Predicting Shapes & Angles (Edexcel International A Level Chemistry)

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Predicting Shapes & Angles

  • You will need to know and explain the shapes of the following molecules:
    • BeCl2
    • BCl3
    • CH4
    • NH3
    • NH4+
    • H2O
    • CO2
    • PCl5 (g)
    • SF6
    • C2H4

  • In order to write the correct shapes and structures of the molecules listed we need to consider the number of valence electrons

BeCl2

  • Beryllium is in group 2, so has 2 valence electrons; Cl is in group 17, so has 7 valence electrons
  • Both electrons are used to form covalent bonds with Cl and there are no lone pairs
  • Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’
  • This gives a linear shape with bond angles of 180°

1-7-6-beryllium-chloride-molecule

BCl3

  • Boron is in group 13, so has 3 valence electrons; Cl is in group 17, so has 7 valence electrons
  • All 3 electrons are used to form covalent bonds with Cl and there are no lone pairs
  • Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’
  • This gives a trigonal planar shape with bond angles of 120°

1-7-6-boron-trichloride-molecule

CH4 & NH3

  • Carbon is in group 14, so has 4 valence electrons; H is in group 1, so has 1 valence electron
  • All 4 electrons are used to form covalent bonds with H
  • This gives a tetrahedral arrangement with a bond angle of 109.5°
  • Nitrogen is in group 15, so has 5 valence electrons
  • Only 3 electrons are used to form covalent bonds with H and 2 are unbonded as a lone pair
  • This gives a trigonal pyramid arrangement with a bond angle of 107° Chemical Bonding Shapes of Molecules 2 (2)

NH4+

  • Nitrogen is in group 15, so has 5 valence electrons; H is in group 1, so has 1 valence electron
  • Only 3 electrons are used to form covalent bonds with H and 2 are used to form a dative covalent bond
  • This gives a tetrahedral arrangement 
  • With a bond angle of 109.5° (similar to CH4)

Chemical Bonding Dative Covalent Bonding Ammonium ionThe NH4+ ion 

Other examples

1-7-6-bf3-and-co2-molecules

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

PCl5 (g)

    • Phosphorus is in group 15, so has 5 valence electrons; Cl is in group 17, so has 7 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
    • With bond angles of 120° and 90°

PCl5 moleculePhosphorus pentachloride molecule 

C2H4

  • Each carbon atom is in group 14, so has 4 valence electrons, each hydrogen atom is in group 1 so has 1 valence electron
  • This gives a total of 12 electrons
  • There are 4 C-H bonds which use 8 electrons 
  • This leaves 4 electrons which are involved in the C=C
  • Therefore the overall shape is trigonal planar as there are three regions of electron density for each carbon atom

Chemical Bonding Double Covalent Bonding EtheneEthene molecule

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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.