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If we now turn to Group VI. we shall find that the 410 first member of the group, oxygen, to some extent summarises in itself, or is typical of, the properties of all the other members; and that the other elements placed in this group fall into two well marked families, one of which contains the even-series members and the other contains the odd-series members of the group.

M. E. C.

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CHAPTER XX.

THE ELEMENTS OF GROUP VI. 411

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12 (Even series. 0=15.96 Cr=52:4 Mo= 95.8

W=1836 U=239-9 Group VI.

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lodd series. S=31.98 Se=78.8 Te=125
Even-series CHROMIUM.

MOLYBDENUM.
TUNGSTEN.

URANIUM. elements (omitting oxygen) Atomic weights

524
95.8
1836

239.9
The molecular weights of these elements are unknown.
Sp. grs. (approx.)

67
8.5
18.5

18.7 Melting points Above 2000°.

Infusible at full Softens and agglom- A full red-heat. white-heat.

erates at white-heat. Sp. heats

•10 (?)
•072 (?)
0334

-028 Atom. weights

7.7
113
9.7

12-9 spec. graus. Occurrence and Chrome-ironstone, Occurs in small Occurs very sparing- Sparingly distripreparation. (FeoCr203) and lead quantities as oxide ly, as tungstate of buted, as oxide in

chromate, &c. occur and sulphide, also as Ca, of Fe and Mn, pitchblende, as uranin a few rocks; not lead or cobalt mo- and of Pb; also as ite of Ca and of Cu, widely distributed. lybdate.

oxide.

as carbonate of I Obtained by deoxi- Obtained by reduc- Obtained by reduc- and Ca, &c. dising Cr202 by C, or ing oxide or chloride ing the oxide or chlo- Obtained by reducby action of K on by H, or the oxide ride in H.

ing the chloride by Crącle, or by elec- by C or KCN.

Na. trolysis of Cr,C14

containing Cr Cle. Colour, appear- Very hard, brittle, Ashen-grey powder; Resembles iron in White, lustrous, ance, fc. powder composed of when compressed, is colour and lustre; metal; softer than

minute brilliant tin- a silver-white, lus- hard and brittle;alsó steel; malleable, but white crystals. trous, hard, brittle, obtained as a brown cannot be beaten Descriptions differ infusible, metal. amorphous powder. into thin plates; also much; probably the

obtained as a greymetal has not yet

black powder. been obtained ap

proximately pure. General chemical Burns in stream of Not oxidised in air Burns in air at red- Slowly tarnishes in properties. 0; heated in air is su- at ordinary tempera- heat; unchanged at air; oxidised rapidly

perficially oxidised; ture but burns at low ordinary tempera- in air at 150°—20°. oxidised by molten red-heat.

tures.

Combines with (1 KNO3 or RCI03. No reaction with Oxidised to W03 by and Br when heated, Easily dissolved by HCIAT, HFAq, or hot HNO3Aq, and very slowly with dilute HCIAq or H2SO4Aq; oxidised HCA or H.S04.

hot I vapour. H.S04Aq, but does by conc. II NO, to Dissolves in hot Dissolves in most not react with hot M003; oxidised by KOHAq to form K aqueous acids with conc. HNO3.

molten KOH, but tungstate and H. evolution of H and Combines readily no reaction with Combines with Cl at formation of salts. with Cl and I when KOHAq.

high temperatures. Forms two series of heated.

Combines with C1 Does not seem to salts, members of Decomposes steam and Br, but not di- form salts by replac- one of which alwars slowly at a red-heat. rectly with I. ing H of acids. contain ( in addition Replaces H of acids Salts formed by re- WO3 is an anhy- to U and the acil forming two series of placing H of acids dride, and it also radicle.

by Mo are scarcely combines with other UO, is an anhydride. Cr03 is an anhy- known.

more negative anhy- Atom of U is tetradride ; Cr203 seems Mo03 is an anhy- drides, e.g. S03. valent in gaseous to form a few salts dride, and also com- Atom of Wis penta- molecules. by heating with bines with many

and hexavalent in
basic oxides.

more negative anhy- gaseous molecules.
Atom of Cr is per- drides, e.g. P205.
haps hexavalent in Atom of Mo is pen-
gaseous molecules.

tavalent in gaseous
molecules.

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General formulae and chemical characters of compounds. 412 The compositions of the more important compounds of these four metals are expressed by the following formulae ; but representatives of each formula are not known for all the elements, thus sesquioxides, M,0g, of tungsten or uranium have not been obtained, and the formula MX, is represented by WCl, only.

Oxides. MO, M,03, MO, MO,: hydrates of some of
these are known.
Sulphides. MS, MS, MS, MS, MS,

,
Haloid compounds; chiefly chlorides. MX, MX,,
MX, MX,, MX.
Åcids. HMO,, H,

MO,, &c., H,MS,
Salts. MSO, Ń2NO, &c.; M,(80),*M,(NO3),, &c. when
M M=Cr. M(SO), M(NỞ), M6,(S0,), &c. when M=U.
Salts of Mo and W are scarcely known.

The oxides MO are scarcely known; hydrates of CrO and 413 Moo are obtained by adding a solution of potash in air-free water to solutions of Cr, Cl, and Mo,cl, respectively. These hydrates are rapidly oxidised in the air; neither yields corresponding salts by its reactions with acids.

