p-Block Elements

Syllabus Unit: Inorganic Chemistry

Why it matters

Chapter overview

The p-block spans groups 13 to 18 and is the most chemically diverse region of the periodic table — metals, metalloids and non-metals sit side by side. JEE tests it around one recurring theme: the first member of each group (B, C, N, O, F) behaves anomalously compared to its heavier congeners due to its small size, absence of d-orbitals, and strong pπ-pπ bonding ability, while the heavier members increasingly show the inert pair effect and catenation.

Chemistry Syllabus Structure Inorganic Chemistry ~33% weightage

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Groups 13 & 14

1BoronElectron-deficient, Lewis acid

2AluminiumAmphoteric oxide

3CarbonStrongest catenation

4LeadInert pair effect (Pb2+)

1Groups 13 & 14: Boron & Carbon Families

Boron and carbon head chemically dissimilar families — boron's small, electron-deficient atoms form Lewis-acidic covalent compounds, while carbon's unmatched catenation and multiple-bonding ability builds the whole of organic chemistry.

Learning Objectives

State the general electronic configuration of groups 13 (ns2np1) and 14 (ns2np2) and explain the growing inert pair effect down each group.
Explain why boron compounds like BF3 and BCl3 are electron-deficient Lewis acids.
Compare the structures of diamond, graphite and fullerene (C60).
Define catenation and explain why carbon shows it far more strongly than silicon.

Key Terms

Inert pair effectThe reluctance of the heaviest p-block elements' ns2 electrons to participate in bonding, favouring lower oxidation states such as Tl+ and Pb2+.

Electron-deficient compoundA compound whose central atom has fewer than 8 valence electrons (e.g. BF3), making it a strong Lewis acid.

CatenationThe self-linking of like atoms into chains or rings; strongest in carbon due to the high strength of the C-C bond.

AllotropyThe existence of an element in two or more structurally distinct forms, such as diamond and graphite for carbon.

Group 13 (B, Al, Ga, In, Tl) has configuration ns2np1; group 14 (C, Si, Ge, Sn, Pb) has ns2np2. Down both groups, the inert pair effect grows stronger, making the oxidation state two below the group valence increasingly favoured for the heaviest members.

Boron's electron-deficient chemistry

Boron has only 3 valence electrons, so its trihalides (BF3, BCl3) and hydrides are electron-deficient and act as strong Lewis acids, readily accepting an electron pair. Diborane (B2H6) achieves stability using 4 normal terminal B-H bonds plus 2 bridging 3-centre-2-electron B-H-B 'banana bonds'.

Common MistakeDown group 13, Tl+ is more stable than Tl3+ (inert pair effect) — the opposite of what simple group-valence reasoning (Tl should be +3 like B, Al) would suggest.

Carbon's allotropes and catenation

Diamond — each C is sp3-hybridised, bonded to 4 others in a rigid 3D network; hardest known natural substance, poor conductor.
Graphite — sp2-hybridised C in flat hexagonal sheets held by weak van der Waals forces between layers; slippery and conducts electricity via delocalised π electrons.
Fullerene (C60) — sp2 carbon atoms arranged in a closed cage of fused hexagons/pentagons, resembling a soccer ball.

Carbon shows far stronger catenation than silicon because the C-C bond (348 kJ/mol) is much stronger than Si-Si (about 226 kJ/mol), and carbon lacks accessible d-orbitals that would otherwise let larger atoms react further and break long chains.