Hund's Rule of Maximum Spin Multiplicity

Hund's rule of maximum spin multiplicity is an observational rule which states that Pairing of electrons in degenerate orbitals is not possible until all available orbitals contain one electron each.

The Aufbau principle let's us build up an atoms electronic configuration by placing electrons into orbitals of every increasing energy. Hund's Rule tells us about how the electrons in an atom should be placed into degenerate orbitals. Technically Hund's Rules say that the ground state configuration should maximize the multiplicity of a configuration. What does that mean? The simple rule is that electrons should be placed into separate orbitals before going into the same orbital. This is because these best represent the configurations that are the lowest energy when we do the quantum mechanical calculation.

These pictures with up and down arrows for electron spin are a gross simplification of the realities of quantum mechanics. However, they do allow us to predict some measurable behavior for atoms such as the number of unpaired electrons. Thus Hund's rule has implications for predicting the properties of atoms as paired and unpaired electrons have distinct properties (in particular with interactions with magnetic fields).

Substances with unpaired electrons are called "paramagnetic" as they are attracted towards a magnetic field. Substances will all paired electrons are "diamagnetic" as they are repelled from a magnetic field.

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.

So when working with the p sublevel, electrons fill like this....up, up, up...down, down, down...take a look: