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JEE Mains Chemistry · Teaching notes

Chemical Bonding and Molecular Structure — JEE Mains Chemistry

Chemical Bonding and Molecular Structure has 176 past-year questions from 2021 to 2026, and 54 of them ask for a number rather than an option. Most of those numbers are counts: how many species in a list are linear, polar or paramagnetic, how many lone pairs sit on a central atom, how many σ and π bonds a chain holds. Each count rests on a short routine done the same way every time, so a careful count of electrons and electron pairs is worth more here than any formula. The rest of the chapter is orders and exceptions worth knowing by heart, such as NF₃ against NH₃ or O₂⁺ against O₂⁻.

Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.

Subtopic notes

Formula & revision sheet

8 formulas · 12 reference tables · 42 gotchas across all subtopics — the exam-eve cheat-sheet

Lewis Structures, Formal Charge and the Octet Rule

Formulas (1)

Reference tables (2)

The octet rule and its three exceptions5 rows
TypeElectrons on the central atomExamples
Obeys the octet rule8CH4\mathrm{CH_4}, CO2\mathrm{CO_2}, CCl4\mathrm{CCl_4}, NH3\mathrm{NH_3}, SiF4\mathrm{SiF_4}, H2S\mathrm{H_2S}
Incomplete octet4 for Be, 6 for B and AlBeF2\mathrm{BeF_2}, BeH2\mathrm{BeH_2} (4); BF3\mathrm{BF_3}, BCl3\mathrm{BCl_3}, AlCl3\mathrm{AlCl_3} (6)
Electron deficientBridging B–H–B bonds hold 2 electrons over 3 atomsB2H6\mathrm{B_2H_6}; BCl3\mathrm{BCl_3} is also called electron deficient
Odd-electron speciesAn odd total: NO 11, NO₂ 17, ClO₂ 19NO\mathrm{NO}, NO2\mathrm{NO_2}, ClO2\mathrm{ClO_2}
These are also the paramagnetic oxides: an odd electron cannot pair.
Expanded octet10 or 12 (14 in IF₇)PCl5\mathrm{PCl_5}, SF4\mathrm{SF_4} (10); SF6\mathrm{SF_6}, H2SO4\mathrm{H_2SO_4}, SO3\mathrm{SO_3} (12); IF7\mathrm{IF_7} (14)
Only period 3 and heavier atoms can expand the octet; N, O and F never do.
Lewis acids and Lewis bases6 rows
SpeciesRoleReason
BF3\mathrm{BF_3}, BCl3\mathrm{BCl_3}Lewis acidB has 6 electrons and an empty p orbital; sp2sp^2, trigonal planar
AlCl3\mathrm{AlCl_3}Lewis acidAl has 6 electrons; it dimerises to Al2Cl6\mathrm{Al_2Cl_6} to fill the gap
BI3\mathrm{BI_3}Strongest boron halide acidBack-bonding from large I into B is weakest
NH3\mathrm{NH_3}, NF3\mathrm{NF_3}Lewis baseOne lone pair on N; sp3sp^3, pyramidal
SF4\mathrm{SF_4}, ClF3\mathrm{ClF_3}Lewis baseOne lone pair on S; two on Cl
PCl5\mathrm{PCl_5}Not a Lewis baseAll five P electrons are in bonds; no lone pair
PCl₅ can accept a pair (forming PCl₆⁻), but it cannot donate one.
An empty orbital makes an acid; a lone pair on the central atom makes a base.

