VSEPR & HybridizationJEE Main

Concept

VSEPR theory predicts a molecule's shape by arranging electron pairs around the central atom as far apart as possible. The steric number (bonding + lone pairs) fixes the hybridization and electron geometry; lone pairs then distort the visible molecular shape.

Key formula

Derivation

Electron pairs repel, so $n$ pairs adopt the geometry that maximises their separation: 2 → linear (sp), 3 → trigonal planar (sp²), 4 → tetrahedral (sp³), 5 → trigonal bipyramidal (sp³d), 6 → octahedral (sp³d²).

Lone pairs repel more strongly than bonding pairs, so they squeeze bond angles (CH₄ 109.5° → NH₃ 107° → H₂O 104.5°) and change the named shape even when the electron geometry is the same.

Scenarios to explore

• VSEPR & Hybridization — Molecular shape from bonding & lone pairs.
• BeCl₂ — linear — Two bonds, no lone pairs: 180°, sp.
• BF₃ — trigonal planar — Three bonds, no lone pairs: 120°, sp².
• H₂O — bent — Two bonds and two lone pairs squeeze the angle to about 104.5°.
• NH₃ — trigonal pyramidal — Three bonds and one lone pair: a pyramid, sp³.
• SF₄ — see-saw — Four bonds and one lone pair: the lone pair sits equatorial.
• PCl₅ — trigonal bipyramidal — Five bonds: sp³d, with distinct axial and equatorial positions.
• ClF₃ — T-shaped — Three bonds and two lone pairs, both equatorial.
• XeF₂ — linear again — Two bonds and three lone pairs: the lone pairs take the equator.
• SF₆ — octahedral — Six bonds, 90° between neighbours: sp³d².
• XeF₄ — square planar — Six electron pairs, two lone pairs opposite each other.

Real-world applications

• Predicting polarity and dipole moments.
• Explaining reactivity and physical properties.
• Rationalising IR/NMR spectra from structure.

JEE exam tips

• Lone-pair repulsion order: lp–lp > lp–bp > bp–bp — this sets the angle squeezing.
• Steric number maps directly to hybridization: 2→sp, 3→sp², 4→sp³, 5→sp³d, 6→sp³d².

Common mistakes

• Counting lone pairs toward the molecular shape name (only bonded atoms define it).
• Forgetting lone pairs still occupy hybrid orbitals (so they count for hybridization).
• Treating a double bond as two regions — it counts as one electron domain.

Exam traps to avoid

• H₂O and CO₂ both have two bonds, but lone pairs make water bent and CO₂ linear.
• Square planar (XeF₄) and tetrahedral both have 4 bonds — lone pairs decide which.