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highest sulphide of chromium known with certainty, but the important sulphides of the other elements are MS, and MS, Again the chlorides of chromium are CrCl, and Crci,, but the chlorides MCI, MClz, and even MCI,, are characteristic of the other elements of the series.

The oxides MO, are all acidic; but the acidic character becomes less marked as the atomic weight of M increases. This decrease in acidic character is shewn by the production of such a salt as UO,(SO) from the oxide UO, and by the fact that the most definite and stable molybdates, tungstates, and (probably) uranates, belong to the form XMO,..MOG(X = K,, Ba, &c.; M = Mo, W, U), whereas the most marked chromates are the normal salts X CroIn other words, the combination of a relatively large quantity of the acidic oxides MO, with basic oxides seems to be necessary for the production of stable salts when MO, is MoO, WO,, or UO.

Thio-salts, usually of the composition MXS (M=K,, Ba, &c. X = Cr, Mo, W), of all the elements except uranium are known.

The salts of molybdenum and tungsten have been so little examined that no generalisations regarding them

can be made ; salts of chromium and uranium are numerous, many of them are basic, and several form double salts chiefly by combining with salts of the alkali and alkaline earth metals.

The odd-series members of Group VI.are SULPHUR, SELENION, and TELLURIUM. The properties of these elements have been already considered (Chap. XI. pars. 170 to 179); it will suffice to summarise these properties here.

Sulphur and selenion are distinctly non-metallic in their . chemical properties; tellurium inclines towards the metals but it is decidedly less metallic than chromium, molybdenum, tungsten, or uranium.

The existence of stable gaseous hydrides MH,; the distinctly acidic functions of the oxides MO, and MO,; the non-existence of salts produced by replacing the hydrogen of acids by M; the existence of strong acids H MO, and H MOL, each giving a series of definite salts; these among other properties, mark the non-metallic character of the elements sulphur, selenion, and tellurium.

The negative character of sulphur and selenion is further marked by the fact that these elements exhibit allotropy; by their physical properties; by the possibility of forming oxy

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chlorides directly from the acids H MO, and H,MO,; by the slightly acidic functions of the hydrides MH,

Tellurium does not exist in more than one form; the acids H Teo, and H.Teo, are not produced by the direct interactions of the oxides Teo, and Teo, with water; the hydride TeH, shews no acidic properties; the anhydride Teo, combines with some acids (e.g. Teo,. 2HCl is known); some of the physical properties of tellurium approximate to those of the metals.

The existence of the stable gasifiable tetrachloride TeCl, ; the formation not only of ditellurates MTe, but also of tetratellurates MT 0, and of salts of the form MTO ; (M = K,, Ba, &c.); "the unreadiness to enter into chemica reaction with alkalis or alkaline carbonates of the oxide Teog; these are some of the properties in which tellurium approaches the higher members (W and U) of the even series of Group VI. OXYGEN is the first member of the even-series of the

group now under consideration. Oxygen is a typical non-metallic or negative element both in its chemical and physical properties (s. Chap. VIII.). Nevertheless the properties of some of the compounds of this element suggest the properties of the other elements of the group in which oxygen occurs. forms two compounds with hydrogen, water H O and hydrogen peroxide H,0, but neither is acidic; oxygen combines with the positive elements to form oxides the composition of which is frequently similar to that of the sulphides and selenides of the same elements, compare for instance the formulae MO and MS where M = Cu, Fe, Mg, Ni, Co, Mn, Ca, Ba, Sr, K, Na, &c.; most of the oxides of positive elements are basic, some however as we have seen are acidic; most of the sulphides of positive elements interact with acids to form salts and hydrogen sulphide, some however interact with alkaline sulphides to form thio-salts.

