The structure of nuclei begins with lower-order nuclei, such as deuterium, tritium, and helium He−3, which evolve into the helium nucleus He−4, and then the first upper-order oxygen nucleus O−16 that has four helium nuclei He−4 in a column of strong negative electric field. Furthermore, the second upper-order calcium nucleus Ca is based on the fundamental natural phenomenon of mirror symmetry, by repeating the structure of the first upper-order oxygen nucleus and its half, i.e., at a 2.5 factor. The same principle applies to the third upper-order tin nucleus Sn, which emerges from the second upper-order calcium nucleus Ca, according to mirror symmetry and the same 2.5 factor. It is noted that the tin nucleus Sn will further form the basis for the structure of all heavy nuclei up to the radioactive uranium nucleus U − 235. This is the simple and elegant structure model, according to which nuclei consist of fixed helium nuclei He − 4 (plus deuterium, tritium, and helium He − 3, all evolving into helium He − 4), while the bonding neutrons rotate around them.