(a) Pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied.
(b)In the ground state of the atoms, the orbitals are filled in order of their increasing energies.
(c) Orbitals with lower value of ((n+l) have lower energy.If two orbitals have the same value of (n+l) then orbital with lower value of n will have lower energy.
