-quanta orbits, giving rise to the rare earths. This occurs at a place in the periodic system where the third stage in the development of an electronic group with
-quanta orbits, indicated by the outer frame, has already begun. In this connection it will be seen that the inner frame encloses a smaller number of elements than is usually attributed to the family of the rare earths. At the end of this group an uncertainty exists, due to the fact that no element of atomic number
is known with certainty. However, as indicated in [Fig. 1], we must conclude from the theory that the group with
-quanta orbits is finally completed in lutetium (
). This element therefore ought to be the last in the sequence of consecutive elements with similar properties in the first half of the sixth period, and at the place