Every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin.
Single electrons occupy all empty orbitals before they start to form pairs in orbitals. Two electrons in the same orbital have a repulsion between them due to their negative charge. The more stable configurations is with single electrons in different orbitals.
Evidence for Hund’s Rule
The first ionisation energy for the elements in period 3 has a general increase.
Sulphurs value is below that of phosphorus. The highest energy electrons are both in 3p sub levels this is evidence for Hund’s Rule.
Phosphorus has 3 electrons in its 3p sub level whereas sulphur has 4.
The lowest first ionisation energy for sulphur is because it has a pair of electrons in one of the 3p orbitals. Mutual repulsions between these two electrons make it easier to remove one from the other.
Phosphorus ionisation energy = 1012 KJ mol-1
Sulphur ionisation energy = 1000 KJ mol-1