The f_7/2 shell constitutes an ideal benchmark to investigate different nuclear-structure properties that can be studied in detail by means of the large-scale shell model. In particular, the systematic study of differences in excitation energy in isobaric nuclei has allowed to develop a theoretical method to deduce from the data, changes in the shape of the nuclei along a rotational band, the mechanism of the backbending, and the need to include an isospin-symmetry-breaking term in the nuclear interaction. Recently, these studies have been extended to other mass regions. In particular, I show here some results obtained in the sd shell where the method deduced for the f_7/2 nuclei seems to work well. A new approach based on a charge-dependent realistic nucleon-nucleon interaction is introduced. It allows to deduce the neutron skin at every excited state from the mirror energy differences. Moreover, a strong correlation between the skin and the difference of occupation numbers of neutrons and protons in the s_1/2 orbit is found.