p electron bond order
We can also define a quantity which measures the strength of a p-electron bond between two atoms. This quantity depends on the electron density which is shared between two atoms.
To calculate the p-bond order between a pair of atoms, add together the contributions to bonding from all the occupied p MOs.
p electron bond order for a-b bond (pab):
again ni is the number of electrons in molecular orbital i. If all the occupied molecular orbitals have two electrons in them then ni = 2 for all the MOs, and: