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, BBг, 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

450

451

of sulphuric acid with calcium fluoride CaF,; thus
CaF2+ H2SO Aq = CaSO, + 2HFAq (compare preparation of
HCl, HBr, and HI, Chap. XI. par. 153). Hydrogen fluoride
is a colourless strongly smelling and irritating gas at temps.
above 20o, and a light mobile liquid at temps. under 20o.
The vapour-density points to the existence of the gaseous
molecule HF only at fairly high temperatures, and to the
existence of the gaseous molecule HF, at temps. not very
far above 20o. Liquid hydrogen fluoride chars organic matter
rapidly, and dissolves many bodies which are insoluble in all
other acids, e.g. strongly heated silica, titanium oxide, boron,
silicon, &c. An aqueous solution of this compound interacts
with metals and basic oxides similarly to aqueous solutions of
hydrochloric, hydrobromic, and hydriodic, acids; fluorides,
Ba Bi
salts of the form MF (M = K, Na,,, &c.), are produced.
The metallic fluorides shew great readiness to combine with
hydrogen fluoride and produce double compounds; e.g. KF. HF,
BiF. 3HF, &c. Some of the fluorides of non-metallic elements
also combine with hydrogen fluoride; the products in some
cases react as acids; thus SiF.2HF is a dibasic acid
(HSiF, fluosilicic acid), and BF. HF is a monobasic acid
(HBF, fuoboric acid). A few similar compounds of hydrogen
chloride and bromide are known, e.g. HAuCl, and HAuBr,,
both of which react as monobasic acids (s. par. 441.)

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3'

Hydrofluoric acid, HFAq, is an extremely weak acid; its affinity for bases is less than 5 when that of hydrochloric acid is taken as 100 (s. Chap. XIII. pars. 251, 255).

Whether fluorine does or does not interact with water and solutions of alkalis similarly to chlorine, bromine, and iodine, cannot be determined until the properties of fluorine have been more fully investigated.

The chemical properties of fluorine, so far as they have been investigated, shew that this element is very similar to the elements chlorine, bromine, and iodine; but, at the same time, there are fairly marked differences between fluorine and these three elements. No one of the four elements shews any tendencies to react as a metal.

MANGANESE is the second member of the even-series of Group VII. The sketch of the chemical properties of manganese given in pars. 195-199 of Chap. XI. shews that manganese is at once metallic and non-metallic in its chemical functions. The oxides MnO, Mn,O,, and Mn,O, are basic; MnO,

is feebly acidic. A series of manganous salts MnX (X = SO1, 2NO, PO, &c.) exists; a few manganic salts Mn,3X are also known. Permanganic acid, H.Mn,O,, has been isolated and a number of permanganates have been obtained as definite stable salts, generally isomorphous with perchlorates MCIO (M = K2, Ba, &c.). Many manganates, MMnO1, are also known; these salts do not correspond in composition with any salts derived from acids of chlorine, bromine, iodine, or fluorine; they are similar to the sulphates, selenates, and tellurates MXO (M='K„, Ba, &c.; X=S, Se, Te).

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Manganese, then, shews very feeble analogies with the other elements which are placed in the same group with it.

If the three generalisations stated in par. 446 are applied 452 to Group VII., they would lead us to expect that the unknown members of the even series of this group should resemble manganese, but should on the whole be more distinctly metallic than this element; and that the unknown members of the odd series of the group should resemble the halogen elements, but should be less decisively non-metallic than these elements; the unknown members of series 9 and 11, Group VII., might fairly be expected to form a few salts by the interactions of their oxides with acids.

*There are still some doubts whether these elements are or are not mixtures of two or more distinct kinds of matter.