p-Block Elements
p-Block questions focus on periodic trends, anomalous behaviour of the first member, and the shapes and oxoacids of the heavier elements.
Group 15
- N2 is inert: triple bond, small size, no dipole
- NH3 is basic and hydrogen-bonded; HNO3 is a strong oxidiser
- white phosphorus (P4) is reactive and soluble in CS2; red phosphorus is polymeric and less reactive
- H3PO3 is dibasic while H3PO4 is tribasic
- NCl3 hydrolyses to NH3 and HOCl
Group 16
- ozone is bent and diamagnetic, and liberates iodine from acidified KI (a quantitative estimation)
- H2O2 acts as oxidant and reductant; it titrates against acidified KMnO4
- SO2 bleaches by reduction (temporary), Cl2 bleaches by oxidation
- H2SO4 is both an oxidising agent and a dehydrating agent; contact process gives it
Group 17 and 18
- Cl2 with cold dilute NaOH gives NaCl and NaOCl; with hot concentrated NaOH it gives NaClO3
- acid strength: HClO4 > HClO3 > HClO2 > HOCl, because extra oxygen atoms delocalise and stabilise the conjugate base
- XeF2 is linear, XeF4 square planar, XeF6 a distorted octahedron
- inert pair effect stabilises lower oxidation states down groups: Tl(I), Pb(II), Bi(III)
Worked example
Comparing HOCl and HClO4: both have an O–Cl bond, but perchlorate has four oxygens. The extra oxygens pull electron density away and spread the negative charge of the anion by resonance, so ClO4⁻ is far more stable than ClO⁻. Hence HClO4 is a much stronger acid.
Traps that cost marks
- Calling H3PO3 tribasic; it has two ionisable OH groups.
- Saying SO2 bleaching is permanent.
- Assuming XeF6 is a regular octahedron; it is distorted by a lone pair.