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oxides and hydroxides, in the formation of hydroxides by the direct union of water with the oxides, in the great stability of their oxides towards reducing agents, in the stability of their salts and the existence of but few basic salts, in the preparation and properties of their sulphides and hydrosulphides, &c. In its physical character however Mg differs from Ca, Sr, and Ba, and resembles Be, and to some extent Zn and Cd. Beryllium, although occurring in Series 2, is decidedly more like the odd-series, than the evenseries, members of the group; the properties of the oxide and hydroxide of Be are very similar to those of the corresponding compounds of Zn and Cd; in the comparatively large number of basic salts which it forms, and in the readiness with which oxyhaloid compounds of Be are produced, the metal also resembles the odd-series members, especially Hg. Some of the haloid compounds of Be, Zn, Cd, and Hg, are gasifiable without decomposition at workable temperatures; no compounds of the other elements of the group have been gasified.

Mercury is distinguished from the other members of the group by the fact that it forms two series of compounds, mercurous HgX, and mercuric HgX,,

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also by its physical properties, &c.

Looked at broadly, Ca, Sr, and Ba, are the most positive members of the group, next to them comes Mg, and Hg is the least positive. It is to be noted that the arrangement of Group II. in accordance with the periodic law shews a gap in Series 9; if an element is discovered to fill this gap it will probably resemble mercury on the one hand and zinc and cadmium on the other.

Beryllium to some extent summarises in itself the properties of the other members of the group.

It cannot be said that the even-series members of Group II. form a sub-group or family distinctly marked off from the odd-series members of the group; nor can it be asserted that there is a gradual change of properties from the first to the last member of the group. All the members shew distinct relations to each other; neither the family-character nor the group-character preponderates.

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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Atom. weights spec. gravs. Occurrence and preparation.

Colour, appearance, &c.

General chemical properties.

Chrome-ironstone, (FeO Cr2O3) and lead chromate, &c. occur in a few rocks; not widely distributed. Obtained by deoxidising Cr2O3 by C, or by action of K on Cr2Cle, or by electrolysis of Cr2Cl4 containing Cr2Cl6. Very hard, brittle, powder composed of minute brilliant tinwhite crystals. Descriptions differ much; probably the metal has not yet been obtained approximately pure. Burns in stream of O; heated in air is superficially oxidised; oxidised by molten KNO3 or KClO3. Easily dissolved by dilute HCIAq or H2SO4Aq, but does not react with hot conc. HNO3. Combines readily with Cl and I when heated. Decomposes steam slowly at a red-heat. Replaces H of acids forming two series of salts.

Crog is an anhydride; Cr2O3 seems to form a few salts by heating with

basic oxides.
Atom of Cr is per-
haps hexavalent in
gaseous molecules.

Infusible at full white-heat.

*072 (?)

11.3

Occurs in small
quantities as oxide
and sulphide, also as
lead or cobalt mo-
lybdate.

Obtained by reduc-
ing oxide or chloride
by H, or the oxide
by C or KCN.

Ashen-grey powder; when compressed, is a silver-white, lustrous, hard, brittle, infusible, metal.

Not oxidised in air at ordinary temperature but burns at low red-heat. No reaction with HCIAq, HFAq, or H2SO4Aq; oxidised by conc. HNO3 to MoOg; oxidised by molten KOH, but no reaction with KOHAq. Combines with Cl and Br, but not directly with I. Salts formed by replacing H of acids by Mo are scarcely known.

MoO, is an anhydride, and also combines with many. more negative anhydrides, e.g. P205. Atom of Mo is pentavalent in gaseous molecules.

18.5 Softens and agglomerates at white-heat.

⚫0334

9.7

Occurs very sparingly, as tungstate of Ca, of Fe and Mn, and of Pb; also as oxide. Obtained by reducing the oxide or chloride in H.

Resembles iron in colour and lustre; hard and brittle; also obtained as a brown amorphous powder.

Burns in air at redheat; unchanged at ordinary temperatures.

