Shapes of molecules and ions

Cards (33)

  • 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°