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The p-Block Elements

Groups thirteen to eighteen: trends, the inert pair effect, oxoacids, interhalogens and xenon compounds. Exact recall of structures and reactions.

Questions in the bank
185
q/paper in 2025–26
1.09
Calculation
14%
Notes pages
8

Strand: Structure and recall

When you’ll see it

A statement about a group 13 to 18 element or compound: a trend and its exception, an oxoacid's structure or basicity, an oxide's nature, an interhalogen or xenon fluoride shape, or a salt-analysis test.

How this chapter is tested

Nearly every question is multiple choice, and most are lists of statements where each claim is a trend or one of its exceptions. Check each statement on its own before looking at the options; a statement can name a true fact and attach the wrong reason.

The exceptions repeat across the groups and have three causes. Poor d and f shielding makes Ga smaller than Al and lifts the ionisation enthalpy of Tl and Pb. The inert pair effect makes the lower state more stable down groups 13 to 16, so Tl³⁺ and Pb⁴⁺ oxidise while Sn²⁺ reduces. The small first member of each group has no d orbitals and crowded lone pairs: BF₆³⁻ and NCl₅ do not exist, the N–N and F–F bonds are weak, and Cl, not F, has the most negative electron gain enthalpy.

The few counting questions follow fixed rules. The basicity of a phosphorus oxoacid is its count of P–OH groups. The lone pairs on an interhalogen's centre are (7 − n)/2 and on xenon (8 − n)/2. The π bonds in a sulphur oxoacid equal its S=O count.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Group 13 trends and the inert pair

    Radius B < Ga < Al < In < Tl; first ionisation enthalpy lowest at In; +1 grows stable down to Tl, so Tl³⁺ oxidises; TlI₃ is Tl⁺I₃⁻ and GaAlCl₄ is Ga⁺[AlCl₄]⁻.

  • Boron and aluminium compounds

    Borax is Na₂[B₄O₅(OH)₄]·8H₂O; boric acid is a monobasic Lewis acid; diborane has two three-centre two-electron bridges and is not planar; Lewis acidity BF₃ < BCl₃ < BBr₃ < BI₃; boron's covalency stops at four.

  • Group 14

    Carbon forms pπ–pπ bonds and stops at covalency four; [SiF₆]²⁻ exists, [SiCl₆]²⁻ does not; C₆₀ has twenty hexagons and twelve pentagons; Sn²⁺ reduces, Pb⁴⁺ oxidises; PbCrO₄ dissolves in NaOH as Na₂[Pb(OH)₄].

  • Group 15 and its hydrides

    N–N is weaker but shorter than P–P; +5 grows less stable down the group; from NH₃ to BiH₃ stability and basicity fall and reducing power rises; boiling point PH₃ < AsH₃ < NH₃ < SbH₃.

  • Nitrogen and its oxides

    N₂O and NO are neutral, the rest acidic; N₂O, N₂O₃ and N₂O₄ have an N–N bond, N₂O₅ an oxygen bridge; NO₂ is the odd-electron oxide; the brown ring holds NO; Ostwald oxidises NH₃ to NO.

  • Phosphorus and its oxoacids

    Basicity = P–OH groups (H₃PO₂ one, H₃PO₃ two, H₃PO₄ three); any P–H bond makes the acid a reductant; P₄ with hot NaOH gives PH₃ and NaH₂PO₂; H₄P₂O₇ has one P–O–P bridge.

  • Group 16 and sulphur

    Oxygen shows −2, −1, +1 and +2; the hydrides grow more acidic and more reducing from H₂O to H₂Te; H₂S₂O₇ has an S–O–S bridge and H₂S₂O₈ a peroxo link, both at +6; I₂ stops thiosulphate at tetrathionate, Br₂ takes it to sulphate.

  • Halogens, interhalogens and xenon

    Bond enthalpy Cl₂ > Br₂ > F₂ > I₂; HF boils highest, HI melts highest; cold dilute alkali gives ClO⁻, hot concentrated ClO₃⁻; XX′₃ is T-shaped, XX′₅ square pyramidal; XeF₂ linear, XeF₄ square planar.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • The smooth group order

    B < Al < Ga < In < Tl for atomic radius is wrong, because Ga is smaller than Al. A steady boiling-point rise from NH₃ is wrong too: PH₃ boils lowest.

  • Back-bonding read as acid strength

    Back-bonding is strongest in BF₃, and that is what makes BF₃ the weakest Lewis acid of the boron trihalides, not the strongest.

  • P–H hydrogens counted as acidic

    H₃PO₃ is dibasic and H₃PO₂ monobasic, because a hydrogen on phosphorus never ionises. H₃PO₂ with NaOH gives NaH₂PO₂.

  • Oxo and peroxo bridges swapped

    Oleum, H₂S₂O₇, has S–O–S; Marshall's acid, H₂S₂O₈, has O–O. N₂O₄ has an N–N bond and no bridging oxygen; N₂O₅ has the bridge.

  • Bonds counted as oxidation state

    Boron makes four bonds in [BF₄]⁻ but is +3; sulphur in H₂S₂O₈ is +6, because the two peroxo oxygens are −1. Balance the charge, do not count the bonds.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

The p-Block Elements notes

Drill every The p-Block Elements question

185 questions from the bank, across 8 subtopics.

Drill the 185 questions

Drill one subtopic at a time

The 8 subtopics, in teaching order.

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