VSEPR: Valence Shell Electron Pair Repulsion.
Assumptions about the nature of the bonding
The underlying assumptions made by the VSEPR method are the following.
Atoms in a molecule are bound together by electron pairs. These are called bonding pairs. More than one set of bonding pairs of electrons may bind any two atoms together (multiple bonding).
Some atoms in a molecule may also possess pairs of electrons not involved in bonding. These are called lone pairs or non-bonded pairs.
The bonding pairs and lone pairs around any particular atom in a molecule adopt positions in which their mutual interactions are minimized. The logic here is simple. Electron pairs are negatively charged and will get as far apart from each other as possible.
Lone pairs occupy more space than bonding electron pairs.
Double bonds occupy more space than single bonds.