Ask a Teacher

explain Hund's rule of maximum multiplicity?

Hund's rule of maximum multiplicity gives idea for filling electrons into the orbitals having equal energy .The rule states that '' When more than one orbitals of equal energies  are available ,then the  electrons will first occupy these orbitals separately with parallel spins.The pairing of electrons will start only after all the orbitals of a given sub level are singly occupied''.

Let us take an example in which three electrons are to be filled into  three p-orbitals in two different ways as shown below .

Explanation  : Two electrons with parallel spins ,tend to be as far  apart as possible in order to minimize the electrostatic repulsion.Therefore ,the electrons prefer to occupy the orbitals singly as far as possible .When all the orbitals get singly occupied ,then the incoming electron has two choices :either to pair up with the other electron or to go to the next higher orbital .When a vacant orbital of suitable energy is not available ,then the incoming electron will have no choice except to pair up with another electron .