Hund's First rule

For a given n and l the screening effect is similar for different m values and thus these orbitals remain degenerate in several electron atoms. In the ground state of boron (1s)2(2s)2(2p)1 any one of the three m values (-1, 0, +1) for the p electron has similar energy. But in carbon (1s)2(2s)2(2p)2 the different ways of placing two electrons in the three 2p orbitals do not have similar energy, as the electrons may repel each other to dissimilar extents. Putting two electrons in an orbital with similar m incurs more repulsion than having distinct m values. In the latter case, the exclusion principle Makes no restriction on the spin direction (ms values), but it is found that if the electrons have parallel spin (same ms), there is less repulsion. This is shortened in Hund's first rule:

1. The ground state has as several electrons as possible in different orbitals, and with parallel spin, when electrons are placed in a set of degenerate orbitals.

The mathematical formulation of many-electron wave functions accounts for the rule by presentation that electrons with parallel spin tend to avoid each other in a way that cannot be described classically. exchange energy is the reduction of electron repulsion that results from this effect.