The compounds of oxygen with chlorine CIO, and (1) CIO, do not resemble the chlorides of the other members of Group VI. in composition, but the oxide OCI, is analogous in composition to McI, when M = S, Se, Te, Cr, Mo, or W. The compounds of oxygen with chlorine are very easily decomposed by heat and reagents generally ; in this they resemble the compounds of sulphur and selenion with chlorine. The existence of the gaseous molecules 0, and 0g, Se, and Sez, S, and Se, emphasises the resemblance between oxygen, sulphur,

Thus oxygen

and selenion. Oxygen forms a solid, stable, compound with iodine, 0,1,; although the composition of this compound is not similar to that of the iodides of the other elements of Group VI., the fact of the existence of this stable compound suggests the existence of the stable iodides of tellurium, chromium, and tungsten. The existence of many definite and stable compounds of oxygen with non-metallic elements (oxides of As, B, C, I, N, P, Si) shews that oxygen is to some extent positive in its chemical properties : the compositions of these oxides are very frequently similar to those of the sulphides of the same elements when sulphides of these elements exist.

Group VI., then, is evidently divided into two well marked sub-groups or families; one of these families consists of the elements chromium, molybdenum, tungsten, and uranium; the other is formed of sulphur, selenion, and tellurium; oxygen, which is the first member of the group, to some extent summarises the properties of both families, but at the same time it differs from all the other members of the group. At the same time the elements of Group VI. taken as a whole more closely resemble one another than they resemble any other elements.

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If we now turn to Group V. we shall find a group of ten elements shewing a gradation of properties from the first to the last member; we shall find that the group-character preponderates over the family-character, so that although the evenseries members are on the whole more like each other than they are like the odd-series members, yet it is not possible to divide Group V., as we have divided Group VI., into two distinct sub-groups or families.

51-2

6.5 (?)

424

2
4
6
8
10

12 Even series. N=14.01 V=512 Nb= 94 Di=144 Ta=182 Group v.

3
5
7
9

11 Odd series. P=30.96 As=74.9 Sb=120 Er=166 Bi=208 Even-series NITROGEN. VANADIUM. NIOBIUM.

DIDYMIUM*. TANTALUM. elements Atomic weights 14.01

94
144

182 The molecular weight of nitrogen is 28.02; the molecular weights of the other elements are unknown. Sp. grs. (approx.) .97 if air=1;

5.5
7 (?)

11 (?) liquid ; s.G.=.885 Sp. heats not determined. not determined.

·0456

not determined. Atom, weights

903
13:4 (?)
22:3 (?)

16:5(?) spec. gravs.

The melting points of most of these elements have not been determined. Occurrence and In large quanti- In a few minerals, not widely distributed, chiefly as oxides and sulphides. preparation. ties in air.

Prepared by Prepared by Prepared by
Many com-

Prepared by

heating
pounds, especi- heating VCl, in repeatedly in
long, continued heating NbC15 heating Diči

with potassium; K,TaF, with ally ammonia

hydrogen. hydrogen. and nitrates,

or by electrolys- potassium.

ing molten
also occur in

DiC1z.
large quantities
and widely dis-
tributed.
Prepared by
removing 0

froin air. Appearance, and Colourless, Grey, crystal- Steel-grey, lus- White solid Grey, lustrous, general physical tasteless, odour- line, powder. trous, solid.

with slightly solid; not yet properties. less, gas.

yellow tinge. obtained apLiquefied at

Ductile, hard. proximately very low tem

pure.
perature and
under great
pressure ; liquid
boils at abt.

- 1950 General chemical Combines di- Burns in air to Burns in air to Burns in air to Burns in air to properties. rectly with few V,0, then V203, N5205.

Di203.

Ta205 if any elements and finally Combines di- Oxides seem to Combines diat ordinary

V.205. Burns rectly with Cl to act only as rectly with CI temperatures; in či to VC14. form NbC15. bases; Di205 to form TaC15. but at very

Combines di- Dissolved by acts as a per- Dissolved only high temperarectly with N conc. H2SO4 not oxide.

by conc. HFAQ tures combines forming VN. by HCIAQ or No compounds or by mixture directly with B, Dissolved by HNO3Aq. have been of H2SO4A9 Si, Cr, Mg, V, conc. H,804 Nb20; is an an- gasified. and HFĂg. and a few other and HNOzXq. hydride. No hydride Ta205 is an anelements. Molten alkalis Salts of Nb are known.

hydride, and has If electric dis- forni vanadates. not known but

apparently no charge is passed Some oxides are the subject has

basic properties. through mix- basic, others are not been

No hydride ture of N with basic and also thoroughly in

known.
0, or H, a very acidic.
vestigated.

