38 SECTIONAL ADDRESSES. 
increases by unity as we pass from a lighter to a heavier element in 
the alkali group. 
Another interesting feature of the classification is that in the genesis 
of the different kinds of atoms provision is made for the appearance 
at certain stages of sets of homologous elements such as those of the 
iron, palladium, platinum, and rare-earth groups. For example, the 
appearance of the iron group accompanies the establishment in the 
normal atom of an inner group of orbits of the 3, type beginning with 
the element scandium. These 3, orbits begin in the fourth period and 
differentiate it from the second and third because for the first time the 
charge on the nucleus is sufficiently great to make it possible for the 
successive atoms to differ by an extra electron in such an inner group 
instead of in an outer one. The appearance of the palladium group also 
is associated with the beginning of a development of inner orbits of the 
4, type at a stage in the binding process when the outer orbits of the next 
lighter atoms consist of 5, quantum orbits. The appearance of the 
platinum and rare-earth groups of elements, too, it will be seen, is 
associated with the beginnings of developments of inner orbits of the 
5, and 4, types respectively at stages in the binding process when 
the outer orbits of the neighbouring lighter elements are of the 6, type. 
732i ———" Na 
 4Be 12 Mg 
5B 13 A? 
6c 14 Si 
, 
2We. IN ISP 
“80 ——4S 
OIF 170? 
(0Ne 148A 
FIG. 4.—Elements. 
