, the negative charge is delocalised over more equivalent resonance forms, so the anion is stabilised and the O–H bond is released more readily. Order: HClO < HClO₂ < HClO₃ < HClO₄.
Traps that cost marks
- Writing NCl₅ or PH₅ as stable compounds.
- Forgetting that H₃PO₃ is dibasic, not tribasic.
- Treating all noble gases as completely unreactive; xenon forms fluorides.
- Reversing the oxidising and dehydrating roles of concentrated H₂SO₄.
- Mixing up bond angles: NH₃ is larger than H₂O but smaller than CH₄.