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group except Er. Hydrates of many of these oxides are known.

Sulphides. No sulphides of niobium or erbium have yet been prepared. MS, ; M = any element of the group except N, Ta, (Nb or Er). M.S,; M =P, V, As, Sb. A few other sulphides are known, e.g. N S, Ta,S.

Haloid compounds. "No haloid compounds of erbium have yet been prepared; the haloid compounds of nitrogen are very unstable and explode with great violence, their composition is still doubtful.

MX,; M = any element of the group except Ta (? N) or Er.
MX; ; M = P, Nb, Sb, Ta; X = Cl and in some cases also
Br or I. A few other haloid compounds exist, e.g. P. I,
VCI,
Åcids.

The following are the chief compounds of hydrogen with oxygen and an element of Group V. which are acids (when Aq is added to a formula it indicates that the acid is known only in aqueous solution).

HNOAq, HNO,Aq, HNO3. H PO,Aq, H PO,Aq, H PO, HPO, H,PO, ÉVO, Hvo. H AsO, Aq, HÅSOM HASO, HASO, H SbO HSbO3, H.Sbo," H,Sb,0% HTa,0.

Sálts. The only elements of the group which are known to form series of definite salts by replacing the hydrogen of acids are vanadium, didymium, erbium, and bismuth. The salts of vanadium are all basic salts, they belong for the most part to the series VOX and (VO),X,; the salts of didymium and erbium belong to the series M 3X; the normal salts of bismuth belong to the series Bi 3X, but besides these a great many basic salts exist. (X=SO, 2NO,, &c.). Antimony forms a few compounds which may be regarded as basic salts, especially «Sb, Oz.YSO; and Sb,Oz:YN,0; ; tartar-emetic, KSC,H,O,, is probably to be classed as a double antimonypotassium salt.

The following are the principal compounds of elements of 426 Group V. which have been gasified, and the relative densities of the vapours of which have been determined ; NH, PH, AsHz, PC1,, PI,, PF,, VCI, AsCl, AsIy, NbCl,, SbCl,, SbI TaCl, Bici. The statements made in the table in par. 424 regarding the valencies of the atoms of the elements of Group V. are based on the existence and compositions of these gaseous molecules.

Arsenious and antimonious oxides have been gasified; the

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compositions of the gaseous molecules of these compounds are expressed by the formulae As.. and SbO The formulae of the following oxides of nitrogen are molecular NO, NO, NO, NO, A few oxychlorides, e.g. NbOCI,, have also been gasified.

The formulae of the other compounds are not necessarily molecular ; they are the simplest formulae that can be given, consistently with the determined values of the atomic weights of the elements, and with the reactions of the compounds.

The hydrides MH, are gases under ordinary conditions.

Ammonia, NH, is obtained in small quantities by the direct union of nitrogen and hydrogen under the influence of the induced electric discharge; also in many reactions in which hydrogen is produced in contact with nitrogen, e.g. when steam and nitrogen are passed over hot iron, or when nitrogen is produced in contact with hydrogen, e.g. when hydrogen and nitric oxide are passed over hot finely divided platinum. Ammonia is usually prepared by heating a mixture of ammonium chloride and lime;

2NH4Cl + CaO = CaCl, + H,O+ 2NH,. Phosphoretted hydrogen or phosphine, PH,, may be obtained by heating a mixture of phosphonium iodide and an alkali; PHI + KOHAq = KIAq+H,0+PH,; it is however more usually prepared by heating a solution of an alkali with phosphorus; 3KOHAq + 3H,0 + 4P = 3KH PO, Aq + PH, (potassium hypophosphite remains in solution, phosphine is evolved as a gas).

Arsenuretted hydrogen or arsine, Ash, is obtained by producing hydrogen in contact with a solution of an arsenic compound. 'Antimonuretted hydrogen, or stibine, SbH, is obtained by a similar method. Thus, M.0,Aq+aM,O, Aq+12H+H=2MH,+3H,O+H+M O Aq (the hydrogen must be produced in contact with the M,03Aq; M=As or Sb).

