Chemical Bonding and Molecular Structure
Why This Chapter Matters
Chemical bonding is tested in JEE every year — 6-10 marks. VSEPR theory, hybridisation, bond polarity, and molecular orbital theory are the main focus areas.
Core Concepts
1. Types of Bonds
Ionic: Metal + non-metal. Electron transfer. High mp/bp, conducts when dissolved.
Covalent: Non-metal + non-metal. Electron sharing. Lower mp/bp.
Coordinate/Dative: Both electrons from one atom (donor to acceptor). e.g., NH₃→BF₃
Metallic: Electron sea model. Good conductors.
Hydrogen bond: X-H···Y (X,Y = F,O,N). Explains high bp of water, HF.
2. VSEPR Theory
Electron pairs (bonding + lone pairs) repel → adopt geometry to minimise repulsion.
Lone pairs repel MORE than bonding pairs.
| Electron pairs | Geometry | Bond angle |
|---|
|---|---|---|
| 2 | Linear | 180° |
|---|---|---|
| 3 | Trigonal planar | 120° |
| 4 | Tetrahedral | 109.5° |
| 5 | Trigonal bipyramidal | 90°, 120° |
| 6 | Octahedral | 90° |
With lone pairs: geometry (name) of molecule changes but electron geometry stays.
H₂O: 4 pairs (2 bond + 2 lone) → bent, angle 104.5° (< 109.5° due to LP repulsion)
NH₃: 4 pairs (3 bond + 1 lone) → pyramidal, angle 107°
3. Hybridisation
sp: linear (BeCl₂, C₂H₂)
sp²: trigonal planar (BF₃, C₂H₄, benzene)
sp³: tetrahedral (CH₄, NH₃, H₂O)
sp³d: trigonal bipyramidal (PCl₅)
sp³d²: octahedral (SF₆)
Hybridisation = bond pairs + lone pairs on central atom (for valence shell)
4. Bond Parameters
Bond length: covalent radius sum. Inversely proportional to bond order.
Bond energy: energy to break one mole of bonds. Higher bond order → more energy needed.
Bond order = (bonding e⁻ - antibonding e⁻)/2
Triple bond > double bond > single bond (in terms of energy and strength)
5. Molecular Orbital Theory (MO Theory)
Atomic orbitals combine to form molecular orbitals.
σ and σ (antibonding, marked with )
Filling order: σ1s, σ1s, σ2s, σ2s, σ2p, π2p, π2p, σ2p
Bond order = (N_bonding - N_antibonding)/2
If BO > 0: stable molecule | BO = 0: molecule doesn't exist
O₂: BO = 2, paramagnetic (2 unpaired electrons in π*)
N₂: BO = 3 (most stable diatomic)
6. Polarity
Bond polarity: difference in electronegativity → dipole moment (μ = q × d)
Molecular polarity: vector sum of all bond dipoles.
CO₂: linear, dipoles cancel → non-polar
H₂O: bent → polar
CCl₄: tetrahedral, dipoles cancel → non-polar
CHCl₃: dipoles don't cancel → polar
PYQs
2024: Hybridisation and shape of XeF₄?
Xe: 8 valence e. 4 bonds (F) + 2 lone pairs = 6 pairs → sp³d² hybridisation.
4 bonding + 2 LP (opposite faces) → square planar geometry.
2023: Which has higher bond angle: H₂O or NH₃?
Both sp³-ish. NH₃ has 1 LP, H₂O has 2 LP.
More LP → more repulsion → more compression of bond angle.
NH₃: 107° > H₂O: 104.5°
2022: Bond order of O₂⁺, O₂, O₂⁻?
O₂ has 16 electrons. MO: BO of O₂ = 2. O₂⁺ (remove 1e from antibonding) BO=2.5. O₂⁻ (add 1e to antibonding) BO=1.5.