In this table the elements arranged in the order of 

 their atomic weights fall into eight groups where the 

 known oxides progress regularly, with the exception of 

 two or three elements, from R 2 in Group I to R 2 7 in 

 Group VII, while in Group VIII two oxides (of ruthen- 

 ium and osmium) are known which carry the progression 

 to R0 4 . 



It was pointed out by Mendeleeff that, with the excep- 

 tion of series 1 and 2 at the top of the table, the alternate 

 members of the groups show particularly close relation- 

 ships. These subordinate groups, marked A and B, in 

 most cases show remarkable analogies and gradations in 

 their .properties, for example, in the alkali-metals from 

 lithium to caesium, and in the halogens from fluorine to 

 iodine. The two divisions of a group do not usually 

 show very close relations to each other, except in their 

 valency, and they even display, in several instances, 

 opposite gradations in chemical activity in the order of 

 their atomic weights. For instance, caesium stands at 

 the electro-positive end, while gold stands at the electro- 

 negative end of its subordinate group. The difference 

 between the two divisions is very great in Groups VI and 

 VII, but it is extreme in Group VIII, where heavy metals 

 are on one side and inactive gases on the other. Many 

 authorities separate these gases into a " Group 0" by 

 themselves at the left-hand side of the table, but this does 

 not change their relative positions, and the plan may be 

 objected to on the ground that many vacant places are 

 thus left in the groups VIII and 0. 



The periodic law has been useful in rectifying certain 

 atomic weights. At the outset Mendeleeff was obliged to 

 change beryllium from 14-5 (assuming Be 2 3 ) to 9 

 (assuming BeO), and later the atomic weights of indium 

 and uranium were changed to make them fit the system. 

 All of these changes have been confirmed by physical 

 means. 



Mendeleeff found a number of vacant places in his 

 table, and was thus able to render further service to 

 chemical science by predicting the properties of undis- 

 covered elements, and his predictions were very closely 

 confirmed by the later discovery of scandium, gallium, 

 and germanium. The table indicates that there are still 



