Rules for Filling Orbitals
   1. Lower-energy orbitals fill first.
   2. An orbital can hold only 2 electrons with opposite
       spins (Pauli Exclusion Principle).
   3. If 2 or more degenerate orbitals available, 1 electron
       goes in each until all are half-full (Hund's Rule).

   Degenerate: Orbitals with same energy level!
    Theoretically, electrons in same shell possess the same energy.
    This is not true for multielectron atoms.
    The reason is SHIELDING!

   Shielding results from repulsion of electrons either in the same
   shell or inner shell. We will consider shielding from same shell.
   Energies of 3^rd Shell:  3s < 3p < 3d
   These differences are due to shielding!

   For electrons in 3^rd shell:
   3p shielded by 3s
   3d shielded by 3s/3p
    Beacause electrons in 3s shield electrons in 3p from the nucleus,
    additional energy needed to maintain electrons in 3p orbital.
   Because both 3s/3p shield 3d from nucleus, even more energy
   needed to maintain electrons in 3d orbital.