APPENDIX 361
display the valence of 3 which is explained by the transfer of one
electron from the sheath to the fourth kernel layer giving the
structure Ce 68 2-8-18-19-8-3.
Praseodymium PrB9 never displays the valence of 5, but it and
the succeeding elements up through lutecium LU71 display almost
without exception the valence of 3. Each additional electron in
this so-called transition series, LaB7 to Lu 7 i, sinks into the fourth
kernel layer until in lutecium this layer is filled to a maximum of
- Lu 7 i 2-8-18-32-8-3. The next element, hafnium, should
retain four electrons in its valence layer and display an invariable
valence of 4.
It is one of the great triumphs of theory that Bohr's predictions
concerning element 72 were proved to be accurate. Bohr con-
cluded from reasonings along the lines we have indicated that the
element of atomic number 72 should have a valence of 4 and
resemble zirconium and thorium in chemical properties, instead
of having a valence 3 and resembling the rare earth elements as
had previously been supposed. He instigated a search in his
laboratories at Copenhagen for this element, and very shortly it
was found in some of the minerals that were tested and it proved
to be present in the earth in fair abundance. It proved to have
the properties predicted, and it was named hafnium in honor of
the country in which it was discovered.
The shorter transition series in the eighteen element periods are
indicated in the table by printing the symbols of the elements in
heavy type or italics.
Subordinate Periods. Within the long periods are clearly
recognizable subordinate periods, which were already indicated
in the older Mendelejeff and Lothar Meyer charts of the periodic
system (see back cover page). Closer study seems to indicate
that the elements of the subordinate periods are related to the
elements Ni, Pd, Er, Pt in the same way that the elements of the
two short periods are related to He and Ne. The number of
electrons in these apparently key elements is such as might give
the following structures.
Ni 28
Pd 46
Er 08
Pt78
2
2
2
2
8
8
8
8
18
18
18
18
18
32
32
8
18
