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Chemical Bonding & Molecular Structure revision notes

Every idea in the Chemical Bonding & Molecular Structure chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 28 and download the PDF.

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Lewis structures, octet rule and formal charge

Valence electrons and lone pairs
7× asked, last 2026
N=∑V−q,lone pairs=N2−bondsN = \sum V - q,\qquad \text{lone pairs} = \frac{N}{2} - \text{bonds}
qq = signed charge (NOX2X−\ce{NO2-}: 18). Double bond = 2 bonds. Non-bonded e⁻ =N−2×=N-2\timesbonds; one atom: lp e⁻ + 2 per bond.
SpeciesNNBondsLone pairs (all atoms)
HX2O\ce{H2O}822
CFX4\ce{CF4}32412
NOX2X−\ce{NO2-}183 (one N=O counts 2)6
  • Not the central atom only. 'molecules with two lone pairs' counts every atom unless 'central' is written. Each terminal F or Cl carries 3 lone pairs, so CFX4\ce{CF4} has 12 while HX2O\ce{H2O} has 2.
Octet rule and its exceptions
9× asked, last 2026
Kossel–Lewis: gain, lose or share electrons to reach 8 (H, Li: 2); period 2 never exceeds 8. Covalency = bonds formed.
ExceptionExamples
Incomplete octetBeClX2\ce{BeCl2}, BFX3\ce{BF3}, BX2HX6\ce{B2H6}
Odd-electronNO\ce{NO}, NOX2\ce{NO2}, ClOX2\ce{ClO2}
Expanded octet (period 3 on)PClX5\ce{PCl5}, SFX6\ce{SF6}, HX2SOX4\ce{H2SO4}, SOX2\ce{SO2}
  • Not 'fewer than 8 ⇒ incomplete octet'. Check the total first: an odd count (NO\ce{NO} 11, NOX2\ce{NO2} 17) makes it odd-electron. Ionic LiH\ce{LiH} is not electron-deficient: LiX+\ce{Li+} and HX−\ce{H-} have full duets.
Formal charge
2× asked, last 2026
FC=V−L−12B\text{FC} = V - L - \tfrac12 B
VV: valence e⁻ of the free atom, LL: lone-pair e⁻, BB: bonding e⁻. FCs add up to the ion charge; prefer the smallest FCs.
  • Not −1 on the central atom because the species looks negative. Work atom by atom. Ozone's central O (1 lone pair, 3 bonds): 6−2−3=+16-2-3=+1; the single-bonded end O is −1-1. N in HNOX3\ce{HNO3} is +1+1 the same way.
Counting σ and π bonds
9× asked, last 2025
double=1σ+1π,triple=1σ+2π;σ=atoms−1+rings\text{double}=1\sigma+1\pi,\quad \text{triple}=1\sigma+2\pi;\qquad \sigma = \text{atoms}-1+\text{rings}
The σ formula holds for one connected molecule. A ring of nn atoms has nn ring bonds (SX8\ce{S8} crown: 8 S–S).
O S O O H O H
H₂SO₄: 7 atoms → 6 σ; 2 S=O → 2 π
O H N + O O -
HNO₃: 5 atoms → 4 σ; N stops at 8 → 1 π
C₂H₄: 6 atoms → 5 σ, 1 π
  • Oxoacids of period-3+ atoms: each acidic H sits on its own O; other non-bridging O are =O\ce{=O} (one π each): HX2SOX4\ce{H2SO4} has 6 σ, 2 π. N cannot pass 8, so HNOX3\ce{HNO3} has 1 π (NX+−OX−\ce{N+-O-}). P–H in HX3POX3\ce{H3PO3} is not acidic.
  • Not 'σ = single bonds only'. Every double and triple bond holds one σ too. Take σ = atoms − 1 (+1 per ring), then add the π; count H from the formula. CX2HX4\ce{C2H4}: 6 atoms, so 5 σ and 1 π.

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Practise Chemical Bonding & Molecular Structure

24 more concepts in 6 topics

Ionic bonding and Fajans' rules; Bond parameters, resonance and dipole moment; VSEPR theory and shapes; Valence bond theory and hybridisation; Molecular orbital theory; and Hydrogen bonding and intermolecular forces. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Ionic bonding and Fajans' rules

  • Ionic bond and lattice enthalpy2×
  • Fajans' rules: covalent character7×

Bond parameters, resonance and dipole moment

  • Bond length and bond enthalpy5×
  • Resonance and bond order of a hybrid2×
  • Dipole moment2×
  • Net dipole and molecular symmetry11×
  • NH₃ vs NF₃ and dipole orders8×

VSEPR theory and shapes

  • Steric number and lone pairs18×
  • Lone-pair repulsion and placement5×
  • Shapes: steric number 2 to 414×
  • Shapes: steric number 514×
  • Shapes: steric number 6 and 714×
  • Bond angle comparisons8×

Valence bond theory and hybridisation

  • Orbital overlap: σ and π bonds1×
  • Hybridisation from steric number17×
  • d orbitals used in hybridisation5×
  • Hybridisation in carbon compounds3×

Molecular orbital theory

  • LCAO: forming molecular orbitals8×
  • MO energy order (s–p mixing)8×
  • Bond order and isoelectronic species15×
  • Magnetism and the O₂ family21×

Hydrogen bonding and intermolecular forces

  • Hydrogen bond8×
  • Intermolecular vs intramolecular H-bond3×
  • Effects of H-bonding on properties8×
  • Also remember1

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