orbit instead of in a
orbit. We shall thus expect in the sixth period to meet with a new stage in the development of the group with
-quanta orbits. Once this point has been reached we must be prepared to find with increasing atomic number a number of elements following one another, which as in the family of the iron metals have very nearly the same properties. The similarity will, however, be still more pronounced, since in this case we are concerned with the successive transformation of a configuration of electrons which lies deeper in the interior of the atom. You will have already guessed that what I have in view is a simple explanation of the occurrence of the family of rare earths at the beginning of the sixth period. As in the case of the transformation and completion of the group of
-quanta orbits in the fourth period and the partial completion of groups of
-quanta orbits in the fifth period, we may immediately deduce from the length of the sixth period the number of electrons, namely