Shapes of molecules and ions

• Shapes of molecules and ions

  The shape of a covalent molecule or an ion depends on the repulsion of the electron pairs around a central atom. The electron pairs are charge clouds around an atom and they repel each other as far as possible. There are two types of electron pairs, a bonding pair of electrons and a lone pair (non-bonding pair) of electrons.
• Determining the shape of a molecule or ion


  1. The total number of electron pairs around a central atom

  2. The number of bonding pairs of electrons

  3. The number of lone pairs of electrons
• Lone pairs are more compact to the central atom so they have a greater repulsive effect on the other pairs of electrons.
• Bonding pair of electrons

  A pair of electrons shared between two atoms
• Lone pair of electrons

  A pair of unshared electrons in the outer shell of an atom
• The order of strength of the repulsions experienced by the electron pairs is: lone pair - lone pair > lone pair - bonding pair > bonding pair - bonding pair
• The molecule or ion will take up a shape that minimises these repulsions
• Information that may be asked about shapes of molecules and ions


  • The shape
  • The name of the shape
  • The bond angle
  • An explanation of the shape
• Molecules with only bonding pairs of electrons


  • Beryllium chloride (linear, 180°)
  • Boron trifluoride (trigonal planar, 120°)
  • Methane (tetrahedral, 109.5°)
  • Carbon dioxide (linear, 180°)
• Drawing three-dimensional shapes


  • Normal lines (-) for bonds in the plane of the page
  • Solid wedge lines (→) for bonds coming towards the viewer out of the plane of the page
  • Dashed lines (---) for bonds going backwards from the plane of the paper
• Molecules with bonding pairs and lone pairs of electrons

  • Ammonia (pyramidal, 107°)
  • Water (bent, 104.5°)
  • Phosphorus pentafluoride (trigonal bipyramidal, 90° and 120°)
  • Sulfur hexafluoride (octahedral, 90°)
• A double bonding pair or a triple bonding pair of electrons repel in the same way as a single bonding pair
• Nitrogen atom in NH3

  Three bonding pairs of electrons and one lone pair of electrons
• Arrangement of electron pairs around nitrogen atom in NH3


  • Tetrahedral, but with no atom attached to the lone pair so it looks like a pyramid
• Lone pair of electrons

  Causes extra repulsion, squeezing the bonding pairs of electrons closer together and decreasing the bond angles
• Shape of NH3 is pyramidal
• Water (H2O)


  Basic arrangement of electron pairs is tetrahedral around the oxygen atom, but with no atom attached to the lone pairs so it appears bent
• Lone pairs of electrons in H2O

  Cause extra repulsion, squeezing the bonds closer and decreasing the bond angle to 104.5°
• Shape of H2O is bent
• Coordinate bond


  When a coordinate bond forms, a lone pair of electrons is contributed into a bonding pair of electrons
• Formation of the ammonium ion (NH4+)


  1. Ammonia (NH3) reacts with hydrogen ions

  2. Causes a change in shape from pyramidal to tetrahedral
• Sketch of the shape of the ammonium ion (NH4+)
• Bond angle of the ammonium ion (NH4+) is 109.5°
• Shape of the ammonium ion (NH4+) is tetrahedral
• Formation of the hydronium ion (H3O+)


  1. H2O reacts with H+

  2. Causes a change in shape from bent to pyramidal
• Shape of the hydronium ion (H3O+) is pyramidal
• BF4- ion


  Four bonding pairs of electrons around the central boron atom, takes up a tetrahedral shape
• Bond angle of the BF4- ion is 109.5°
• Shape of the BF4- ion is tetrahedral
• Bromine trifluoride (BrF3)


  Bromine has one unpaired electron, so it forms three bonding pairs of electrons and two lone pairs of electrons
• The overall shape of BrF3 is described as T-shaped
• The bond angle in BrF3 is 86°
• BeF2 has two lone pairs of electrons and two bonding pairs of electrons, giving a bent (or V-shaped) structure with a bond angle of 104.5°