MATTER 77 



Besides the basic arithmetical progression and 

 the increase in atomic weight, the series of the ele- 

 ments shows a periodic recm-rence of similarities of 

 properties, which latter have led to the grouping as 

 found in the periodic table. The periodic table 

 now shows a grouping into eight periods, with the 

 rare gases placed as the first period or (preferably) as 

 the eighth period. However, this does not fully con- 

 vey the periodicity shown by the elements. For on 

 the basis of similarities of properties, the elements 

 also show first a period of two elements, hydrogen 

 and helium, followed by two small periods of eight 

 elements each. Then come larger periods of eighteen 

 elements, with interruptions (the rare earths, from 

 cerium to lutecium) . Finally, there is the great period 

 of thirty -two elements, which is followed by a period 

 of only six more elements, breaking off with uranium. 



To account for the fact that the series of the ele- 

 ments ends with uranium, when the period of six 

 elements of which uranium is the last, could easily 

 be conceived extended to include more elements, 

 possibly to the rounding out of the period to thirty- 

 two, Sommerfeld suggests that any possible elements 

 that may have followed uranium in this period are 

 now non-existent due to their radioactive decom- 

 position. 



However that may be, it is certain that the ele- 



