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
Lewis Structures, Formal Charge and the Octet Rule
19 PYQsA Lewis structure shares out every valence electron as bond pairs and lone pairs; from it you read the lone-pair count, each atom's formal charge and whether the octet rule holds.
Ionic Bonding, Lattice Enthalpy and Fajans' Rules
9 PYQsAn ionic solid is held by its lattice enthalpy, found from a Born-Haber cycle, and Fajans' rules say how far a small or highly charged cation distorts the anion and gives the bond covalent character.
Bond Length, Bond Angle and Resonance
15 PYQsBond length shrinks as bond order rises, resonance averages the bond order over equivalent positions, and bond angles open or close with the repulsion between electron pairs.
VSEPR Theory and Lone Pairs
16 PYQsVSEPR theory counts the electron pairs on the central atom, bond pairs plus lone pairs, and places them as far apart as possible; lone pairs take the positions where they meet the fewest bond pairs at 90°.
Shapes of Molecules and Ions
27 PYQsThe shape of a molecule is named from its atoms only: fix the arrangement from the total electron pairs, then remove the lone pairs, so AX₃E is pyramidal and AX₂E₂ is bent.
Hybridisation and Sigma and Pi Bonds
27 PYQsEvery bonded pair of atoms shares one σ bond and any extra bonds are π; the central atom mixes one hybrid orbital for each σ bond and lone pair, so the steric number names the hybridisation.
Molecular Orbital Theory
36 PYQsAtomic orbitals of the same symmetry combine into bonding and antibonding molecular orbitals; filling them in order gives the bond order, ½(Nb − Na), and the number of unpaired electrons.
Dipole Moment, Hydrogen Bonding and Intermolecular Forces
27 PYQsA molecule's dipole moment is the vector sum of its bond dipoles and lone-pair moments, so symmetry can cancel it; hydrogen bonds and other intermolecular forces then decide how molecules hold together.
Formula & revision sheet
8 formulas · 12 reference tables · 42 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
8 formulas · 12 reference tables · 42 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (2)
The octet rule and its three exceptions5 rows
| Type | Electrons on the central atom | Examples |
|---|---|---|
| Obeys the octet rule | 8 | , , , , , |
| Incomplete octet | 4 for Be, 6 for B and Al | , (4); , , (6) |
| Electron deficient | Bridging B–H–B bonds hold 2 electrons over 3 atoms | ; is also called electron deficient |
| Odd-electron species | An odd total: NO 11, NO₂ 17, ClO₂ 19 | , , These are also the paramagnetic oxides: an odd electron cannot pair. |
| Expanded octet | 10 or 12 (14 in IF₇) | , (10); , , (12); (14) |
Lewis acids and Lewis bases6 rows
| Species | Role | Reason |
|---|---|---|
| , | Lewis acid | B has 6 electrons and an empty p orbital; , trigonal planar |
| Lewis acid | Al has 6 electrons; it dimerises to to fill the gap | |
| Strongest boron halide acid | Back-bonding from large I into B is weakest | |
| , | Lewis base | One lone pair on N; , pyramidal |
| , | Lewis base | One lone pair on S; two on Cl |
| Not a Lewis base | All five P electrons are in bonds; no lone pair PCl₅ can accept a pair (forming PCl₆⁻), but it cannot donate one. |
Watch out for (6)
- Count every atom, not just the centre→ Counting lone pairs and formal charge
- The charge changes the electron count→ Counting lone pairs and formal charge
- Sulphur acids and oxides are expanded→ The octet rule and its three exceptions
