There are prominent differences between the three ele- 443 ments, copper, silver, and gold, placed in the odd series of Group I. Gold is marked off from the others by the great instability of its salts, by the acidic character of its hydrated oxide Au,O, and its sulphide Au,S,, as well as by the existence of the compounds Au,X, (X=0, Cl,, &c.) and the acids HAUC, and HAuBr, which find no analogues among the compounds of copper or silver. Although gold possesses the physical characters of a metal in a very marked way, yet chemically considered it rather inclines to the non-metals.

The existence and solubility in water of the oxide Au2O, and the existence of aurous chloride AuCl (or Au,Cl) suggest a slight resemblance between gold and the alkali metals.

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The similarities between silver and the alkali metals, although feeble, are more distinct than those between these metals and either copper or gold. The compositions of the silver compounds and of the compounds of the alkali metals are expressible by the formula M,X, where X=0, C1,, SO, 2NO,, &c. and M = Ag, Li, Na, K, Rb, or Cs. In the general formula for the aluminium alums, M,SO,. Al,3SO,. 24H,O, M may be any alkali metal except lithium, or it may represent Ag. Moist silver oxide Ag2O may often be used, especially in organic chemistry, in place of a solution of caustic potash or soda; it reacts as an alkali. The insolubility in water of the haloid compounds, and the comparative insolubility of many salts, of silver, as well as the physical characters of the silver compounds, mark off these compounds from those of the alkali metals.

The cuprous compounds belong to the form M.X, which includes the compounds of the alkali metals and the silver compounds, but the marked compounds of copper are the cupric compounds CuX; almost the only silver compound of this form is the unstable peroxide AgO (or Ag,O,); the gold compounds belonging to this form are also very few in number (AuO, AuSO,).

When the compounds of gold have been more fully examined it is probable that further resemblances will be found between the chemical characters of gold and those of the other elements of Group I.

HYDROGEN is placed in Group I, series 1. The chemical 444 characters of hydrogen are peculiar. It is at once a typical metal and a typical non-metal. (For an account of some of the properties of hydrogen, s. Chap. VIII.) In the electrical series

hydrogen stands midway between the metallic and the nonmetallic elements. It forms stable compounds with all the distinctly non-metallic elements (? with boron); it also forms compounds with some of the metals and metal-like elements (e.g. Cu,H,, ASH,, SbH). In the acids hydrogen plays the part of a metal, as it can be replaced from acids by metals with formation of compounds analogous in composition to the acids. In the hydrocarbons (CH, CH, CH, &c. &c.) hydrogen acts as a non-metal, as it can be replaced by nonmetallic elements such as chlorine, bromine, &c. The physical properties of hydrogen are very different from those which characterise the metals as a class, yet it seems to form alloys with such metals as palladium and platinum.

To mark the peculiar chemical relationships of hydrogen this element is placed apart from the others in Series 1, of which series it is the only member.

445 In Group I, then, the family-character plainly predominates over the group-character: there is the family of the alkali metals-lithium, sodium, potassium, rubidium, and caesium; silver is more or less allied to this family; copper, silver, and gold form another family the members of which differ very considerably from each other; for some reasons silver and copper are placed in one family, and gold is separated from them; hydrogen must be placed apart from all the other elements.

It must be remembered that Group I. includes three elements which have not yet been isolated.

446 In Chap. XVIII. (par. 392) it was remarked that the properties of an element are determined by considering (1) the properties of the group to which it belongs, (2) the properties of the series in which it finds 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 the properties of the other members of the groups and series to which these elements belong, and (5) the relations between the group and series in which the given element occurs and other groups and series.

Let us now apply these statements to the elements copper, silver, and gold, the positions of which in Group I. seem peculiar.

We have already considered the properties of the group to which these elements belong, and the position occupied by each element.

What are the properties of the series in which each of the three elements finds a place, and what is the position of each element in the series?

Copper is the first member of Series 5; it is succeeded by the metallic elements zinc, gallium, and germanium, which are followed by the metal-like non-metal arsenic, and the nonmetallic elements selenion and bromine. Between series 4 and 5 are placed the distinctly metallic elements iron, nickel, and cobalt, forming a section of Group VIII. (s. Tables I. and II., pars. 390 and 394).