Atom pentavalittle NO2, or V acts both as a No hydride

lent in gaseous NH3, is formed. metallic and a known with cer

molecules.
Strongly nega- non-metallic tainty.
tive.
element.

Atom pentava-
Oxides are gene- No hydride lent in gaseous
rally anhy-
known.

molecules.
drides.

Atom is tetra-
Hydride, NH3, (and perhaps
is strongly alka- also tri-) valent
line.

in gaseous
Atom trivalent molecules.
in gaseous

molecules. * There is sone doubt whether the body known as didymium is or is not a mixture of two or more elements; many of the properties of compounds of didymium are probably the properties of mixtures.

5.7

9.9

6.7 430°

270°

Odd-series PHOSPHORUS. ARSENIC.

ANTIMONY. ERBIUM.

BISMUTH. elements Atomic weights 30-96

74.9

120
166

208 Molecular weights 123.84 and 61.92 299•6 and 149.8 ?360 or 240

unknown Sp. grs. (approx.) 1.9 Melting points 45°

500° (under (approx.)

pressure) Atomic weights

16:3
13.2
18

20:5 spec. graus. Sp. heats .202

083
053 not determined.

*0308 Occurrence and Many phos

As203 and

Chief naturally Er,03 occurs in Bismuth is preparation. phates occur As2S3 occur; occurring com- small quantities found native;

very widely dis- also compounds pound is Sb2S3; in ytterbite, a also Bi203 and tributed in of As with Te found in com- Swedish min

Bi,S3, &c., but rocks and and S, with Ni, paratively small eral.

not in large waters; also in Co, &c. occur in quantities in

quantities. bones and in small quantities very various Metal not yet Prepared by parts of plants. widely distri- parts.

obtained. heating Biyog Compounds of buted. Prepared by

with charcoal.
P with C, N, and Prepared by heating Sb203
O occur in nerve heating A8,03 with charcoal.
and brain

with charcoal.
matter.
Prepared by
heating
Ca(PO3)2 with

charcoal.
Appearance, and Soft wax-like Grey, hard, Grey, lustrous,

Grey, with general physical solid.

britile, solid.
brittle, very

faintly reddish properties. Crystalline. Crystalline. crystalline,

tinge; crystalNon-conductor Fair conductor solid.

lises easily; of electricity. of electricity. Fair conductor

brittle. of electricity.

Bad conductor

of electricity. General chemical Burns in air to Burns in air to Burns in air to Oxide, Er203, is Burns in air to properties. P203 and P205. As2O3.

Sb203

basic, forming Bi203. Combines di- Combines di- Combines di- salts, e.g.

Combines directly with ci, rectly with Ci, rectly with Ci, Er23S04. rectly with CI, Br, and I.

Br, and I. Br, and I. No hydride Br, and I.
Oxidised by Oxidised by Oxidised by known.

Oxidised by
HNO3.
HNO3.
HNO3.

No oxide known HNO3, and at Does not form No salts known A few salts de- to act as an an- same time salts by inter- derived from rived from acids hydride. Bi.3NO3 is acting with acids.

seem to exist. No compound formed. acids.

Hydride, AsH3, Hydride, SbH3, yet gasified. Many salts Hydride, PH3, is scarcely if at not alkaline.

known derived resembles NH3 all alkaline. Oxides are

from acids. but is less alka- Oxides are anhydrides, but

No hydride line. anhydrides. also slightly

known. Oxides are Atom trivalent basic.

Oxides are anhydrides. in gaseous Atom trivalent

basic; Bi205 Atom trivalent molecules. in gaseous

shews very and pentavalent Arsenic exists niolecules.

slight acidic in gaseous in allotropic

functions. molecules. forms (s. Chap.

Atom trivalent Phosphorus XI., par. 222).

in gaseous shews allotropy

molecules. (s. Chap. XI,

par. 220). 425 General formulae and chemical characters of compounds of

elements of Group V.

Hydrides. MHz; M=N, P, As, Sb.

Oxides. MO; M =N, V. M,0,; M = N, V, Nb, Bi.
M,Oz; M = any metal of the group except Nb and Ta. M,O,;
M = N, P, V, Nb, Sb, Ta, Bi. MO.; M = any element of the

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