In the model of atomic nuclei presented here, we assume cubic protons and neutrons, with a classical structure of positrons and electrons. With this concept, stable nuclei can be constructed on a purely electrostatic basis, without the postulate of a strong nuclear force nor quarks and gluons, by assuming that the electrons are located on average 1/3 between neighboring nucleons, so that the 1/6 e+ of the neighboring positron charges are compensated and, in addition, stable electrostatic binding is generated. This also leads to the neutron rule, which states that 1/3 neutron must be available for each contact surface (inner surface). The structure of the nuclei is based on the simple principle of a modular system that consists only of the four basic building blocks D, T, He4, Be9 and single nucleons. When the basic building blocks are combined, new structures are created along the stacking direction, the mass defects of which are known or can be easily calculated. From this, the mass defects of the nuclides can be derived quite accurately. The relative errors of these calculations are smaller than those of the Bethe-Weizsäcker model by a factor of 10 and are excellent, especially in the range of smaller nuclei, where the droplet model only provides moderately good results. The basic building blocks mentioned above can be joined in different ways, which causes different structures along the stacking sequences and thus different mass defects. Our model thus leads to a substructure of the isotopes, which we have called isomeric structural variants or, more briefly, structural isomers. These differ by about 1 — 30 10-30 kg. Their mass differences are thus smaller by a factor of 100 — 1000 than those of the isotopes. Nevertheless, we assume that these structural isomers can be isolated and quantified, which would not only be extremely important for the verification of this model, but would also enable a very precise calculation of the isotope masses. Another very interesting point is that the composition of the isotopes with even and those with odd mass numbers follows completely different structural lines. The former all consist of stacked α particles and, where necessary, additional single nucleons. The latter isotopes are all derived from N15, which is formed by two intertwined Be9 rings. Adding two protons and neutrons produces the F19, a 33 cube with missing cornerstones, which can now be extended as desired by adding nucleons in pairs at the periphery, thus representing the core structure of all isotopes with an odd mass number.