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Learn7405Paper 1 · Paper 1: Physical and inorganic chemistryBondingShapes of molecules and polarity
Bonding · Subtopic 2 of 3

Shapes of Molecules and Polarity

VSEPR theory: electron pairs around a central atom repel and get as far apart as possible; lone pairs repel more than bonding pairs, closing bond angles by about 2.5° each. 2 pairs: linear 180°; 3: trigonal planar 120°; 4: tetrahedral 109.5° (NH₃ pyramidal 107°, H₂O non-linear 104.5°); 5: trigonal bipyramidal 90°/120°; 6: octahedral 90°. Electronegativity is an atom's ability to attract the bonding electrons in a covalent bond; it increases across a period and up a group (F highest). Different electronegativities give polar bonds; a molecule is polar only if its dipoles don't cancel.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH11AQA 7405 Paper 1≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Predict shapes and bond angles using VSEPR.
  • Explain the effect of lone pairs.
  • Define electronegativity and describe its trends.
  • Decide whether a molecule is polar.
Pause and think

CO₂ has two polar C=O bonds, yet the molecule is non-polar. How?

Section 2 of 14

Core idea

Start hereShape is everything

Carbon dioxide and water both have polar bonds, but one is a non-polar gas and the other a polar liquid. The difference is shape.

The big idea

Electron pairs around a central atom repel to get as far apart as possible; lone pairs repel more. Shape and electronegativity together decide whether a molecule is polar.

1. Predicting shapes

VSEPR decision

Count pairs, then place atoms.

Worked exampleChlorine trifluoride

ClF3\mathrm{ClF_3}: Cl has 7 outer electrons, 3 used in bonds.

Goal
Number of pairs and shape.
Strategy
Bonding + lone pairs.
  1. 3 bonding pairs+2 lone pairs=53\ \text{bonding pairs}+2\ \text{lone pairs}=5
    (7−3)/2=2(7-3)/2=2 lone pairs.
  2. lone pairs equatorial\text{lone pairs equatorial}
    Minimises 90° repulsions.
AnswerT-shaped, angles slightly under 90°.
✓ CheckMatches experiment ✓.
Key takeawayPolar molecules Polar bonds + unsymmetrical shape = polar molecule (H₂O, NH₃, CHCl₃). Symmetrical shapes cancel dipoles (CO₂, BF₃, CCl₄, SF₆).
Pause and think

Why is BF₃ trigonal planar but NF₃ pyramidal?

Summary

  • Count pairs.
  • Lone pairs squeeze angles.
  • Symmetry decides polarity.
Key words
VSEPR
Valence shell electron pair repulsion theory.
Electronegativity
Ability of an atom to attract bonding electrons.
Dipole
Separation of charge in a bond or molecule.
HistoryGillespie and Nyholm. Ronald Gillespie and Ronald Nyholm developed VSEPR theory in 1957 as a simple way to predict molecular shapes without quantum mechanics.
Fun factBent water matters. If water were linear like CO₂, it would be non-polar, wouldn't dissolve salts, and would boil far below 0 °C. Life as we know it depends on that 104.5° angle.
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