Oxidised to WOg by hot HNO3Aq, HCIAq or H2SO4. Dissolves in hot KOHAq to form K tungstate and H. Combines with Cl at high temperatures. Does not seem to form salts by replacing H of acids. WO3 is an anhydride, and it also combines with other more negative anhydrides, e.g. SO3. Atom of W is pentaand hexavalent in gaseous molecules.

12.9

Sparingly distributed, as oxide in pitchblende, as uranite of Ca and of Cu, as carbonate of U and Ca, &c. Obtained by reducing the chloride by Na.

White, lustrous, metal; softer than steel; malleable, but cannot be beaten into thin plates; also obtained as a greyblack powder.

Slowly tarnishes in air; oxidised rapidly in air at 150°-200°. Combines with Cl and Br when heated, and very slowly with hot I vapour. Dissolves in most aqueous acids with evolution of H and formation of salts. Forms two series of salts, members of one of which always contain O in addition to U and the acid radicle. UO, is an anhydride. Atom of U is tetravalent in gaseous molecules.

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,O,, of tungsten or uranium have not been obtained, and the formula MX, is represented by WCl only.

Oxides. MO, M,O,, MO,, MO,: hydrates of some of these are known.

Sulphides. MS, M,S,, MS,, MS, MS.

Haloid compounds; chiefly chlorides. MX,, MX,, MX,, MX,, MX,

Ácids. HMO,, H,M,O,, &c., H2MS.

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Salts. MSO, M2NO, &c.; M(SO), M, (NO), &c. when M=Cr. M(SO), M(NO), MO (SO), &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 CrCl and Mo,Cl, respectively. These hydrates are rapidly oxidised in the air; neither yields corresponding salts by its reactions with acids.

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2 3

The sesquioxides MO, are stable compounds when M = Cr or Mo (chromic and molybdic oxides); no sesquioxide of W or U is known. Cr,O, is prepared by precipitating a solution of a chromic salt by ammonia, washing, drying, and heating; in the case of Mo,O,, the hydrated oxide preciptated by potash is heated in hydrogen. Both oxides form dark coloured solids, insoluble or nearly insoluble in acids. Hydrated Cr2O (Cr 0.3H,O) dissolves readily in acids forming chromic salts, e.g. Cr,3SO. This oxide also seems to combine with a few basic oxides, e.g. with CaO. It is therefore basic but also slightly acidic.

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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,O,); MoO, and WO, by heating MOO, and WO, in hydrogen to low redness, or by digesting a solution of MoO,, or WO,, in hydrochloric acid with copper or zinc and then precipitating by ammonia. UO, may be prepared by digesting UCl, with water.

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2

CrO, is decomposed by heat at 300° with evolution of oxygen and production of Cr,O,; the other oxides MO, are oxidised to MO,, MoO, by heating with nitric acid, WỖ, by

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2

414

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(SO), but no definite salts have yet been certainly obtained corresponding to any of the other oxides MO, although such salts seem to exist.

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3

The trioxides 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,)(NO3)2

CrO, is very soluble in water forming a markedly acid liquid; under special conditions the hydrate CrO,.HO-i.e. HCrO, chromic acid-can be obtained from this liquid.

3

CrO, interacts with acids to form oxygen and salts corresponding with the oxide Cr2O,; thus

2

2CrO ̧ + 3H2SOAq = Cr ̧(SO)„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, Cr2O2 and oxygen are produced.

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MOO, is much less soluble in water than CrO,; WO̟, 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 SO, PO, BO, &c. The oxide UO, interacts with a few acids to form salts (s. Salts, par. 417).

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3

3

3

The oxides M2O, 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. 39 UO, is the most stable towards heat and reducing agents, and Cro, is the least stable.

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2

The most important sulphides are Cr,S,; MOS,, MOS, MOS,; WS,, WS,; and US, Chromic sulphide, Cr,S,, is prepared by passing sulphuretted hydrogen over hot chromic oxide (Cr2O); it is not obtainable by reactions between compounds

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