THE PRESENT CONCEPTION OF MATTER 8i 



But the discovery of the inert gases in the atmosphere since 

 then has added another vertical column (Group o) to the table, 

 or, to use the same analogy, another note to the scale. In the 

 table (see fig.) the symbol and the atomic weight are written 

 underneath the element, whilst above them is the atomic 

 number, the most important constant of the element, and one 

 which will be considered later. 



But the Periodic Table is somewhat more difficult than 

 would appear from this statement. In three cases pairs of 

 elements are arranged in the inverse order of their atomic 

 weights, argon before potassium, cobalt before nickel, and 

 tellurium before iodine, this arrangement being necessary to 

 avoid the breaking up of similar families, and is due, as was 

 recently pointed out by Aston in the Journal of the Chemical 

 Society, simply to the existence of isotopes. 



The law holds very well indeed for the first two horizontal 

 lines and half-way through the third, but then come ten elements 

 from Vanadium to Germanium before the characteristics of 

 the preceding ones fully reappear again ; these may best be 

 regarded as being interpolated into the table. Then we go 

 along normally from Arsenic to Zirconium through the full 

 period of eight, before we find ten again interpolated from 

 Niobium to Tin. But after the fifty-sixth element. Barium, 

 the table completely and utterly breaks down, and we have 

 a group of sixteen, known as the " Rare-earth metals," all so 

 very much alike that the separation of one from another is one 

 of the most difficult tasks the chemist has to face. These are 

 merely written continuously in the table without attempting 

 to fit them into the Groups. 



Following them is a group of ten from Tantalum to Lead, 

 and then the table runs on normally to the end. The thirty 

 elements in the table that we have regarded as being inter- 

 polated are well-defined metals and differ among themselves 

 less markedly from member to member along the horizontal 

 lines than do the other elements in other parts of the table. 

 To make them fit properly we make a ninth group, known as 

 No. VIII (since there is also a No. o), and this contains three 

 sets of three each. 



The resemblances between the members of the groups 

 may be brought out still more clearly by dividing some of the 

 latter into two sub-groups. Thus, for example, in Group I 

 we have the so-called alkali metals in one column and copper, 

 silver, and gold in the other. The individual members of each 

 sub-group are most closely connected with the other individuals 

 in the same vertical column, but the sub-groups are themselves 

 connected by virtue of compounds that may be formed from the 

 substances in them. 



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