Here we introduce the deformed shell model in the form of the Nilsson model. We note that the spherical shell model predicts the angular momenta of 21Ne and other nuclei wrongly. One of its limitations arises from the form of the Harmonic oscillator (H.O.) potential: V=1/2 Kr^2, which presents spherical symmetry. However, once we deformed the H.O. along the quadrupole degree of freedom -- i.e., oblate (or lentil) and prolate (or rugby ball) shapes -- we have new single particle energies as a function of quadrupole deformation, which depend on the particular orientation of the proton or neutron going around the nuclear core. In such a way that the ground state and first excitation in 21Ne are predicted correctly for a prolate deformation.
