Chemical Bonding & Molecular Structure

Concept overview

Chemical bonding is the framework that lets you predict a molecule's shape, polarity and reactivity from its Lewis structure. Three complementary theories together cover every JEE bonding question: VSEPR for shape, Valence Bond Theory (VBT) with hybridisation for bond-angle explanations, and Molecular Orbital Theory (MOT) for magnetic behaviour and bond order.

VSEPR — Valence Shell Electron Pair Repulsion — says that electron pairs around a central atom arrange themselves to minimise repulsion. Count the electron pairs (bond pairs + lone pairs) around the central atom to get the electron-pair geometry: 2 → linear, 3 → trigonal planar, 4 → tetrahedral, 5 → trigonal bipyramidal, 6 → octahedral. Lone pairs repel more than bond pairs, so their presence bends the molecular geometry away from the ideal (H₂O is bent, not linear; NH₃ is trigonal pyramidal, not tetrahedral).

Hybridisation gives a quantitative version of VSEPR from a valence-bond perspective. sp (linear), sp² (trigonal), sp³ (tetrahedral), sp³d (trigonal bipyramidal), and sp³d² (octahedral) match VSEPR one-for-one. Multiple bonds involve unhybridised p orbitals — C=C is one σ (sp²-sp²) plus one π (p-p sideways overlap); C≡C is one σ plus two π.

MOT is essential for diatomic molecules where VBT fails — most famously O₂, which VBT predicts as diamagnetic but is actually paramagnetic. MOT correctly assigns O₂ two unpaired electrons in π* orbitals, matching the observation. Bond order in MOT = ½ × (electrons in bonding − electrons in antibonding). Bond order tracks with bond strength and short bond length.

Ionic bonding, covalent bonding, coordinate bonds, dipole moments, hydrogen bonding and Fajans' rules (predicting when an ionic bond gains covalent character) complete the JEE syllabus. Master these six frameworks and every JEE bonding question becomes a look-up.

Key formulas & concepts

• VSEPR geometries: linear (2), trigonal planar (3), tetrahedral (4), TBP (5), octahedral (6)
• Hybridisation: sp, sp², sp³, sp³d, sp³d² → 2, 3, 4, 5, 6 electron pairs
• Bond order (MOT): ½ × (bonding electrons − antibonding electrons)
• Bond angle H₂O vs NH₃ vs CH₄: 104.5° < 107° < 109.5° (lone pairs bend angle in)
• Dipole moment: μ = q × d (Debye = 3.336 × 10⁻³⁰ C·m)
• Fajans' rules: Covalent character ↑ with: small cation, large anion, high charge on either, high-polarisability anion
• O₂ magnetic behaviour: Paramagnetic (MOT correctly predicts 2 unpaired π* electrons; VBT does not)

Common mistakes to avoid

• Assigning tetrahedral geometry to water — the electron-pair geometry is tetrahedral but the molecular geometry (only bonded atoms) is bent.
• Predicting O₂ as diamagnetic based on Lewis structure — MOT correctly shows O₂ is paramagnetic.
• Using Fajans' rules without considering polarisability — a large anion (I⁻) is more polarisable than a small anion (F⁻).
• Getting hybridisation wrong for molecules with expanded octets — PCl₅ is sp³d, not sp³.

JEE Main relevance

JEE Main asks 3–4 bonding-related questions every paper — one shape/hybridisation, one bond-order via MOT, one dipole-moment or polarity, and often one Fajans'-rules or lattice-energy problem. This is the single largest chapter in the Inorganic JEE Main paper.

JEE Advanced relevance

Advanced problems combine MOT with magnetic-moment calculation, or bond-order-vs-bond-length reasoning. Comparing bond angles in a family of hydrides (NH₃ vs PH₃ vs AsH₃ vs SbH₃) is another Advanced staple, and requires understanding the balance between s-character and inter-electronic repulsion.

Previous-year questions (PYQ)

Chemical Bonding is the second-largest JEE Chemistry chapter in the PYQ bank. Split practice: hybridisation and shape as one archetype, MOT diagrams as another, Fajans and lattice energy as a third.