Electron Pair Repulsion Theory
EPR theory states that electron pairs will arrange themselves around a central atom in order to be as far apart from each other as possible. This minimises the repulsion between negatively charged electrons.
We will look at the shapes of molecules containing up to 6 electron pairs. Remember, these are just the shapes for regular molecules. Those with lone pairs will adopt a different shape, which we will see later.
2 electron pairs - will adopt a linear shape, with a bond angle of 180 degrees between the central atom and the other atoms.
3 electron pairs - will adopt a trigonal planar shape, with a bond angle of 120 degrees between the central atom and the other atoms.
Shapes of Regular Molecules
4 electron pairs: adopts a tetrahedral shape, with a bond angle of 109 degrees.
5 electron pairs: adopts a trigonal bipyramidal shape, with bond angles of 90 degrees and 120 degrees.
6 electron pairs: adopts an octahedral shape, with bond angles that are all 90 degrees.
Some molecules have a departure from the expected bond angle because they have lone pairs of electrons, which repel more than bonding pairs - this reduces the bond angle.
Water: contains two lone pairs of electrons around the oxygen atom, which repel more than bonding pairs. It adopts a bent or V-shaped structure, with an angle of 105 degrees.
Ammonia: has a lone pair of electrons around the the nitrogen atom, which repels more than the bonding pairs. It adopts a pyramidal shape, with an angle of 107 degrees.