Opposite Spin

                  Electrons with opposed spins are represented by arrows pointing in opposite directions. 

Hund Rule

                Two electrons usually do not occupy a given orbital in a sub-shell until all of the orbitals of the sub-shell have at-least one electron.

Pauli Exclusion Principle

              No orbital can contain more than two electrons and these electrons must have opposite spin.

Orbital

        Orbital is the mathematical expression describing the behavior of an electron moving in the vicinity of a positively charged nucleus.

M - Shell

        A third shell with a maximum of eighteen electrons.

L - Shell

        Second shell with a maximum of eight electrons.

K - Shell

     First shell with a maximum of two electrons.