Watch out for (6)

Ionic Bonding, Lattice Enthalpy and Fajans' Rules

Formulas (1)

Reference tables (1)

Fajans' rules and covalent character5 rows
RuleOrder of covalent characterWhy
Smaller cationLiCl>NaCl>KCl>CsCl\mathrm{LiCl > NaCl > KCl > CsCl}Li+\mathrm{Li^+} is the smallest and most polarising
Higher cation chargeAlCl3>MgCl2>NaCl\mathrm{AlCl_3 > MgCl_2 > NaCl}; SnCl4>SnCl2\mathrm{SnCl_4 > SnCl_2}More charge on a smaller ion
Larger anionCaI2>CaBr2>CaCl2>CaF2\mathrm{CaI_2 > CaBr_2 > CaCl_2 > CaF_2}; KI>KF\mathrm{KI > KF}I−\mathrm{I^-} has the largest, softest cloud
18-electron cationCuCl>NaCl\mathrm{CuCl > NaCl}; AgCl>KCl\mathrm{AgCl > KCl}d electrons shield the nuclear charge poorly
Electronegativity differenceIonic character: N2<ClF3<SO2<K2O<LiF\mathrm{N_2 < ClF_3 < SO_2 < K_2O < LiF}Δχ\Delta\chi is 0 for N2\mathrm{N_2}, about 0.8 for Cl–F, 0.9 for S–O
The same polarisation that adds covalent character lowers the melting point and the solubility in water.

Watch out for (4)

Bond Length, Bond Angle and Resonance

Formulas (1)

Reference tables (2)

Bond length and what sets it10 rows
BondTypical length (pm)Note
C–H109Shortest here: hydrogen is tiny
C≡C120Triple bond
C=C134Double bond
C–C154Single bond
C≡N116Shorter than C=O despite N being larger than C
C=O122Carbonyl
C–O143Alcohols and ethers
O=O121In O₂
O–O148In H₂O₂
P–Cl in PCl₅219 axial, 204 equatorialAxial bonds are the longer, weaker pair
Calling the axial bonds of PCl₅ stronger is a standard wrong statement.
For the same pair of atoms: triple shorter than double, double shorter than single.
Bond angles and lone-pair repulsion8 rows
SpeciesPairs on the centreBond angle
BF3\mathrm{BF_3}3 bond, 0 lone120°
SO2\mathrm{SO_2}2 bond (plus π), 1 loneabout 119°
CH4\mathrm{CH_4}4 bond, 0 lone109.5°
NH3\mathrm{NH_3}3 bond, 1 lone107°
H2O\mathrm{H_2O}2 bond, 2 lone104.5°
NF3\mathrm{NF_3}3 bond, 1 lone102°
PF3\mathrm{PF_3}3 bond, 1 loneabout 98°
ClF3\mathrm{ClF_3}3 bond, 2 loneabout 87.5° (axial F–Cl–equatorial F)
The two lone pairs bend the axial F atoms back below 90°.
More lone pairs on the centre, or more electronegative outer atoms, means a smaller angle.

Watch out for (6)

VSEPR Theory and Lone Pairs

Formulas (1)

Reference tables (1)

Where lone pairs sit: equatorial and trans7 rows
SpeciesBond pairs, lone pairsLone pairs sitShape
SF4\mathrm{SF_4}, SeF4\mathrm{SeF_4}4, 1EquatorialSee-saw
ClF3\mathrm{ClF_3}, BrF3\mathrm{BrF_3}3, 2Both equatorialT-shaped (bent T), about 87.5°
XeF2\mathrm{XeF_2}, I3−\mathrm{I_3^-}, ICl2−\mathrm{ICl_2^-}2, 3All three equatorialLinear, 180°
XeO2F2\mathrm{XeO_2F_2}4, 1Equatorial, with the two OSee-saw, F atoms axial
BrF5\mathrm{BrF_5}, IF5\mathrm{IF_5}5, 1Any one octahedral siteSquare pyramidal
XeF4\mathrm{XeF_4}, BrF4−\mathrm{BrF_4^-}4, 2Trans, opposite each otherSquare planar, 90°
BrF2+\mathrm{BrF_2^+}2, 2Two corners of a tetrahedronBent
Five pairs: lone pairs equatorial. Six pairs: two lone pairs trans.

