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Chemistry 3% exam weight

p-Block

Part of the JEE Main study roadmap. Chemistry topic chem-016 of Chemistry.

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p-Block

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The p-block spans Groups 13–18 of the periodic table. Their outer electrons fill the np sub-shell, giving a valence configuration of ns²np¹–⁶. The block contains the most structurally diverse chemistry tested in JEE Main: from boranes and interhalogens to oxoacids and xenon fluorides.

  • First-element anomaly: B, C, N, O, and F deviate sharply from their groups because of small size, high electronegativity, and the absence of d-orbitals.
  • Inert pair effect: the ns² pair resists participation in bonding down Groups 13–15, so Tl⁺, Pb²⁺, Bi³⁺ become the dominant cations.
  • pπ–pπ multiple bonds are strong only between Period 2 atoms (C=C, C≡C, N≡N, O=O). Beyond Period 2, pπ–dπ overlap dominates, which is why Si=O is fleeting but P=O, S=O, Cl=O are common.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Electronic Configuration and Group-Wise Character

Across the block the ns²np¹–⁶ valence pattern produces a steady shift from strongly electropositive (Al, Ga) to strongly electronegative (O, F, Cl) behaviour. Metallicity increases down each group, so Group 13 is metallic except for boron, while Group 17 stays non-metallic throughout.

Anomalous Behaviour of the Period-2 Elements

PropertyWhy Period-2 differs
Small atomic radiusHigh charge density, strong polarising power
No available d-orbitalsMaximum covalency = 4, no expanded octet
High electronegativityHydrides (H₂O, HF, NH₃) associate via H-bonding
pπ–pπ overlapStrong multiple bonds (C=C, N≡N, O=O)

Hybridisation and Shapes (Most-Tested Cluster)

JEE Main repeatedly asks about shape, hybridisation, and lone pairs. The three workhorse hybridisations are sp³ (tetrahedral, e.g. NH₄⁺, ClO₄⁻), sp³d (trigonal bipyramidal, e.g. PCl₅), and sp³d² (octahedral, e.g. SF₆).

  • BF₃: sp², trigonal planar — B has only 6 valence electrons, not 8.
  • PCl₃ vs PCl₅: PCl₃ is sp³ pyramidal; PCl₅ is sp³d trigonal bipyramidal with two distinct P–Cl bond lengths.
  • H₃PO₃ has the structure HP(O)(OH)₂, giving only 2 acidic protons, not 3. H₃PO₂ has only 1 acidic proton.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Inert Pair Effect in Detail

The relativistic stabilisation of the ns² pair grows with atomic number, so the n − 1 oxidation state becomes more stable down a group. Concretely, Tl⁺ (thallous) dominates over Tl³⁺, Pb²⁺ over Pb⁴⁺, and Bi³⁺ over Bi⁵⁺ in aqueous chemistry. This is why TlCl, PbCl₂, BiCl₃ are the commonly encountered halides rather than TlCl₃, PbCl₄, BiCl₅.

Halogen oxidising power falls down the group: F₂ > Cl₂ > Br₂ > I₂. The F–F bond is weaker than Cl–Cl because lone-pair repulsion in the tiny F₂ molecule outweighs the bond energy gain. Interhalogens are written XY, XY₃, XY₅, XY₇, and the central halogen always uses its higher (n) oxidation state — never positive for F.

Group-18 (Noble Gas) Compounds

Only xenon forms a reliable fluoride series under heating with F₂:

CompoundHybridisationShapeLone pairs on Xe
XeF₂sp³dLinear3
XeF₄sp³d²Square planar2
XeF₆sp³d³Distorted octahedral1

Common Traps in MCQs

  • Writing NO₂ as bent ignores that the neutral molecule is an odd-electron radical with bond order 2.5 and one unpaired electron.
  • Treating O₂ as diamagnetic overlooks the MO picture: two unpaired electrons in the π* level.
  • Quoting H₃PO₃ as tribasic is the single most common oxoacid error.
  • JEE Main weightage: 1–2 questions per paper, almost always from Groups 15–17 structures or inert pair effect.

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