391 392 393 class if similarity of properties is to be the distinguishing mark of the members of a class. Omitting hydrogen, it may be said that the properties of the members of a series vary much from the first to the last member, and that each series is to a great extent a repetition of that which precedes it; and that the properties of the members of a group vary from the first to the last member, but that all the members are more like each other than they are like any other elements. The properties of any element may be determined by considering (1) the properties of the group to which it belongs, (2) the properties of the series in which it occupies a place, (3) the position of the element in the group and in the series, (4) the relations between the properties of elements situated similarly to the given element and those of the other members of the group and of the series in which these elements occur, (5) the relations of the group and the series in which the specified element occurs with other groups and series. We cannot thoroughly grasp these generalisations until we have examined in some detail the properties of several groups and series, but their general bearing will be made more intelligible by glancing at the position assigned to one specified element in the scheme of classification based on the periodic law. Let us choose the element antimony. The chemical properties of an element may be fairly summarised by stating (a) the compositions, and (b) the properties, of its more important compounds, such as oxides, hydrides, haloid compounds, salts, &c. 2 2 We must look in the first place at the properties of the group of elements to which antimony belongs. The compositions of the typical compounds of the elements of this group are represented by the general formulae M,O,, MO, MH, MCI,, MBr,, MI, MOCl, and, in the cases where the elements form salts, M.3SO,, M,6NO,, &c. The oxides are acidic, but become less acidic and more basic as the atomic weight of M increases. The hydrides, when the elements combine with hydrogen, are gases, solutions of which in water are more or less alkaline; these hydrides are oxidised by mixing with oxygen and heating, giving M.O, (or M ̧0 ̧) and H2O. 3 We must look in the second place at the properties of the series in which antimony finds a place. Arranging the elements of the series in order of increasing atomic weights we find them forming oxides, haloid compounds, hydrides, salts, thus ; 3 -no hydrides 2 2 5 2 7 Sb,O, and SbO, Te O, and TeO, IO, and (?) 1,0, 3 SbH, TeH2 IH ICI: 3 Ag, SO &c., CdSO, &c., In, 3SO, &c., Sn(SO) and Sn(SO4),&c., (1)Sb,3SO, salts of Te very unstable, no salts of I. A glance at the third determining condition, viz. the position of antimony in the group and series, shews; (1) that this element comes in Group V. following after elements whose oxides are acidic, most of which form hydrides, and which as a class do not form stable salts by replacing the hydrogen of acids, and most of which combine with hydrogen and oxygen to form well marked acids; (2) that the element is succeeded in Group V. by elements whose highest oxides are feebly acidic but at the same time interact with acids to form salts, which do not form hydrides, and the compounds of which with hydrogen and oxygen are easily separated into oxides and water and on the whole are basic rather than acidic. 2 2 If we now choose tin as an element similarly situated to antimony, we find that this element forms the oxides SnO and SnO,; that the elements preceding and succeeding it in Group IV. form oxides of similar composition; that the oxides CO and Sio, are decidedly acidic, the oxides TiO, GeO, and ZrO, are less acidic and are also basic, and that CeO2 PbO, and ThO, are basic with perhaps very feebly marked acidic properties; that the only elements of the group which form hydrides are carbon and silicon; and that all the elements of the group except carbon and silicon replace the hydrogen of acids and thereby produce salts. Looking back on these facts we see that antimony is well placed in Group V., Series 7; that it is more metallic than the elements, taken as a whole, which precede it, and less metallic than those which succeed it, in the group; that it is more negative or nonmetallic than the elements which precede it in the series, and less negative than those which come after it; that its highest typical oxide (Sb,O) is composed of a greater number of oxygen atoms relatively to the atoms of antimony than the highest oxide of the elements which precede it in 394 Series 7, (Ag,O, Cd,O,, In,O,, Sn,O,), but that it is succeeded in the series by elements whose highest typical oxides (TeО„ IO) are composed of a greater number of atoms of oxygen relatively to the atoms of the other element than Sb2O.. Finally, we ought to examine the relations of Group V. and Series 7 to other groups and series. It is impossible to do this at present except in the merest outline. Looking at this subject broadly, it may be said that the members of Group I. shew greater differences among themselves, and the members of Group VII. are more closely related among themselves, than the members of the intermediate groups; and that the variation of properties from the first to the last member of a series is very marked in Series 2, but becomes, on the whole, less marked as we pass through Series 3, 4, 5, to Series 11. If we may apply so vague a generalisation as this, we should conclude that antimony ought to exhibit very well marked analogies with the other members of the group in which it occurs; and that although it must widely differ from the other elements in its series, yet it will probably not differ to so very marked an extent as, say, nitrogen differs from the other members of Series 2, or phosphorus from the other members of Series 3. These tentative and somewhat vaguely worded conclusions, are fairly borne out by the actual relations between antimony, the members of Group V. on the one hand, and the members of Series 7 on the other hand. But In the sketch which has now been given of the periodic law, each group of elements has been treated as a whole. the more detailed study of these groups will shew us that each, except Group VIII., is more or less sharply divided into two sub-groups; one sub-group contains the elements belonging to even series, the other sub-group contains the elements which are placed in odd series. This division of the groups into sub-groups is sometimes marked, e.g. in Group VI.; sometimes it is almost hidden by the distinct way in which all the members of the group are stamped with the characteristics of the groups, e.g. in Group V. Some groups, for instance Group II., exhibit very clearly both the general characteristics of the group, and the division into two sub-groups the members of either of which are more like each other than they are like the members of the other subgroup. This division into sub-groups is rendered very clear in the annexed table. |