The sesquioxides M,0, are stable compounds when M = Cr or Mo (chromic and molybdic oxides); no sesquioxide of W or U is known. Cr,0, is prepared by precipitating a solution

0 of a chromic salt by ammonia, washing, drying, and heating ; in the case of Mo, 0, the hydrated oxide preciptated by potash is heated in hydrogen. Both oxides form dark coloured solids, insoluble or nearly insoluble in acids. Hydrated Cr 0 (Cr 0.3H,O) dissolves readily in acids forming chromic

This oxide also seems to combine with a few basic oxides, e.g. with Cao. It is therefore basic but also slightly acidic.

The dioxides MO, are dark coloured solids, obtained by reducing the oxides MO directly or indirectly. 'Cro, is more easily formed by passing nitric oxide into an aqueous solution of potassium dichromate (K,Cr,0,-); Moo, and Wo, by heating M00, and Wo; in hydrogen to low redness, or by digesting a solution of Moog, or WOg, in hydrochloric acid with copper or zinc and then precipitating by ammonia. UO, may be prepared by digesting UCI, with water.

Cro, is decomposed by heat at 300° with evolution of oxygen and production of CrO;; the other oxides MO, are oxidised to MO, MOO, by heating with nitric acid, wo, by

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heating in air, and UO, by the action of air at ordinary temperatures. These oxides MO are slightly soluble in acids ; UO, gives salts, e.g. U(SO2),, but no definite salts have yet been certainly obtained corresponding to any of the other oxides MO,, although such salts seem to exist.

The triocides Mo, are anhydrides. Cro, is prepared by adding a sufficient quantity of concentrated sulphuric acid to a solution of potassium dichromate, Moo, and wo, are obtained by oxidising the lower oxides, or better from ammonium molybdate and tungstate, respectively, by heating with nitric acid and then washing out the ammonium nitrate formed. UO, is obtained by heating uranyl nitrate

(UO)(NO) Cro, is very soluble in water forming a markedly acid liquid ; under special conditions the hydrate Crog.H, O-i.e. H Cro, chromic acidcan be obtained from this liquid.

Cro, interacts with acids to form oxygen and salts corresponding with the oxide Cr 0,; thus

2Cr0, +3H,80 Aq=Cr,(80.),Aq + 3H,0 + 30. This oxide readily parts with part of its oxygen and therefore acts as an oxidiser, e.g. when it is heated, Cr,O, and oxygen are produced.

M00, is much less soluble in water than Cr0,; W0, is only very slightly soluble in water; and UO, is insoluble

. Hydrates of these oxides exist and exhibit acidic properties (s. Acids, par. 416), but none of them is obtained by the direct addition of water to the oxide. The oxides Moo, and WO; form various complex compounds with several anhydrides such as SO3, P, O., B,O, &c. The oxide UO, interacts with a few acids to form salts (s. Salts, par. 417).

The oxides M,O, and Mo, are on the whole basic; the oxides MO, are acidic, but their acidic character is less marked as the atomic weight of M increases. The change from MO, to MO, is effected the more easily and directly the greater the atomic weight of M. Of the oxides MO, UO, shews the most clearly marked basic character. Of the oxides MO.,, UO, is the most stable towards heat and reducing agents, and

Cro, is the least stable. 414 The most important sulphides are Cr,Sz; MoS,, Mos,

Mos: WS, Ws; and US Chromic sulphide, Crs, is prepared' by passing sulphuretted hydrogen over hot chromic oxide (Cr,0); it is not obtainable by reactions between compounds

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in solution. This sulphide is feebly acidic; it combines with certain more basic sulphides, e.g. with ZnS, CaS, &c.

When excess of sulphuretted hydrogen is passed into the solution of an alkaline molybdate, e.g. K, M00,, and an acid is then added, molybdenum trisulphide Mós,, is precipitated. This sulphide is distinctly acidic; it interacts with K,S, &c. to form thio- (or sulpho-) salts, e.g. K,MOS, When potassium thiomolybdate, K, MOS, is heated with

, MoS, a salt having the composition K MOS, is formed; this salt interacts with acids to produce molybdenum tetrasulphide Mos, Molybdenum disulphide, MoS,, is obtained by heating together Mo0, and sulphur.

Tungsten trisulphide WS, is obtained similarly to Mosz. The disulphide WS, is formed by heating together tungsten and sulphur. WS, is distinctly acidic, forming thiotungstates, e.g. K, WS, BaWS, &c.

Uranium disulphide, US,, is formed similarly to WS,; it exhibits no acidic functions.

As CrS, has not not yet been prepared it is difficult to compare corresponding sulphides of the four elements; but on the whole it appears that the sulphides become more acidic as the atomic weight of the metals increases.

The haloid compounds of the four elements we are 415 considering are important. Their compositions are shewn by the following formulae; CrCl, CrCl, CrBr,, CrBry, Cri,, Crly, CrF,; Moci,, MoCl,, Moci, MoCl., MoBr,, MoBrz, MoBr.

re; wci,, wci," WCI,, wci, WBr,, WBr, WI,; Uči,, UCI, UCI, U Br, UF2. The following have been gasified; MoCl, wci, wởi, uci, UBr,; these formulae are therefore molecular. The formulae of the other compounds are the simplest that can be given, but they are not necessarily molecular.

Chromic chloride, CrCl,, is obtained by heating an intimate mixture of chromic oxide and carbon in chlorine; chromous chloride, CrCl,, is formed by heating CrCl, in hydrogen. The higher chloride is stable in the air, but when strongly heated it gives Cr,g; solutions of this chloride when heated give

Og precipitates of various oxychlorides Cr,O,C1,; the most important oxychloride of chromium is Cro.cl Chromic chloride forms either violet crystals by subliming in chlorine or hydrochloric acid gas, or a greenish solid by dissolving chromic hydrate in hydrochloric acid, evaporating nearly to dryness, and heating in chlorine. The violet form is almost insoluble

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