Shapes of Covalent Compounds (Edexcel AS Chemistry)

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Electron Pair Repulsion & Shape

  • 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 & Angles

  • 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

Worked example

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 used 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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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.