- Odd electrons mean an exception, even with a small atom→ The octet rule and its three exceptions
- Electronegativity does not rank the boron halides→ Lewis acids and Lewis bases
- Amphoteric water is a Brønsted idea→ Lewis acids and Lewis bases
Reference tables (1)
Fajans' rules and covalent character5 rows
| Rule | Order of covalent character | Why |
|---|---|---|
| Smaller cation | is the smallest and most polarising | |
| Higher cation charge | ; | More charge on a smaller ion |
| Larger anion | ; | has the largest, softest cloud |
| 18-electron cation | ; | d electrons shield the nuclear charge poorly |
| Electronegativity difference | Ionic character: | is 0 for , about 0.8 for Cl–F, 0.9 for S–O |
Watch out for (4)
- Half the bond enthalpy, not all of it→ The Born-Haber cycle and lattice enthalpy
- Check which way the lattice step runs→ The Born-Haber cycle and lattice enthalpy
- A bigger cation means LESS covalent→ Fajans' rules and covalent character
- Rank electron gain by magnitude→ Fajans' rules and covalent character
Reference tables (2)
Bond length and what sets it10 rows
| Bond | Typical length (pm) | Note |
|---|---|---|
| C–H | 109 | Shortest here: hydrogen is tiny |
| C≡C | 120 | Triple bond |
| C=C | 134 | Double bond |
| C–C | 154 | Single bond |
| C≡N | 116 | Shorter than C=O despite N being larger than C |
| C=O | 122 | Carbonyl |
| C–O | 143 | Alcohols and ethers |
| O=O | 121 | In O₂ |
| O–O | 148 | In H₂O₂ |
| P–Cl in PCl₅ | 219 axial, 204 equatorial | Axial bonds are the longer, weaker pair Calling the axial bonds of PCl₅ stronger is a standard wrong statement. |
Bond angles and lone-pair repulsion8 rows
| Species | Pairs on the centre | Bond angle |
|---|---|---|
| 3 bond, 0 lone | 120° | |
| 2 bond (plus π), 1 lone | about 119° | |
| 4 bond, 0 lone | 109.5° | |
| 3 bond, 1 lone | 107° | |
| 2 bond, 2 lone | 104.5° | |
| 3 bond, 1 lone | 102° | |
| 3 bond, 1 lone | about 98° | |
| 3 bond, 2 lone | about 87.5° (axial F–Cl–equatorial F) The two lone pairs bend the axial F atoms back below 90°. |
Watch out for (6)
- The hybrid does not flip between forms→ Resonance and fractional bond order
- Resonance, not repulsion, sets ozone's bond length→ Resonance and fractional bond order
- Bond length is not set by bond order alone→ Bond length and what sets it
- See-saw and trigonal bipyramid give unequal bonds→ Bond length and what sets it
- Both SO₂ and H₂O are bent, at very different angles→ Bond angles and lone-pair repulsion
- Fluorine closes the angle; chlorine opens it→ Bond angles and lone-pair repulsion
Reference tables (1)
Where lone pairs sit: equatorial and trans7 rows
| Species | Bond pairs, lone pairs | Lone pairs sit | Shape |
|---|---|---|---|
| , | 4, 1 | Equatorial | See-saw |
| , | 3, 2 | Both equatorial | T-shaped (bent T), about 87.5° |
| , , | 2, 3 | All three equatorial | Linear, 180° |
| 4, 1 | Equatorial, with the two O | See-saw, F atoms axial | |
| , | 5, 1 | Any one octahedral site | Square pyramidal |
| , | 4, 2 | Trans, opposite each other | Square planar, 90° |
| 2, 2 | Two corners of a tetrahedron | Bent |
Watch out for (4)
- A double bond to oxygen uses two electrons→ Counting lone pairs on the central atom
- Know what the question calls a bond pair→ Counting lone pairs on the central atom
- Lone pairs are never axial in a trigonal bipyramid→ Where lone pairs sit: equatorial and trans
- Three lone pairs make a straight molecule→ Where lone pairs sit: equatorial and trans
Reference tables (1)
Shapes from the AXE formula15 rows
| Type | Pairs (X + E) | Shape | Examples |
