All revision notes topics

Shapes of molecules and ionsEdexcel A-Level Chemistry: Revision notes

Section 1

Electron-pair repulsion theory

The shape of a simple molecule or ion is decided by the number of electron pairs in the outer shell of the central atom. Pairs of electrons are negatively charged, so they repel and arrange themselves as far apart as possible, to minimise repulsion.

To find the shape:

  1. Count the outer electrons of the central atom (adjust for any charge on an ion).
  2. Count the bonding pairs (a double or triple bond counts as one region of electron density).
  3. Remaining electrons form lone pairs: (outer electrons − electrons used in bonds) ÷ 2.
  4. The total number of pairs fixes the arrangement; lone pairs then decide the molecular shape.
Key termselectron-pair repulsionbonding pair
Common mistake

Do not say that atoms or bonds repel. It is the electron pairs that repel.

Section 2

Shapes with no lone pairs

With only bonding pairs, all the repulsions are equal and the bond angles are regular:

  • 2 pairs: linear, 180°, e.g. BeCl₂, CO₂
  • 3 pairs: trigonal planar, 120°, e.g. BCl₃
  • 4 pairs: tetrahedral, 109.5°, e.g. CH₄, NH₄⁺
  • 5 pairs: trigonal bipyramidal, 90° and 120°, e.g. PCl₅
  • 6 pairs: octahedral, 90°, e.g. SF₆

The same ideas apply to organic molecules: each carbon in an alkane is tetrahedral (109.5°), each carbon in C=C of ethene is trigonal planar (120°), and the carbons in ethyne are linear (180°).

Key termstrigonal planartetrahedraloctahedral

Section 3

Lone pairs and the shapes they give

A lone pair is held closer to the central atom, so it repels more strongly. The order is lone pair/lone pair > lone pair/bonding pair > bonding pair/bonding pair. Lone pairs therefore squeeze bonding pairs closer, so bond angles get smaller.

  • CH₄ (4 bp): 109.5°
  • NH₃ (3 bp, 1 lp): pyramidal, 107°
  • H₂O (2 bp, 2 lp): bent, 104.5°
  • NH₄⁺ (4 bp): tetrahedral, 109.5°; the dative bond is identical to the others.

The name of the shape describes only the atoms, not the lone pairs, so water is bent even though its four pairs are arranged tetrahedrally.

Key termslone pairpyramidalbent
Exam tip

For a 2-mark angle question: state the number of lone pairs, then that lone pairs repel bonding pairs more strongly than bonding pairs repel each other.

Section 4

Five and six pairs with lone pairs

With five pairs the arrangement is trigonal bipyramidal. Lone pairs occupy the central (equatorial) plane, where they have fewer 90° repulsions.

  • SF₄ (4 bp, 1 lp): see-saw, angles less than 120° and 180°
  • ClF₃ (3 bp, 2 lp): T-shaped, angle slightly less than 90°
  • XeF₂ (2 bp, 3 lp): linear, 180°

With six pairs the arrangement is octahedral:

  • BrF₅ (5 bp, 1 lp): square pyramidal, about 90° (slightly less)
  • XeF₄ (4 bp, 2 lp): square planar, 90°, the lone pairs being opposite each other.
Key termstrigonal bipyramidalsquare planar

Section 5

Predicting shapes of related species

Use the same method for any species. Example: what is the shape of the PCl₄⁺ ion?

  • P has 5 outer electrons; one is lost for the 1+ charge, leaving 4.
  • All 4 form bonds to Cl, so 4 bonding pairs and 0 lone pairs.
  • The shape is tetrahedral, with bond angles of 109.5°.

Example with a lone pair: ICl₂⁻. I has 7 electrons, plus 1 for the charge makes 8; two are used in bonds, leaving 3 lone pairs. Five pairs in total, so a trigonal bipyramidal arrangement with the lone pairs in the central plane, giving a linear ion at 180°.

Key termsanalogous species

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Shapes of molecules and ions

  1. Boron trichloride, BCl₃, is a gas used in the manufacture of semiconductors. It reacts with ammonia, NH₃, to form a solid adduct, H₃N→BCl₃, in which the boron atom is bonded to four atoms.
    Explain why the Cl–B–Cl bond angle changes from 120° in BCl₃ to about 109.5° in the adduct.2 marks
  2. The H–X–H bond angles in three hydrides of second-row elements are measured as 109.5° in methane, CH₄, 107° in ammonia, NH₃, and 104.5° in water, H₂O.
    Explain why the bond angle in water is smaller than the bond angle in ammonia.2 marks
  3. Sulfur hexafluoride, SF₆, is an unreactive gas used to insulate electrical switchgear. Phosphorus pentachloride, PCl₅, is a reagent used in organic chemistry. Both are simple molecules in the gas phase. Phosphorus has five outer electrons, sulfur has six and bromine has seven.
    Deduce the shape of a gaseous PCl₅ molecule and the bond angles in it, explaining your answer.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).