Watch out for (4)

Shapes of Molecules and Ions

Reference tables (1)

Shapes from the AXE formula15 rows
TypePairs (X + E)ShapeExamples
AX₂2LinearBeCl2\mathrm{BeCl_2}, CO2\mathrm{CO_2}, NO2+\mathrm{NO_2^+}, N3−\mathrm{N_3^-}, HC≡C−\mathrm{HC{\equiv}C^-}
AX₃3Trigonal planarBF3\mathrm{BF_3}, SO3\mathrm{SO_3}, NO3−\mathrm{NO_3^-}, CO32−\mathrm{CO_3^{2-}}
AX₂E3BentSO2\mathrm{SO_2}, O3\mathrm{O_3}, NO2−\mathrm{NO_2^-}
AX₄4TetrahedralCH4\mathrm{CH_4}, NH4+\mathrm{NH_4^+}, SO42−\mathrm{SO_4^{2-}}, SO2Cl2\mathrm{SO_2Cl_2}
AX₃E4Trigonal pyramidalNH3\mathrm{NH_3}, H3O+\mathrm{H_3O^+}, SO32−\mathrm{SO_3^{2-}}, ClO3−\mathrm{ClO_3^-}, BrO3−\mathrm{BrO_3^-}, XeO3\mathrm{XeO_3}
AX₂E₂4BentH2O\mathrm{H_2O}, OF2\mathrm{OF_2}, ClO2−\mathrm{ClO_2^-}, BrF2+\mathrm{BrF_2^+}
AX₅5Trigonal bipyramidalPCl5\mathrm{PCl_5}, PF5\mathrm{PF_5}, Fe(CO)5\mathrm{Fe(CO)_5}
AX₄E5See-sawSF4\mathrm{SF_4}, SeF4\mathrm{SeF_4}, IF4+\mathrm{IF_4^+}, XeO2F2\mathrm{XeO_2F_2}
AX₃E₂5T-shapedClF3\mathrm{ClF_3}, BrF3\mathrm{BrF_3}, IF3\mathrm{IF_3}
AX₂E₃5LinearXeF2\mathrm{XeF_2}, I3−\mathrm{I_3^-}, IBr2−\mathrm{IBr_2^-}
AX₆6OctahedralSF6\mathrm{SF_6}, [CrF6]3−\mathrm{[CrF_6]^{3-}}
AX₅E6Square pyramidalBrF5\mathrm{BrF_5}, IF5\mathrm{IF_5}, XeOF4\mathrm{XeOF_4}
AX₄E₂6Square planarXeF4\mathrm{XeF_4}, ICl4−\mathrm{ICl_4^-}, BrF4−\mathrm{BrF_4^-}
AX₇7Pentagonal bipyramidalIF7\mathrm{IF_7}
AX₆E7Distorted octahedralXeF6\mathrm{XeF_6}
Same total of pairs, same arrangement; the lone pairs decide the name of the shape.

Watch out for (4)

Hybridisation and Sigma and Pi Bonds

Formulas (2)

Reference tables (1)

Hybridisation, orientation and complexes8 rows
HybridisationOrientationMain-group examplesComplex examples
spspLinear, 180°BeCl2\mathrm{BeCl_2}, CO2\mathrm{CO_2}, NO2+\mathrm{NO_2^+}[Ag(NH3)2]+\mathrm{[Ag(NH_3)_2]^+}
sp2sp^2Trigonal planar, 120°BF3\mathrm{BF_3}, SO2\mathrm{SO_2}, NO2−\mathrm{NO_2^-}Rare in complexes
sp3sp^3Tetrahedral, 109.5°CH4\mathrm{CH_4}, NH4+\mathrm{NH_4^+}, XeO3\mathrm{XeO_3}Ni(CO)4\mathrm{Ni(CO)_4}, [NiCl4]2−\mathrm{[NiCl_4]^{2-}}
dsp2dsp^2Square planar, 90°None[PtCl4]2−\mathrm{[PtCl_4]^{2-}}, [Ni(CN)4]2−\mathrm{[Ni(CN)_4]^{2-}}
sp3dsp^3dTrigonal bipyramidalPCl5\mathrm{PCl_5}, SF4\mathrm{SF_4}, ClF3\mathrm{ClF_3}, XeF2\mathrm{XeF_2}Fe(CO)5\mathrm{Fe(CO)_5} is often written dsp3dsp^3
sp3d2sp^3d^2Octahedral, 90°SF6\mathrm{SF_6}, BrF5\mathrm{BrF_5}, XeF4\mathrm{XeF_4}[CoF6]3−\mathrm{[CoF_6]^{3-}} (outer orbital)
d2sp3d^2sp^3Octahedral, 90°None[Co(NH3)6]3+\mathrm{[Co(NH_3)_6]^{3+}}, [Fe(CN)6]3−\mathrm{[Fe(CN)_6]^{3-}} (inner orbital)
[Co(NH₃)₆]³⁺ is d²sp³, not sp³d²: a stated match of it with SF₆ is false.
sp3d3sp^3d^3Pentagonal bipyramidalIF7\mathrm{IF_7}, XeF6\mathrm{XeF_6} (distorted)None
The hybridisation fixes the arrangement of pairs; the shape then depends on how many are lone pairs.

