According to this rule, electron pairing will not take place in orbitals of same energy (same sub-shell) until each orbital is first singly filled with parallel spin. In other words in a set of orbitals having same energy (degenerate orbitals), the electrons distribute themselves to occupy separate orbitals with same spin as far as possible. This rule can be illustrated by considering the example of carbon. The atomic number of carbon is 6 and it contains two electrons in 2p subsheIl and these can be distributed in the following three ways:
Since all the three 2p orbitals have same energy, therefore, it does not take any difference as to which of the three orbitals contain electrons. In state (a) both the electrons are in the same orbital. In state (b), the two electrons are present in different orbitals but with opposite spins while in state (c), the electrons are present in different orbitals with same spins. Now, the electrons are charged particles and repel one another. The electron-electron repulsions are minimum when the electrons are as far apart as possible with parallel spins. Thus, state (c) has minimum repulsions and corresponds to lower energy (stable state). This is in accordance with Hund's rule. This principle is very important in guiding the filling of p, d and f subshells, which have more than one type of orbitals.