VSEPR theory

The shape of a molecule or ion is determined by the repulsion between the electron pairs surrounding the central atom. 
Valence Shell Electron-Pair Repulsion Theory
- Only the electron pairs arranged around the central atom in a covalent molecule need to be considered 
- Pairs of electrons repel each other and move as far apart as possible 
- Lone pair-lone pair repulsion is greater than lone pair-bonding pair repulsion which is greater than bonding pair- bonding pair repulsion 
- The actual shape of species is determined by the "atoms" attached round the central atom (and influenced by the lone pairs of electrons) 
- All bonds - single bonds, double bonds and triple bonds - take up the same space.