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Chemical Bonding

VSEPR shapes, hybridisation, bond order from molecular orbital theory, dipole moments and hydrogen bonding. Mostly structure work on given molecules.

Questions in the bank
176
q/paper in 2025–26
1.28
Calculation
35%
Notes pages
8

Strand: Structure and recall

When you’ll see it

A list of molecules or ions to sort by shape, hybridisation, lone pairs, bond order, polarity or magnetism, or two species to compare by bond length, bond angle or dipole moment.

How this chapter is tested

Most numeric answers here are counts from a list: how many species 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 one routine (steric number, AXE type, MO filling) done the same way for every species. One slip in one species changes the count, and there are no options to catch it.

The rest is orders and exceptions to know by heart: NH₃ more polar than NF₃, BF₃ the weakest boron halide acid, O₂⁺ and O₂⁻ with one unpaired electron each, C≡N shorter than C=O, the axial bonds of PCl₅ longer than the equatorial ones. Options often hold the arrangement of electron pairs as a decoy for the shape, or a σ-only count where π pairs are meant.

Molecular orbital theory is the largest single block: bond order ½(Nb − Na) and unpaired electrons from the filling order, with σ2p dropping below π2p from O₂ onwards. Isoelectronic species share a bond order, so learn the ladder from 13 to 18 electrons once.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Lewis structures and formal charge

    Lone pairs = (valence electrons − 2 × bonds)/2, with the charge included; FC = V − L − ½S; octet exceptions are incomplete (BF₃), odd-electron (NO, NO₂, ClO₂) and expanded (PCl₅, SF₆, H₂SO₄).

  • Ionic bonding and Fajans' rules

    Born–Haber uses half the X–X bond enthalpy; covalent character rises with a smaller or higher-charged cation, a larger anion, and an 18-electron cation (CuCl > NaCl).

  • Bond length, angle and resonance

    An equivalent bond's order = bonds shared ÷ positions (CO₃²⁻ 4/3, O₃ 1.5); lp–lp > lp–bp > bp–bp closes angles: CH₄ 109.5°, NH₃ 107°, H₂O 104.5°, OF₂ 103°.

  • VSEPR and lone pairs

    SN = ½(V + M − c + a), with oxygen adding nothing; lone pairs go equatorial in a trigonal bipyramid and trans in an octahedron; XeF₂ has three, XeF₄ two, XeF₆ one.

  • Shapes

    Name the shape from the atoms: AX₃E pyramidal, AX₂E₂ bent, AX₄E see-saw, AX₃E₂ T-shaped, AX₂E₃ linear, AX₅E square pyramidal, AX₄E₂ square planar. Put the charge into the count: I₃⁻ is linear, I₃⁺ bent.

  • Hybridisation and σ and π counts

    SN 2 to 7 gives sp to sp³d³; π bonds add no hybrid orbital; σ bonds = atoms − 1 + rings, every C–H included; complexes with strong ligands use dsp² and d²sp³.

  • Molecular orbital theory

    Bond order ½(Nb − Na); σ2p above π2p up to N₂, below it from O₂; O₂, B₂ and N₂²⁻ have two unpaired electrons; N₂, CO, CN⁻ and NO⁺ have bond order 3 and are diamagnetic.

  • Dipole moment and hydrogen bonding

    Symmetric shapes with identical outer atoms are non-polar (CO₂, BF₃, XeF₄, PCl₅); NH₃ 1.47 D beats NF₃ 0.23 D; o-nitrophenol bonds within itself, so it boils lower and is steam volatile.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • The arrangement offered as the shape

    NH₃ has a tetrahedral arrangement of pairs but is trigonal pyramidal; XeF₄ is octahedral in pairs and square planar in atoms. The arrangement is a standing distractor.

  • π bonds counted as hybrid orbitals

    SO₃ has three S=O bonds but only three σ bonds, so it is sp², not sp³d². Count σ bonds and lone pairs only.

  • Electronegativity used for NF₃ and BF₃

    F is the most electronegative element, yet NF₃ is far less polar than NH₃ and BF₃ is the weakest boron halide acid. The lone-pair moment and back-bonding decide.

  • O₂⁺ and O₂⁻ ranked by unpaired electrons

    Both have exactly one unpaired electron. Their bond orders differ (2.5 against 1.5); their unpaired counts do not.

  • Bond length by bond order alone

    C≡N (116 pm) is shorter than C=O (122 pm), and C–H (109 pm) is shorter than both. Compare bond orders only between the same pair of atoms.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Chemical Bonding notes

Drill every Chemical Bonding question

176 questions from the bank, across 8 subtopics.

Drill the 176 questions

Drill one subtopic at a time

The 8 subtopics, in teaching order.

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