Today the long lasting assumption from antiquity about the indivisibility of atoms remains to be valid only with respect to their chemical quality. At the beginning of the twentieth century discoveries were made to the effect that atoms must have a decided structure and do not appear as compact mass spherules. Fast electrons for example can penetrate thin metal foils, a fact that indicates that atoms consist mainly of empty space. But in this case the mass of the atom must be concentrated in a centre surrounded by force fields responsible for electro-magnetism.