Watch out for (6)

Molecular Orbital Theory

Formulas (1)

Reference tables (2)

Combining atomic orbitals (LCAO)8 rows
Pair of orbitals (axis z)Symmetry of eachDo they combine?
1s and 1sσ and σYes: σ1s and σ*1s
2pz2p_z and 2pz2p_zσ and σYes: σ2p and σ*2p (head-on)
2px2p_x and 2px2p_xπ and πYes: π2p and π*2p (sideways)
2s and 2pz2p_zσ and σYes, if their energies are close
2s and 2py2p_yσ and πNo: zero net overlap
2px2p_x and 2py2p_yπ, but at right anglesNo: they are orthogonal
3dxz3d_{xz} and 2px2p_xπ and πYes: a π overlap
3dxy3d_{xy} and 3dx2−y23d_{x^2-y^2}δ and δ, but rotated 45°No: orthogonal to each other
Both are δ type, yet they cancel; same symmetry label is not enough when the lobes are turned 45°.
Same symmetry about the axis and a matching orientation are both needed for a net overlap.
Unpaired electrons and magnetism10 rows
SpeciesElectronsBond orderUnpaired electronsMagnetism
H2+\mathrm{H_2^+}, He2+\mathrm{He_2^+}1, 30.51Paramagnetic
Li2\mathrm{Li_2}610Diamagnetic
B2\mathrm{B_2}1012Paramagnetic
C2\mathrm{C_2}1220Diamagnetic
C2−\mathrm{C_2^-}, N2+\mathrm{N_2^+}132.51Paramagnetic
N2\mathrm{N_2}, CO, CN−\mathrm{CN^-}, NO+\mathrm{NO^+}1430Diamagnetic
N2−\mathrm{N_2^-}, O2+\mathrm{O_2^+}, NO152.51Paramagnetic
O2\mathrm{O_2}, N22−\mathrm{N_2^{2-}}1622Paramagnetic
O2−\mathrm{O_2^-}171.51Paramagnetic
O22−\mathrm{O_2^{2-}}, F2\mathrm{F_2}1810Diamagnetic
O₂²⁻ has 10 electrons in bonding orbitals and 8 in antibonding ones.
Species with the same electron count have the same bond order and the same number of unpaired electrons.

Watch out for (6)

Dipole Moment, Hydrogen Bonding and Intermolecular Forces

Formulas (1)

Reference tables (2)