|---|---|---|---|
| AX₂ | 2 | Linear | , , , , |
| AX₃ | 3 | Trigonal planar | , , , |
| AX₂E | 3 | Bent | , , |
| AX₄ | 4 | Tetrahedral | , , , |
| AX₃E | 4 | Trigonal pyramidal | , , , , , |
| AX₂E₂ | 4 | Bent | , , , |
| AX₅ | 5 | Trigonal bipyramidal | , , |
| AX₄E | 5 | See-saw | , , , |
| AX₃E₂ | 5 | T-shaped | , , |
| AX₂E₃ | 5 | Linear | , , |
| AX₆ | 6 | Octahedral | , |
| AX₅E | 6 | Square pyramidal | , , |
| AX₄E₂ | 6 | Square planar | , , |
| AX₇ | 7 | Pentagonal bipyramidal | |
| AX₆E | 7 | Distorted octahedral |
Watch out for (4)
- Name the shape from the atoms, not the pairs→ Shapes from the AXE formula
- Same formula type, different shape→ Shapes from the AXE formula
- Charge changes the shape→ Counting species of one shape in a list
- Two nickel complexes, two shapes→ Counting species of one shape in a list
Formulas (2)
Reference tables (1)
Hybridisation, orientation and complexes8 rows
| Hybridisation | Orientation | Main-group examples | Complex examples |
|---|---|---|---|
| Linear, 180° | , , | ||
| Trigonal planar, 120° | , , | Rare in complexes | |
| Tetrahedral, 109.5° | , , | , | |
| Square planar, 90° | None | , | |
| Trigonal bipyramidal | , , , | is often written | |
| Octahedral, 90° | , , | (outer orbital) | |
| Octahedral, 90° | None | , (inner orbital) [Co(NH₃)₆]³⁺ is d²sp³, not sp³d²: a stated match of it with SF₆ is false. | |
| Pentagonal bipyramidal | , (distorted) | None |
Watch out for (6)
- Every C–H bond is a σ bond→ Counting σ and π bonds
- Give σ and π in the order asked→ Counting σ and π bonds
- π bonds do not add hybrid orbitals→ Hybridisation from the steric number
- Five atoms around the centre can still be sp³d²→ Hybridisation from the steric number
- sp³d with a lone pair gives unequal bonds→ Hybridisation, orientation and complexes
- Square planar means dsp², not sp³→ Hybridisation, orientation and complexes
Formulas (1)
Reference tables (2)
Combining atomic orbitals (LCAO)8 rows
| Pair of orbitals (axis z) | Symmetry of each | Do they combine? |
|---|---|---|
| 1s and 1s | σ and σ | Yes: σ1s and σ*1s |
| and | σ and σ | Yes: σ2p and σ*2p (head-on) |
| and | π and π | Yes: π2p and π*2p (sideways) |
| 2s and | σ and σ | Yes, if their energies are close |
| 2s and | σ and π | No: zero net overlap |
| and | π, but at right angles | No: they are orthogonal |
| and | π and π | Yes: a π overlap |
| and | 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°. |
Unpaired electrons and magnetism10 rows
| Species | Electrons | Bond order | Unpaired electrons | Magnetism |
|---|---|---|---|---|
| , | 1, 3 | 0.5 | 1 | Paramagnetic |
| 6 | 1 | 0 | Diamagnetic | |
| 10 | 1 | 2 | Paramagnetic | |
| 12 | 2 | 0 | Diamagnetic | |
| , | 13 | 2.5 | 1 | Paramagnetic |
| , CO, , | 14 | 3 | 0 | Diamagnetic |
| , , NO | 15 | 2.5 | 1 | Paramagnetic |
| , | 16 | 2 | 2 | Paramagnetic |
| 17 | 1.5 | 1 | Paramagnetic | |
| , | 18 | 1 | 0 | Diamagnetic O₂²⁻ has 10 electrons in bonding orbitals and 8 in antibonding ones. |
Watch out for (6)
- Bonding π density is not low above the axis→ Combining atomic orbitals (LCAO)
- Maximum overlap, not minimum→ Combining atomic orbitals (LCAO)
- Count every electron, core included, or none→ Bond order from the MO diagram
- A bond order of zero means no molecule→ Bond order from the MO diagram
- O₂⁺ and O₂⁻ have the same number of unpaired electrons→ Unpaired electrons and magnetism
- N₂²⁻ looks like N₂ but behaves like O₂→ Unpaired electrons and magnetism
Reference tables (2)
Polar or non-polar: when symmetry cancels9 rows
