VSEPR Theory & Molecular Geometry for JEE Main

VSEPR (Valence Shell Electron Pair Repulsion) theory is the fastest tool for predicting molecular shapes, and JEE Main tests it both directly (predict the shape of PCl₅, SF₄, XeF₂) and indirectly (compare bond angles in NH₃ vs H₂O, explain why the bond angle in H₂O is 104.5° and not 109.5°). This guide gives you the VSEPR framework, the hybridisation shortcut, and the full molecule-by-molecule table that covers every JEE target.

VSEPR Rules and the Geometry Prediction Method

VSEPR rules: (1) electron pairs around the central atom repel each other and arrange to maximise inter-pair distance; (2) lone pairs occupy more space than bonding pairs (stronger repulsion); (3) repulsion order: LP-LP > LP-BP > BP-BP; (4) double and triple bonds are treated as single electron domains for geometry purposes. Steps to predict geometry: count bonding pairs (BP) and lone pairs (LP) on central atom. Steric number (SN) = BP + LP. Map SN to the electron geometry, then remove lone pairs for the molecular geometry.

SN = 2: linear (180°, no LP) — examples: BeCl₂, CO₂, HgCl₂. SN = 3: trigonal planar (120°, no LP) — BF₃, AlCl₃, SO₃; bent (< 120°, 1 LP) — SO₂, SnCl₂. SN = 4: tetrahedral (109.5°, no LP) — CH₄, SiCl₄, NH₄⁺; trigonal pyramidal (~107°, 1 LP) — NH₃, PCl₃; bent (~104.5°, 2 LP) — H₂O, H₂S. SN = 5: trigonal bipyramidal (no LP) — PCl₅; see-saw (1 LP, axial) — SF₄; T-shaped (2 LP, axial) — ClF₃; linear (3 LP) — XeF₂, I₃⁻. SN = 6: octahedral (no LP) — SF₆; square pyramidal (1 LP) — BrF₅; square planar (2 LP, trans) — XeF₄, ICl₄⁻. Take a free chemical bonding mock to test your shape-prediction speed. For deeper chemical bonding, see our chemical bonding guide.

Hybridisation and Geometry: The Shortcut

Hybridisation type = steric number: SN=2 → sp, SN=3 → sp², SN=4 → sp³, SN=5 → sp³d, SN=6 → sp³d². This shortcut holds for most JEE molecules. Exceptions to note: NO₂ appears sp² hybridised (SN=3, one LP which is actually a half-filled orbital), giving a bent shape with angle ≈134°. For transition metal complexes, use the d²sp³ (inner orbital) or sp³d² (outer orbital) distinction based on CFSE and spectrochemical series — covered in Coordination Chemistry.

Bond angle trends: as LP count increases in a series (CH₄ → NH₃ → H₂O), bond angle decreases because LP-BP repulsion is greater than BP-BP. So H₂O (104.5°) < NH₃ (107°) < CH₄ (109.5°). Similarly, for hybridisation affecting bond angle: sp (180°) > sp² (120°) > sp³ (109.5°). When multiple factors compete, identify the dominant one. For the complete chemical bonding framework including MO theory, see our molecular orbital theory guide and our hybridisation guide.

Dipole Moments and Polarity

A molecule is polar if: (1) it has polar bonds (significant electronegativity difference), AND (2) the bond dipoles do not cancel due to asymmetric geometry. Symmetrical molecules are non-polar even if bonds are polar: CCl₄ (tetrahedral), BF₃ (trigonal planar), CO₂ (linear), SF₆ (octahedral), XeF₄ (square planar), PCl₅ (trigonal bipyramidal). Asymmetric molecules are polar: CHCl₃, NH₃ (both cases — polar bonds, incomplete cancellation), H₂O, SO₂, SF₄. JEE tests: "Which of the following has a zero dipole moment?" — look for high symmetry.

Comparing dipole moments in a series: NF₃ vs NH₃ — both pyramidal, but in NF₃ the bond dipoles (N→F) point away from the lone pair and partly cancel with the LP vector, giving μ(NF₃) < μ(NH₃). This is the classic JEE counterintuitive question: despite N-F bonds being more polar than N-H bonds, NH₃ has a larger dipole moment. The explanation requires correctly adding the bond dipole vectors with the lone pair contribution. For related inorganic chemistry, see our P-Block elements guide.

Quick Reference Table for JEE

Molecules to memorise with their shapes: ClF₃ (T-shaped, sp³d), I₃⁻ (linear, sp³d²), NO₂⁺ (linear), NO₂⁻ (bent), SO₃²⁻ (trigonal pyramidal), BrF₅ (square pyramidal), [Ni(CN)₄]²⁻ (square planar, dsp²), [Ni(Cl)₄]²⁻ (tetrahedral, sp³). The contrast between nickel complexes — planar for CN⁻ (strong field), tetrahedral for Cl⁻ (weak field) — is a classic JEE question. Build your own molecule shape table and review it weekly until every shape is automatic. For a comprehensive VSEPR and hybridisation revision, see our chemical bonding guide.