Ammonia, NH,, combines with most acids to form ammonium salts; e.g. NH, +HCI = NH,CI; 2NH,+H SO, = (NH),SO, Phosphine, PH,, combines with hydřiodic acid, and with hydrochloric acid under increased pressure, phosphonium salts; e.g. PH, + HI=PH.I. (Regarding ammonium salts s. Chap. XI. pars. 210, 211, and Chap. XVII. par. 371.) Arsine and stibine do not combine with acids.

The hydrides MH, can be oxidised to oxides and water; ammonia is oxidised by mixing it with much oxygen and bringing a flame to the mixture; water, nitrogen, and a little ammonium nitrate (NH, NO,) are formed; phosphine is oxidised to phosphorus pentoxide (P,0) and water by mixing it with air or oxygen and raising the temperature; arsine and stibine are oxidised by mixing with air or oxygen and bringing a flame to the mixture; if much oxygen is present the products are water and arsenious or antimonious oxide; if little oxygen is present, water, arsenic or antimony, and a little arsenious or antimonious oxide, are formed.

Arsine and stibine are decomposed by heat; ammonia and phosphine are much more stable towards heat.

The most important and best studied oxides of the 428 elements of Group V. are those whose composition is expressed by the formulae M,O, and M,05, respectively.

The trioxides M20, may be formed by the direct union of their elements; N5,0, and Ta,0, have not yet been prepared, and as erbium has not been isolated it is uncertain whether the oxide Er,0, would or would not be formed by heating erbium in oxygen. Nitrogen and oxygen combine only when a mixture of the gases is submitted to the continued action of electric sparks, and then only a small quantity of , O, is formed; the other elements of the group are readily oxidised to M,O, by heating in air or oxygen.

The oxides M,0, may be divided into three classes: (1) acidic oxides, N, O, P Оg; (2) basic oxides, Di, 0Er, og

Bi,03; (3) oxides which are both acidic and basic, As,O3, Sb, (? V,03). The oxides N, O, and P, O, dissolve in water to form solutions of nitrous acid ẢNO, and phosphorous acid H PO, respectively; neither oxide shews the smallest tendency to interact with acids and form salts. The oxides Di, 0, Er, and Bio, interact with acids to produce salts; they are así insolubíe in, and unchanged by contact with, water. Arsenious oxide, As, O.,, dissolves in water and the solution interacts with caustic soda or potash to form salts of the composition M. AsO, (M = Na or K); but no oxyacid derived from As, has been obtained. Antimonious oxide, Sb, oy, is slightly soluble in water and the solution reacts with potash or soda to form antimonites M SbO3; the acid H, Sb0, is known as a solid, but it is not obtained by the direct interaction of water with Sb,. Arsenious oxide interacts with concentrated hydrochloric acid to form arsenious chloride, AsCly; it is also said to form a salt AsKC,H,O, by reacting with solution of potassiumhydrogen tartrate; antimonious oxide forms SbCl, by reacting M. E.. C.

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with concentrated hydrochloric acid, it forms SbKCH,O, by dissolving in KHC,7,0 Aq and crystallising, and it probably interacts with concentrated sulphuric acid to form Sb (SO).. Vanadium trioxide V,0, is said to interact with acids to produce a series of unstable, easily oxidised, salts.

The pentoxides, M.Os, can be obtained by heating the elements in oxygen when M=P, V, Nb, or Ta; by evolving oxygen in contact with M,0; when M = As, Sb, Di, or Bi; and by withdrawing water from nitric acid in the case of NO.(2HNO-H,O=N.0).

