THE PERIODIC SYSTEM 277 



in an eighth group. At the time Mendelejeff made the table, 

 three places in the third, long series had to be left blank, as a tri- 

 valent element [Sc] was lacking in the first octave of the series, and 

 a trivalent [Ga] and a quadrivalent one [Ge] in the second. These 

 places have since been filled, as we shall presently see. 



The fourth series, which is a long series exactly similar to the 

 third, contained many blanks at the time of Mendelejeff, but is 

 now nearly complete. It begins with an active alkali metal, 

 rubidium, and ends with iodine, a halogen. The rule of valence 

 is strictly preserved throughout the series, and in general the 

 elements fall below those which they most closely resemble. 



The fifth series is still somewhat incomplete, but the order 

 of the atomic weights and the valence enable us satisfac- 

 torily to place most of those elements which are known. The 

 chemical relations to elements of the fourth series justify the 

 position assigned to each. Caesium, for example, is the most 

 active of the alkali metals; barium has always been classed 

 with strontium, and bismuth with antimony. 



The sixth and last series contains only a few radioactive ele- 

 ments. No element with an atomic weight greater than that of 

 uranium (238) has yet been discovered. 



The most important change made in the table since the time of 

 Mendelejeff is the addition of another group, the family of the 

 inert gases of the atmosphere (see p. 296). These elements were 

 unknown before 1894, but fall logically into a new group at the 

 left hand side of the table as here given. 



