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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.

4

3'

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.

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 HCI, BBr,, SI, BIOCI, 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

2" 2

3

5

2

. 3

4

of sulphuric acid with calcium fluoride CaF, ; thus CaF, +H SO, Aq = CaSO, + 2HFAq (compare preparation of HCI, HBr, and HI, Chap. xi. par. 153). Hydrogen fluoride is a colourless strongly smelling and irritating gas at temps. above 20°, and a light mobile liquid at temps. under 20°. 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 H F, at temps. not very far above 20°. 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, 2, 3, &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 (H SiF., fuosilicic acid), and BF .HF is a monobasic acid HBF

fluoboric acid). A few similar compounds of hydrogen chloride and bromide are known, e.g. HAuCl, and HAuBrą, both of which react as monobasic acids (s. par. 441.)

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 Auorine, 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, MnO,, and MnO, are basic; MnO,

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is feebly acidic. A series of manganous salts MnX (X = SO, 2NO3, {PO, &c.) exists; a few manganic salts Mn,3X are also known. Permanganic acid, H, Mn,, has been isolated and a number of permanganates have been obtained as definite stable salts, generally isomorphous with perchlorates MCI O (M=K,, Ba, &c.). Many manganates, MMnO,, are

,, M 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).

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.

CHAPTER XXIV. .

453

THE ELEMENTS OF GROUP III.

2
4
6
8

10

12 Even series. B=10-9 Sc=44 Y*= 89-6 La= 138.5 Yb*=173 Group III.

3
5
7
9

11
load series. Al=27-02 Ga=69 In=1134

TI=203-64 Even-series Borox.

SCANDIUM

YTTRICM.

LANTHANUM. YTTERBIUM. elements Atomic weights 10-9

44
89-6
138-5

173 The molecular weights of these elements are unknown. Sp. grs. (approx.) 2.5

6:2 Alom. weights 4.3

22:3 8, eu. gruvs. Sp. heats •5 (?) not determined. not determined.

0449 not determined. Appearance, and Dark greenish- Not isolated. Grey powder. Steel-grey pow

Not isolated. general physical brown powder.

der; or, when properties. Non-conductor

compressed, of electricity.

lustrous, greyHas not been

white, hard,par-
melted.

ticles.
Fairly malle-
able and ductile.
Melts at full

red-heat. Occurrence and Chief com

Small quanti

Occurs as sili- Silicate occurs, Small quanti.. preparation pounds occur- ties of silicate cate in small with silicates of ties of silicate

ring in rocks occur in a rare quantities with Ce, Di, Fe, and occur in a rare
and waters are Swedish mine- silicate of scan- Ca, in a few rare Swedish mine-
boric oxide ral.
dium and ytter- minerals.

ral.
B203 and borax Element has bium.

Prepared by re- Element has not Na,B407; not not been iso- Prepared by ducing LaCig been isolated. widely distri- lated.

electrolysing by potassium. buted, nor in

fused
largequantities.

YС13.x Naci,
Prepared by

or by de-
strongly heating

chlorinating the B203 with so

same salt by sodium.

dium. General chemical Burns in air or

Burns when Oxidises in ordiproperties. oxygen to B20.3.

heated in air, nary air to Decomposes

giving Y,03. La203. steam at red

Decomposes Decomposes heat forming

water, rapidly cold water B.0, and H.

when warmed. slowly, hot waOxidised by

Dissolves in di- ter rapidly, with heating with

lute acids, also evolution of hyHNO3, H2SO4

in hot KOHAq, drogen. molten Kon,

with evolution
or molten

of hydrogen.
KNO,
Combines di-
rectly with C1,
Br, I, S, and
also with N.
Atom of boron
is tri-valent in
gaseous mole-

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

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