The nuclear SU$ model is used to describe the basic characteristics of nuclei with .4 = 21. The use of the phenomenological residual interaction (Serber-Gauss and Serber-Yukawa) gives only unsatisfactory agreement with experiment. The residual interactions derived from the Hamada-Johnston and Tabakin realistic potentials, after introducing renormalization corrections with three particles and one hole, and with two particles and two holes in the intermediate states, produce Ne 21 and Na 21 spectra in satisfactory agreement with experiment and also confirm the equivalence of these potentials. More of the electromagnetic characteristics of these nuclei are described very well by the model used.