| Shape | Net dipole | Examples |
|---|---|---|
| Linear AX₂ or AX₂E₃ | Zero | , , , |
| Trigonal planar AX₃ | Zero | , , |
| Tetrahedral AX₄ | Zero | , , |
| Square planar, TBP, octahedral | Zero | , , |
| Bent | Non-zero | , , |
| Pyramidal | Non-zero | , , |
| See-saw, T-shaped, square pyramidal | Non-zero | , , |
| Tetrahedral with mixed atoms | Non-zero | , |
| Heteronuclear diatomic | Non-zero | HF, HCl, HBr H₂ has zero dipole; HF, with the biggest electronegativity gap, has the largest of the hydrogen halides. |
Hydrogen bonding and intermolecular forces7 rows
| Case | Kind of attraction | Effect |
|---|---|---|
| HF | Intermolecular H-bonds, zig-zag chains | The strongest single H-bond; the H sits nearer one F, so the bonds are not symmetrical |
| Ice, water, water with solute | Intermolecular H-bonds | Most in ice (each molecule bonded four ways), fewer in liquid water, fewer again with impurities |
| -Nitrophenol, salicylaldehyde | Intramolecular H-bond | Lower boiling point; steam volatile |
| -Nitrophenol, -hydroxybenzaldehyde | Intermolecular H-bonds | Higher boiling point; not steam volatile |
| None, weak C–H···N, N–H···N | Order of intermolecular H-bond strength | |
| Noble gases, | London forces only | Energy ; grows with molecular size |
| Ar, , , | Van der Waals constant a (about 1.4, 2.3, 5.5, 18 L² bar mol⁻²) | Larger a means stronger attraction between molecules |
Watch out for (6)
- More electronegative F does not mean a bigger dipole→ Dipole moment: size and direction
- The two arrow conventions point opposite ways→ Dipole moment: size and direction
- Polar bonds can give a non-polar molecule→ Polar or non-polar: when symmetry cancels
- Lone pairs do not always make a molecule polar→ Polar or non-polar: when symmetry cancels
- Ortho means intramolecular→ Hydrogen bonding and intermolecular forces
- HF has no intramolecular hydrogen bond→ Hydrogen bonding and intermolecular forces
PYQ weightage by concept
21 concepts · 176 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
21 concepts · 176 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Counting lone pairs and formal charge | 8 | 5% |
| The octet rule and its three exceptions | 6 | 3% |
| Lewis acids and Lewis bases | 5 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Fajans' rules and covalent character | 5 | 3% |
| The Born-Haber cycle and lattice enthalpy | 4 | 2% |
| Concept | PYQs | Share |
|---|---|---|
| Bond angles and lone-pair repulsion | 6 | 3% |
| Bond length and what sets it | 5 | 3% |
| Resonance and fractional bond order | 4 | 2% |
| Concept | PYQs | Share |
|---|---|---|
| Counting lone pairs on the central atom | 10 | 6% |
| Where lone pairs sit: equatorial and trans | 6 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Shapes from the AXE formula | 17 | 10% |
| Counting species of one shape in a list | 10 | 6% |
| Concept | PYQs | Share |
|---|---|---|
| Hybridisation from the steric number | 11 | 6% |
| Counting σ and π bonds | 8 | 5% |
| Hybridisation, orientation and complexes | 8 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Bond order from the MO diagram | 15 | 9% |
| Unpaired electrons and magnetism | 13 | 7% |
| Combining atomic orbitals (LCAO) | 8 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Hydrogen bonding and intermolecular forces | 11 | 6% |
| Polar or non-polar: when symmetry cancels | 9 | 5% |
| Dipole moment: size and direction | 7 | 4% |
Test yourself on Chemical Bonding and Molecular Structure
20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.