According to Hund's rule, electrons must occur singly first before pairing takes place.
If I want to fill six electrons into orbitals, the filling of electrons will be as follows;
1s2 2s2 2p2.
The first four electrons are filled into the 1s and 2s levels having only one orbital each. The fifth and sixth electrons are filled into 2p orbitals. The 2p level have three degenerate orbitals. The two electrons are singly filled into each of the degenerate orbitals in accordance to Hund's rule.
