p-Block Elements
p-Block Elements in this exam covers Group 15–18 trends, the anomalous behaviour of the first member, oxoacids, interhalogens and xenon compounds. The exam often asks you to explain a trend rather than recall a list.
Group trends
- Down Group 15, metallic character increases and catenation decreases; N2 is inert because of its strong triple bond.
- Down Group 17, oxidising power decreases F2 > Cl2 > Br2 > I2, while acid strength in water increases HF < HCl < HBr < HI.
- HF has an unusually high boiling point because of intermolecular hydrogen bonding; HCl is a gas.
Important compounds
- Allotropes: white and red phosphorus; rhombic and monoclinic sulphur.
- Interhalogens such as ClF3, BrF5 and IF7 are more reactive than the parent halogens.
- Xenon forms XeF2 (sp3d, linear), XeF4 (sp3d2, square planar) and XeF6 (sp3d3, distorted octahedral).
- Oxoacid strength for chlorine: HClO4 > HClO3 > HClO2 > HClO.
Worked example
Find the oxidation state of xenon in XeOF4 and predict its shape. Reasoning: oxygen is -2 and each fluorine is -1. Let x be Xe: x + (-2) + 4(-1) = 0, so x = +6. Xenon has six electron pairs: five bonding pairs and one lone pair, giving square pyramidal geometry.
Traps
- Do not say HF is the strongest hydrohalic acid; it is the weakest.
- F2 is not prepared by electrolysing aqueous HF because F2 reacts with water; anhydrous KHF2 in HF is used.
- The inert-pair effect explains Tl(I) and Pb(II) stability, not a simple loss of reactivity.
- Interhalogens are not mixtures; they are definite compounds.