The periodic table of the elements in its most complete form as it has been developed since Mendeleev's time is shown above. It summarizes the observed chemical behavior of atoms. In essence it is a series of eight-element rows, representing shells containing one to eight electrons. To this extent, the eight-electron shell model of previous chapters has been correct. The first row contains only two elements, hydrogen and helium. The second and third rows contain eight elements each.

The fourth row is interrupted after the second element, and a series of ten additional metals is inserted, the transition metals scandium (Sc) through zinc (Zn). Similarly, the fifth row is interrupted by the insertion of another series of ten transition metals, yttrium (Y) through cadmium (Cd). The sixth row has a double interruption: a set of fourteen inner transition metals (also called lanthanides or rare earths), and a series of ten more transition metals.