Before any orbital is doubly occupied, all orbitals in the sub-level are singly occupied.
All electrons in a single-occupancy orbital have the same spin in order to maximize overall spin.
An electron can fill all of its orbitals with the same energy, therefore it won't couple with another electron in an orbital that is only partially filled.
Atoms in their ground state have a lot of unpaired electrons.
The behavior of two electrons in contact would be similar to that of two magnets.
Electronic configuration
According to Hund's maximum multiplicity rule, all electrons entering a subshell fill the subshell's orbitals singly, i.e. unpaired electrons occupy all orbitals first.
To maximize the multiplicity of spin, all such electrons have the same spin.
According to the rule, the lowest energy term in the electronic configuration will have the highest spin multiplicity value.
An atom's electronic configuration in its stable state has a maximum number of electrons.
Electrons can only share an orbital once all degenerate orbitals have been filled with electrons of the same spin.
When electrons enter -subshells, all three -orbitals will have one each, and only the fourth, fifth, and sixth electrons will be able to share the orbitals that are already occupied.
Example: Carbon with atomic number , hence its electronic configuration is .