The next inactive gas is _xenon_ (54), which outside of the 4-quantum group has a group of eight electrons in 5-quantum orbits, four in 5₁ orbits and four in 5₂ orbits. We notice that in xenon the group of 4-quantum orbits still lacks the 4₄ orbits. On the theory we must, therefore, expect to meet a new process of completion and reconstruction when proceeding in the system of the elements. The theoretical argument is similar to that which applies in the case of the completion of the 3-quantum group which takes place in the fourth period of the natural system. In fact, in the formation of the normal atoms of the elements next after xenon, caesium, 55, and barium, 56, the fifty-four electrons first captured will form a xenon configuration, while the fifty-fifth electron will be bound in a 6₁ orbit, consisting of very oblong loops, which represents a much stronger binding than a circular 4₄ orbit. Calculation shows, however, that with increasing nuclear charge there must soon appear an element for which a 4₄ orbit will represent a stronger binding than any other orbit. This is actually the case in _cerium_ (58), and starting from this element we meet a series of elements where, in the normal neutral atom, the 4-quantum group is in a state of reconstruction. This reconstruction must occur far within the atom, since the group of eighteen 4-quantum orbits in xenon is already covered by an outer group of eight 5-quantum orbits. The result is a whole series of elements with very slight outward differences between their neutral atoms, and therefore with very similar properties. This is the _rare earths_ group, which in such a strange way seemed to break down the order of the natural system (cf. p. 21), but which thus finds its natural explanation in the quantum theory of the structure of the atom.