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CHAPTER XXIII.

THE ELEMENTS OF GROUP VII.

GROUP VII. is unfortunately far from complete; it comprises 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.

447

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 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, CrOF, &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.

2

Hydrogen fluoride, HF, is prepared by the interaction 449

450

451

2

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.
2' 3'
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, Auosilicic acid), and BF.HF is a monobasic acid
(HBF, Auoboric 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.)

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,

3

2 3

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 MCI,O, (M = K,, 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).

8

4

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.

453

Group III.

Even-series elements

Atomic weights

4

load

Even series. B=109 Sc=44
3
5
Odd series. Al=27-02 Ga=69
SCANDIUM.

BORON.

Sp. grs. (approx.)

Atom. weights

ses. gravs. Sp. heats Appearance, and general physical properties.

Occurrence and preparation

General chemical properties.

10.9

THE ELEMENTS OF GROUP III.

6
8
Y*= 89-6 La=138.5
7
9
In=1134
YTTRIUM.

2.5

4.3

*5 (?) Dark greenishbrown powder. Non-conductor of electricity. Has not been melted.

Chief com-
pounds occur-
ring in rocks
and waters are
boric oxide
B2O3 and borax
Na2B4O7; not
widely distri-
buted, nor in
largequantities.
Prepared by
strongly heating
B2O3 with so-
dium.

CHAPTER XXIV.

Burns in air or
Oxygen to B20g.
Decomposes
steam at red-
heat forming
B2O3 and H.
Oxidised by
heating with

HNO3, H2SO4
molten

KOH,

or molten

KNO3
Combines di-
rectly with Cl,
Br, I, S, and
also with N.

Atom of boron
is tri-valent in
gaseous mole-

cules.

not determined. not determined.
Not isolated. Grey powder.

Small quanti-
ties of silicate
occur in a rare
Swedish mine-
ral.
Element has
not been iso-
lated.

Occurs as sili-
cate in small
quantities with
silicate of scan-
dium and ytter-
bium.

Prepared by
electrolysing
fused

YC13. NaCl,
or by de-
chlorinating the
same salt by so-
dium.

10 Yb*=173 11

Burns when

heated in air, giving Y203. Decomposes water, rapidly when warmed. Dissolves in dilute acids, also in hot KOHAq, with evolution of hydrogen.

TI=203-64

44

89.6

138-5

The molecular weights of these elements are unknown.

6.2

22.3

*0449 not determined.
Steel-grey pow- Not isolated.
der; or, when
compressed,
lustrous, grey-
white, hard,par-
ticles.

Fairly malle-
able and ductile.
Melts at full
red-heat.
Silicate occurs,
with silicates of
Ce, Di, Fe, and
Ca, in a few rare
minerals.
Prepared by re-
ducing LaCls
by potassium.

LANTHANUM.

12

Oxidises in ordi-
nary air to
La203.
Decomposes
cold water
slowly, hot wa-
ter rapidly, with
evolution of hy-
drogen.

[blocks in formation]

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

Boron is said to exist both as an amorphous powder and as metal-like, hard, lustrous, crystals; but recent investigation shews that the so-called crystalline boron is a compound of boron and aluminium.

General formulae and chemical characters of compounds. 454 The compounds BC, BBr, and BF, have been gasified; these formulae are molecular. No compounds of the other even-series elements have yet been gasified; the formulae given are the simplest that express the compositions of these compounds. Gaseous aluminium and indium chlorides have the molecular composition MCl,, and gallium chloride appears to exist in the gaseous state both as GaCl, and GaCl. There are indications of the existence of a hydride of boron, probably BH; but the compound has not been isolated. The compounds of the elements of this family, with the exception of those of boron, have not been very fully investigated: there are many points in the chemical history of boron which require elucidation.

Oxides: MO.

Haloid compounds: MX,.
Sulphides: M,S,.

2

Acids: HBO, H ̧В ̧O̟, H‚Â ̧O̟,; no acids of other elements are known.

2 2 49

Salts: M3X; X = SO1, 2NO,, PO,, &c.; MSc, Y, La, Yb, not B.

The oxides M.O, with the exception of BO, are obtained 455 by adding an alkali to the solution of a salt, and heating the hydrated oxide which is pptd. BO, is found in waters in various volcanic districts; it is formed by heating boron in oxygen.

2 49

Boric oxide, BO, is acidic; it dissolves in water, and from this solution crystals of ortho-boric acid, H,BO,, separate on evaporation. By heating ortho-boric acid to 100° metaboric acid, H,B,O,, is obtained; and at 160° tetra- or pyro-boric acid, HBO,, is formed. Salts are known derived from each of these acids; borax, one of the most important salts, is sodium tetra-borate Na,BO,; the ortho-borates are very unstable salts, it is doubtful whether they exist in solution in water. A solution of boric oxide in water reacts as if it contained dibasic metaboric acid, HBO. Boric acid is an extremely weak acid. Dilute aqueous solutions of alkali borates precipitate metallic oxides, not borates, from solutions of many metallic salts; that is to say, these dilute solutions behave as if they contained boric acid and free alkali, pro

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