Electron Pair Repulsion Theory

The electron pair repulsion theory can be used to predict the shape and bond angles of covalent molecules and polyatomic ions.

Electron pairs repel each other so they are arranged as far apart as possible. This minimises repulsion and holds the molecule in a particular shape.

The shape of a molecule depends on the number of electron pairs.
A lone pair of electrons is slightly closer to the central atom and therefore takes up more space than a bonded pair of electrons. As a result, repulsion from a lone pair is stronger than from a bonding pair. For each lone pair, the bond angle decreases by about 2.5°Double and triple bonds are treated as a single bonding region. The octet rule and expanded octet rule determines how many valence electrons there are. In the PCl₅ molecule, the central phosphorus atom is bonded to five Cl atoms, thus having 10 bonding electrons and violating the octet rule (trigonal bipyramidal).Some of the third-period elements (Si, P, S, and Cl) bond to more than four other atoms because for n=3, the d sublevel exists, and it has five d orbitals. Although the energy of empty 3d-orbitals is ordinarily higher than that of the 4s orbital, that difference is small and the additional d orbitals can accommodate more electrons. Therefore, the d orbitals participate in bonding with other atoms and an expanded octet is produced.