4. Shapes of molecules

• Work out shape of molecule:

  1. Find central atom
  2. Work out how many electrons are in outer shell if central atom
  3. Add 1 electron for every atom that central atom is bonded to.
  4. Looking at an ion, need to take its charge into account - add 1 electron for each negative charge or subtract 1 for each positive charge
  5. Add up all electrons. Divide by 2 to find number of electron pairs
  6. Compare number of electron pairs to number of bonds to find number of lone pairs and number of bonding pairs of central atom
• Central atoms with 2 electron pairs:

  Bond angle of 180° and linear shape. Because pairs of bonding electrons want to be as far away from each other as possible. E.g. BeCl2
• Central atoms with 3 electron pairs:

  bond angle is 120° . Shape of molecule is called trigonal planar. E.g. BF3
• Central atoms with 4 electron pairs:

  Bond angle are 109.5° - shape of molecule is tetrahedral. E.g NH4+
• If there are 3 bonding pairs of electrons and one lone pair, lone pair repulsion will be greater than bonding pair repulsion and so angles between atoms will change. Smaller bond angle between bonding pairs of electrons and larger bond angles between lone pair and bonding pairs. Bond angle is 107° and shape of molecule is original pyramidal E.g. :NH3
• If there are 2 bonding pairs of electrons and 2 lone pairs of electrons lone pair repulsion will squish bond angle even further. Bond angle will be 104.5° and shape of molecule is bent (non-linear) e.g. H2O
• Central atoms with 5 electron pairs:

  molecule with 5 bonding pairs will be trigonal bipyramidal. Repulsion between Bonding pairs means that 3 of the atoms will form original planar shape with bond angle of 120° and other 2 atoms will be at 90° to them E.g. PCl5

  
• If there are 4 bonding pairs and one lone pair of electrons molecule forms a seesaw shape. Lone pair always positioned where one of the original planar atoms would be in trigonal bipyramidal molecule. E.g. SF4

  
• Central atoms with 6 electron pairs:

  Molecule with 6 bonding pairs will be octahedral. Bond angle of 90°. E.g. SF6
• If there are 5 bonding pairs and one lone pair, molecule forms square pyramidal structure. E.g. CiF5
• If there are 4 bonding pairs and 2 lone pairs of electrons, molecule will be square planar