AQA A Level Chemistry

Revision Notes

1.5.1 Shapes of Simple Molecules & Ions

Electron Pairs

  • The valence shell electron pair repulsion theory (VSEPR) predicts the shape and bond angles of molecules
  • Electrons are negatively charged and will repel other electrons when close to each other
  • In a molecule, the bonding pairs of electrons will repel other electrons around the central atom forcing the molecule to adopt a shape in which these repulsive forces are minimised
  • When determining the shape and bond angles of a molecule, the following VSEPR rules should be considered:
    • Valence shell electrons are those electrons that are found in the outer shell
    • Electron pairs repel each other as they have the same charge
    • Lone pair electrons repel each other more than bonded pairs
    • Repulsion between multiple and single bonds is treated the same as for repulsion between single bonds
    • Repulsion between pairs of double bonds are greater
    • The most stable shape is adopted to minimize the repulsion forces
  • Different types of electron pairs have different repulsive forces
    • Lone pairs of electrons have a more concentrated electron charge cloud than bonding pairs of electrons
    • The cloud charges are wider and closer to the central atom’s nucleus
    • The order of repulsion is therefore: lone pair – lone pair > lone pair – bond pair > bond pair – bond pair

Chemical Bonding Electron Pairs Repulsive Forces, downloadable AS & A Level Chemistry revision notes

Different types of electron pairs have different repulsive forces

Shapes of Molecules & Ions

  • Molecules can adapt the following shapes and bond angles:

Shapes of molecules, downloadable AS & A Level Chemistry revision notes

Molecules of different shapes can adapt with their corresponding bond angles

Examples

Chemical Bonding Shapes of Molecules 2 (1), downloadable AS & A Level Chemistry revision notes

Chemical Bonding Shapes of Molecules 2 (2), downloadable AS & A Level Chemistry revision notes

Chemical Bonding Shapes of Molecules 2 (3), downloadable AS & A Level Chemistry revision notes

Chemical Bonding Shapes of Molecules 2 (4), downloadable AS & A Level Chemistry revision notes

Examples of molecules with different shapes and bond angles

VSEPR & shapes of molecules

Draw the shape of the following molecules:

  1. Phosphorus(V) chloride
  2. N(CH3)3
  3. CCl4

Answer 1:

    • Phosphorus is in group 15, so has 5 valence electrons; Cl is in group 17, so has 17 valence electrons
    • All 5 electrons are used to form covalent bonds with Cl and there are no lone pairs
    • This gives a trigonal (or triangular) bipyramidal shape:

Worked Example Answer 1 - VSEPR for Phosphorous(V) chloride, downloadable AS & A Level Chemistry revision notes

Phosphorus pentachloride or phosphorus (V) chloride

Answer 2: 

    • Nitrogen is in group 15, so has 5 valence electrons; carbon is in group 14, so has 4 valence electrons, 3 of which are already used in the covalent bonds with hydrogen
    • Three of the valence electrons in N are used to form bonding pairs, so there is one lone pair left
    • N(CH3)3 has a triangular pyramid shape:

Worked Example Answer 2 - VSEPR for N(CH3), downloadable AS & A Level Chemistry revision notes

Trimethylamine

Answer 3: 

    • Carbon is in group 14, so has 4 valence electrons; chlorine is in group 17, so has 7 valence electrons
    • All four valence electrons are use to bond with chlorine and there are no lone pairs
    • The shape of CCl4 is tetrahedral

Worked Example Answer 3 - VSEPR for CCl4, downloadable AS & A Level Chemistry revision notes

Tetrachloromethane

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