NEW CPIEMICAL ELEMENTS-^DUSHMAN 



247 



elements lithium (atomic number 3), sodium (atomic number 11), 

 and potassium (atomic number 19) all belong to the alkali metal 

 group. 



The next members of the group of inert gases are krypton (atomic 

 number 36) and xenon (atomic number 54) . Cesium of atomic number 

 55 (that is, 37+18) is a member of the alkali metals group. The 

 periodicity of 18 is succeeded by a period of 32 elements, so that 

 radon (radium emanation) of atomic number 86 is the next and last 

 member of the group of inert gases. 



Figure 1. — Perodic arrangement of elements. 



This periodicity in the distribution of the electrons in shells corre- 

 sponds to a periodicity in chemicals and physical properties of the 

 elements when these elements are arranged in order of increasing 

 atomic number, as shown in the periodic arrangement of elements, 

 figure 1.^ Thus we find that the elements fall into eight groups, with 

 chemically similar elements in each group. 



Such an arrangement was first suggested in 1870 by the Russian 

 chemist Mendeleeff and was based on the arrangement of the elements 

 in order of increasing atomic weight. With further discovery of 

 new elements and more accurate determinations of atomic weights 



•Prom Fundamentals of Atomic Physics, by S. DusLman. New York, 1951. 

 981445—52 17 



