Shapes of molecules and ions

Electron-pair repulsion theory
The shape of a molecule or ion is determined by the number of electron pairs in the outer shell surrounding the central atom.

• As electrons all have a negative charge, each electron pair repels other electron pairs
• The electron pairs push one another as far apart as possible  

Molecules with bonded pairs
A dot-and-cross diagram is used to show the number of electron pairs surrounding the central atom

• Molecules have different shapes when there are different numbers of bonded electron pairs around the central atom
• The shapes and bond angles result from the equal repulsion of the bonded pairs. Each bonded pair is repelled as ar away as possible from other bonded pairs of electrons.
• The resulting shapes may be three-dimensional. 

Representation of 3D structures

• Linear. Bond angle 180'. 2 bonding regions. e.g. CO2
• Trigonal Planar. Bond angle 120' 3 electron pairs around central atom. e.g. BF3
• Tetrahedral. Bond angle 109.5'. 4 electron pairs around central atom. e.g. CH4
• Octahedral. Bond angle 90'. 5 electron pairs around central atom. e.g. SF6

Molecules with lone pairs
A lone pair of electrons is slightly more electron-dense then a bonded pair. Therefore a lone pair repels more than a bonded pair.
The relative strengths are
lone pair/lone pair > lone pair/bonded pair > bonded pair/bonded pair 

• Each lone pair reduces the bond angle by about 2.5'. This is the result of the extra repulsive effect of each lone pair

Molecules with double bonds
A double bond has 4 electrons as two bonded pairs. To work out the shape of a molecule with double bonds, each double bound is treated as a bonded region, in the same way as a bonded pair. Carbon dioxide has 2 double bonds. They will repel one another to be as far apart as possible, resulting in a linear molecule with a bond angle of 180'.

Shapes of ions
The principles discussed can also be applied to any molecular ion.  For NH4+, there are 4 electron pairs around the central atom. The shape will be tetrahedral.

Drawing 3d diagrams

• A normal line shows a bond in the plane of the paper
• A bold wedge shows the bond coming out from the plane of the paper towards you
• A dotted wedge shows the bond going out from the plane of the paper away from you