Silver is the first member of Series 7; it is succeeded by the metals cadmium, indium, and tin, which are followed by the metal-like non-metals antimony and tellurium, and the nonmetallic element iodine. Silver comes immediately after the metals rhodium, ruthenium, and palladium which form a section of Group VIII.

Gold is the first member of Series 11; it is succeeded by the metals mercury, thallium, lead, and bismuth; the sixth and seventh members of this series are not yet isolated. Gold, like copper and silver, immediately succeeds a section of Group VIII., the section, namely, comprising the metals iridium, osmium, and platinum.

The positions of copper, silver, and gold are peculiar; no other known elements are similarly situated in the classificatory scheme based on the periodic law. The change from the last member of an even series to the first member of the next odd series seems to be always less sudden and abrupt than the change from the last member of an odd series to the first member of the succeeding even series. Group VIII. differs considerably from the other groups; each section of it seems to impress its character on the elements which come before and on those which succeed it. We cannot expect the relations of copper, silver, and gold, to lithium, potassium, rubidium, and caesium, to resemble the relations of zinc, cadmium, and mercury, to beryllium, calcium, strontium, and barium, except in a broad and general way. (s. also Chap. xxvI.)

If we consider the relations of group to group and series to series we shall find, speaking broadly, that in the lower groups the first member of the odd series is very like the second and succeeding members of the even series, but that this similarity becomes less and less marked as we pass to the higher groups; magnesium, for instance, closely resembles calcium, strontium, and barium, but sulphur differs consider

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ably from chromium, molybdenum, and tungsten. We shall also find that the resemblances between the first and the succeeding even-series members of a group become on the whole less marked as we pass from lower to higher groups; beryllium, calcium, strontium, and barium, for instance, more nearly resemble each other than do nitrogen, vanadium, niobium, and didymium. Finally we shall find that the first odd-series member of a group is more like the succeeding oddseries members of the same group, when the group is one of the higher than when it is one of the lower groups; thus, the resemblances between sulphur, selenion, and tellurium, are more marked than those between magnesium, zinc, and cadmium.

If we apply these general conclusions to Group I. they would lead us to expect to find (1) marked analogies between sodium, potassium, rubidium, and caesium; (2) lithium fairly closely resembling potassium, rubidium, and caesium; (3) considerable differences between sodium on the one hand, and copper, silver, and gold, on the other hand.

The position given to copper, silver, and gold, is thus seen to be less anomalous than at first sight it appeared to be.

CHAPTER XXIII.

THE ELEMENTS OF GROUP VII.

GROUP VII. is unfortunately far from complete; it com- 447 prises the four distinctly negative and non-metallic elements fluorine, chlorine, bromine, and iodine, and the element manganese which is usually classed with the metals. We have already considered the most important properties of chlorine, bromine, and iodine (Chap. XI. pars. 150-159), and also of manganese (Chap. XI. pars. 194-203); it remains therefore to consider fluorine, and to summarise the properties of all the elements of the group.

FLUORINE. This element is not obtained by a process similar 448 to that whereby chlorine, bromine, and iodine are separated from their compounds. When liquid hydrogen fluoride is electrolysed at a low temperature, a colourless gas is evolved at the positive pole; crystallised silicon and boron burn in this gas to SiF, and BF,, respectively; the gas interacts with water to form ozone and a solution of hydrofluoric acid. This gas is very probably fluorine.

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The chief naturally occurring fluorine compound is fluorspar which is more or less pure calcium fluoride, CaF. The compositions of many fluorine compounds are similar to those of the compounds of chlorine, bromine, and iodine; thus HF, BF, SbF, BioF, CrO,F, &c. are analogous to HCl, BBr, SbI, BioCl, Cro,Cl,, &c. In some cases a stable fluoride is known to which there is no corresponding chloride, bromide, or iodide; thus PF, exists as a gas, but the highest gasifiable chloride of phosphorus is PCI. No oxide or oxyacid of fluorine has yet been obtained; but the reactions of the element itself have scarcely been examined as it has only recently been isolated.

Hydrogen fluoride, HF, is prepared by the interaction 449