All revision notes topics

Shapes of simple molecules and ionsAQA A-Level Chemistry: Revision notes

Section 1

Electron pair repulsion

Pairs of electrons in the outer shell of a central atom behave as charge clouds that repel each other. They arrange themselves as far apart as possible to minimise repulsion, and this fixes the shape. There are bonding pairs (shared in a covalent bond) and lone pairs (not involved in bonding).

Lone pairs are held closer to the central atom, so they repel more strongly:

lone pair-lone pair > lone pair-bond pair > bond pair-bond pair

So each lone pair squeezes the bond angle by about 2.5° relative to the ideal angle.

Key termscharge cloudbonding pairlone pair

Section 2

Method for deducing a shape

  1. Find the number of outer electrons on the central atom (adjust for any charge: add one for each negative charge, subtract one for each positive).
  2. Subtract the electrons used in bonds (one per single bond to a terminal atom) and halve to get the lone pairs.
  3. Count the total pairs (bonding + lone) to get the arrangement.
  4. Name the shape from the atoms only, ignoring lone pairs.

Example, NH₄⁺: N has 5 outer electrons, minus 1 for the positive charge gives 4, all used in four bonds, so four bonding pairs and no lone pair: tetrahedral.

Key termsshapearrangement of pairs
Common mistake

Shape names count atoms only. Water has a tetrahedral arrangement of pairs but its shape is bent.

Section 3

Two, three and four pairs

  • 2 bonding pairs: linear, 180° (BeCl₂, CO₂)
  • 3 bonding pairs: trigonal planar, 120° (BF₃)
  • 4 bonding pairs: tetrahedral, 109.5° (CH₄, NH₄⁺, BF₄⁻)
  • 3 bonding + 1 lone pair: pyramidal, about 107° (NH₃, PCl₃)
  • 2 bonding + 2 lone pairs: bent (non-linear), about 104.5° (H₂O)

The falling angles from CH₄ to NH₃ to H₂O are explained by the increasing number of lone pairs.

Key termslineartrigonal planartetrahedralpyramidalbent
Exam tip

Quote the angles: 109.5° (CH₄), 107° (NH₃), 104.5° (H₂O). Always say that lone pairs repel more than bonding pairs.

Section 4

Five pairs

With five pairs the arrangement is trigonal bipyramidal, with two different bond angles: 120° between the three equatorial positions, 90° between axial and equatorial positions.

  • 5 bonding pairs: trigonal bipyramidal (PCl₅)
  • 4 bonding + 1 lone pair: see-saw (SF₄), the lone pair taking an equatorial position
  • 3 bonding + 2 lone pairs: T-shaped (ClF₃)
  • 2 bonding + 3 lone pairs: linear, 180° (XeF₂), with the lone pairs equatorial, 120° apart
Key termstrigonal bipyramidalsee-sawT-shaped

Section 5

Six pairs

With six pairs the arrangement is octahedral, with all adjacent angles 90°.

  • 6 bonding pairs: octahedral, 90° (SF₆)
  • 5 bonding + 1 lone pair: square pyramidal, about 90° (BrF₅)
  • 4 bonding + 2 lone pairs: square planar, 90° (XeF₄), the lone pairs opposite each other to minimise lone pair-lone pair repulsion
Key termsoctahedralsquare planarsquare pyramidal

Section 6

Ions and unfamiliar species

The same rules apply to ions: adjust the electron count for the charge. NH₄⁺ and BF₄⁻ are tetrahedral; H₃O⁺ is pyramidal (3 bonding pairs, 1 lone pair); ICl₄⁻ is square planar (4 bonding pairs, 2 lone pairs).

In exams you may be given an unfamiliar formula. Work out the electrons on the central atom, the number of bonding and lone pairs, then name the arrangement and the shape, and state the bond angle with a reason. Show that a lone pair reduces the angle by its extra repulsion.

Key termsunfamiliar species

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Shapes of simple molecules and ions

  1. Boron trifluoride, BF₃, is a gas used as a catalyst in organic synthesis. It reacts with fluoride ions to form the tetrafluoroborate ion, BF₄⁻, in which boron is bonded to four fluorine atoms.
    Explain why BF₃ and BF₄⁻ have different shapes.2 marks
  2. Sulfur forms several fluorides. Sulfur hexafluoride, SF₆, is an unreactive gas used as an insulator in electrical switchgear. Sulfur tetrafluoride, SF₄, is a reactive gas used to make fluorinated organic compounds. Sulfur has six electrons in its outer shell.
    Deduce the number of bonding pairs and lone pairs of electrons around sulfur in SF₄ and name the shape of the molecule.2 marks
  3. Phosphorus forms two chlorides, PCl₃ and PCl₅. Phosphorus has five electrons in its outer shell. In the vapour phase both compounds exist as separate molecules.
    Deduce the shape of a PCl₃ molecule and explain why the Cl–P–Cl bond angle is about 107° rather than 109.5°.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).