< ;« *'.
f < t
I; |ij
fourth and fifth periods a single frame indicating the final
pletion. of the electronic group with 3-quanta orbits, an'
last stage but one in the development of the group with 4-c
orbits respectively. In the sixth period it has been necess:
introduce two frames, of which the inner one indicates tb
stage of the evolution of the group with 4-quanta orbits, givii
to the rare earths. This occurs at a place in the periodic s
where the third stage in the development of an electronic
with 5-quanta orbits, indicated by the outer frame, has a
begun. In this connection it will be seen that the inner
encloses a smaller number of elements than is usually attr
to the family of the rare earths. At the end of this gr<
uncertainty exists, due to the fact that no element of
number 72 is known with certainty. However, as indies
fig, 1, we must conclude from the theory that the grou
4-quanta orbits is finally completed in lutetium (71). This e
therefore ought to be the last in the sequence of cons<
elements with similar properties in the first half of the sixth
and at the place 72 an element must be expected -which
chemical and physical properties is homologous with zirconi
thorium. This, which is already indited on Julius Thorns*
table, has also been pointed out by Bury. [Quite recently Da
has in an investigation of the X-ray spectrum excited in prep*
containing rare earths, observed certain faint lines which he j
to an element of atomic number 72. This element is ident
him as the element celtium, belonging to the family of rare
the existence of which had previously been suspected by
Quite apart from the difficulties which this result, if correcl
entail for atomic theories, it would, since the rare earths a<
to chemical view possess three valencies, imply a rise in
valency of two units when passing from the element 72
next element 73, tantalum. This would mean an except]
the otherwise general rule, that the valency never incr<
more than one unit when passing from one element to the
the periodic table ] In the case of the incomplete seventl
the full drawn frame indicates the third stage in the deve
of the electronic group with 6-quanta orbits, which must
actinium. The dotted frame indicates the last stage bui