Quantum Numbers and Electronic Configuration

The following rules help us to predict electronic configuration
of atoms with the help of quantum numbers
1. Electrons are added into orbitals in the way that gives the
lowest total energy for the atom.
2. Electrons are assigned to orbitals in order of increasing
value of (n+l).
3. For subshells with the same value of (n+l), electrons are
assigned first to the subshell with lower n.
4. No two electrons in an atom may have identical sets of four
quantum numbers.
5. Electrons occupy all the orbitals of a given subshell singly
before pairing begins. These unpaired electrons have parallel
spins. (Hund Rule). Hund rule states that for degenerate
orbitals, the lowest energy is attained when the number of
electrons with the same spin is maximized.