An attempt is made to correlate the kinks in the mass defect curve with the energy differences between isobars, both as obtained from direct measurements and also from the shift of the isotopic number to higher values with increasing number of particles. Since the single-particle picture is known to be an insufficient approximation, the symmetry property of the wave function, resulting from the use of a symmetric Hamiltonian is utilized. The average interaction between symmetrically and antisymmetrically coupled particles (L+L' and L-L') is determined mainly from the kinks in the mass defect curve and enables one to calculate the energy differences between isobars. The energy change at the end of the shell is obtained from experimental data. It should enable one to get some idea of the probabilities with which the particles are in excited configurations. For heavier elements, the formula obtained here should naturally be identical with Weizsäcker's semi-empirical formula and the connection between both is discussed.