Polar or non-polar: when symmetry cancels9 rows
ShapeNet dipoleExamples
Linear AX₂ or AX₂E₃ZeroCO2\mathrm{CO_2}, BeF2\mathrm{BeF_2}, BeCl2\mathrm{BeCl_2}, XeF2\mathrm{XeF_2}
Trigonal planar AX₃ZeroBF3\mathrm{BF_3}, BCl3\mathrm{BCl_3}, SO3\mathrm{SO_3}
Tetrahedral AX₄ZeroCH4\mathrm{CH_4}, CCl4\mathrm{CCl_4}, SiF4\mathrm{SiF_4}
Square planar, TBP, octahedralZeroXeF4\mathrm{XeF_4}, PCl5\mathrm{PCl_5}, SF6\mathrm{SF_6}
BentNon-zeroH2O\mathrm{H_2O}, H2S\mathrm{H_2S}, SO2\mathrm{SO_2}
PyramidalNon-zeroNH3\mathrm{NH_3}, NF3\mathrm{NF_3}, PCl3\mathrm{PCl_3}
See-saw, T-shaped, square pyramidalNon-zeroSF4\mathrm{SF_4}, ClF3\mathrm{ClF_3}, BrF5\mathrm{BrF_5}
Tetrahedral with mixed atomsNon-zeroCHCl3\mathrm{CHCl_3}, CH2Cl2\mathrm{CH_2Cl_2}
Heteronuclear diatomicNon-zeroHF, HCl, HBr
H₂ has zero dipole; HF, with the biggest electronegativity gap, has the largest of the hydrogen halides.
A lone pair on the centre breaks the symmetry unless the lone pairs themselves are placed symmetrically, as in XeF₂ and XeF₄.
Hydrogen bonding and intermolecular forces7 rows
CaseKind of attractionEffect
HFIntermolecular H-bonds, zig-zag chainsThe strongest single H-bond; the H sits nearer one F, so the bonds are not symmetrical
Ice, water, water with soluteIntermolecular H-bondsMost in ice (each molecule bonded four ways), fewer in liquid water, fewer again with impurities
oo-Nitrophenol, salicylaldehydeIntramolecular H-bondLower boiling point; steam volatile
pp-Nitrophenol, pp-hydroxybenzaldehydeIntermolecular H-bondsHigher boiling point; not steam volatile
CH4<HCN<NH3\mathrm{CH_4 < HCN < NH_3}None, weak C–H···N, N–H···NOrder of intermolecular H-bond strength
Noble gases, CH4\mathrm{CH_4}London forces onlyEnergy ∝1/r6\propto 1/r^6; grows with molecular size
Ar, CH4\mathrm{CH_4}, H2O\mathrm{H_2O}, C6H6\mathrm{C_6H_6}Van der Waals constant a (about 1.4, 2.3, 5.5, 18 L² bar mol⁻²)Larger a means stronger attraction between molecules
H bonded to F, O or N gives a hydrogen bond; where it forms, inside or between molecules, decides the boiling point.

Watch out for (6)

PYQ weightage by concept

21 concepts · 176 PYQs — where the marks actually sit, so you know what to drill first

Lewis Structures, Formal Charge and the Octet Rule19 PYQs · 11%
ConceptPYQsShare
Counting lone pairs and formal charge85%
The octet rule and its three exceptions63%
Lewis acids and Lewis bases53%
Ionic Bonding, Lattice Enthalpy and Fajans' Rules9 PYQs · 5%
ConceptPYQsShare
Fajans' rules and covalent character53%
The Born-Haber cycle and lattice enthalpy42%
Bond Length, Bond Angle and Resonance15 PYQs · 9%
ConceptPYQsShare
Bond angles and lone-pair repulsion63%
Bond length and what sets it53%
Resonance and fractional bond order42%
VSEPR Theory and Lone Pairs16 PYQs · 9%
ConceptPYQsShare
Counting lone pairs on the central atom106%
Where lone pairs sit: equatorial and trans63%
Shapes of Molecules and Ions27 PYQs · 15%
ConceptPYQsShare
Shapes from the AXE formula1710%
Counting species of one shape in a list106%
Hybridisation and Sigma and Pi Bonds27 PYQs · 15%
ConceptPYQsShare
Hybridisation from the steric number116%
Counting σ and π bonds85%
Hybridisation, orientation and complexes85%
Molecular Orbital Theory36 PYQs · 20%
ConceptPYQsShare
Bond order from the MO diagram159%
Unpaired electrons and magnetism137%
Combining atomic orbitals (LCAO)85%
Dipole Moment, Hydrogen Bonding and Intermolecular Forces27 PYQs · 15%
ConceptPYQsShare
Hydrogen bonding and intermolecular forces116%
Polar or non-polar: when symmetry cancels95%
Dipole moment: size and direction74%

Test yourself on Chemical Bonding and Molecular Structure

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