The oxides NO, and P,0; dissolve in water to form acid solutions from which the acids HNO, (from NO.), HPO H.PO, and HP,0, (from P,0) can be obtained. The oxides As,, and Sbo are slightly soluble in water forming acid solutions from which salts can be obtained, e.g. Ag, Aso, KSbO; V.O is scarcely soluble, Nb 0,, Ta, 0,, Di, O, and Bio are insoluble, in water; salts of the forms MVO,, MTaO,, and MNbO, &c. are obtained by heating the pentoxides of V, Ta, and Nb, with molten potash or soda; no salts derived from Di,O, or Bi,O, are known; Bi,O; however dissolves in much molten potash and possibly forms very easily decomposed salts.

Of the remaining oxides, nitrous oxide No, nitric oxide NO, nitrogen dioxide NO,, and nitrogen tetroxide NOx, are gases; they are all obtainable by reactions between aqueous solutions of nitric acid and various reducing agents; the oxide NO, exists at moderately high temperatures, when this oxide is cooled it becomes very dark reddish black in colour and the vapour density now shews that its molecular composition is expressed by the formula NOAll the oxides of nitrogen are acidic; none shews any basic properties.

The haloid compounds are generally obtained by the direct union of their elements; also in many cases by dissolving the trioxides, M.Og, in haloid acid and evaporating. Those elements of the group which form pentachlorides MC15

, are P, Nb, Sb, and Ta; the compounds MCl, are produced by heating these elements in excess of chlorine. Trichlorides MC1,, are formed by heating arsenic or bismuth in chlorine; vanadium heated in a stream of chlorine forms the tetrachloride VCI, Pentahaloid compounds (MX;) of nitrogen, vanadium, arsenic, didymium, and bismuth, have not yet been obtained; no trihaloid compound (MX) of tantalum is known; haloid compounds of erbium have not as yet been isolated. Vanadium

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is the only member of the group which forms a haloid compound of the form MX, stable in the state of gas. Niobium and tantalum pentachlorides can be gasified; the pentachlorides of phosphorus and antimony are dissociated to MC1, + Cl, when heated.

The haloid compounds are all decomposed by water, generally forming solutions of haloid acid and the hydrated oxide of the element; thus PCl, gives H,PO,Aq, Asci, gives As 0 Aq, VCI, gives V.0,Aq, NbCl, gives Nb, XH,O, Tači; gives Ta, O.XH,O, Sbči, gives Sb.O,Aq if much warm water is used but solid SbỚCl if less water is added, in each case HCIAq is also formed ; BiCl, gives BioCl and HCIAq whatever be the quantity, or the temperature, of the water employed.

Many oxyhaloid compounds of Group V. exist; the more important belong to the forms MOCI and MOCI..

The sulphides, MS, of the lower members of the group 430 generally interact with alkaline sulphides to form thio- (or sulpho-) salts; thus As, S, dissolves in ammonium sulphide solution to form a solution of ammonium thio-arsenite, (NH2),AsSz; Sb S, under similar conditions forms a solution of NË Sbs. The best studied and apparently most distinctly acidic sulphides are As S, and SbS; PS, and VS, also interact with alkaline sulphides to form thiosalts. Ta S, is not known, but Ta S, is said not to react with alkaline sulphides. Bi,S, shews no acidic properties. Nitrogen forms the sulphide NS,; it is prepared by passing ammonia into sulphur dichloride (SCI); it is very easily decomposed by heat. Phosphorus forms several sulphides, P,S, PS, PS, P.S., &c.; they are generally obtained by direct union of phosphorus and sulphur. The highest sulphide of vanadiụm VS. is formed by passing sulphuretted hydrogen over hot V,05; Ta S, is produced by passing carbon disulphide (CS) over hot Taos The sulphides As,S,, Sb S, and Bi,S, are produced as solid precipitates when sulphuretted hydrogen is passed into acidulated solutions of the oxides M,O,

The acids formed by the combination of elements of 431 Group V. with hydrogen and oxygen are numerous and important. The following table shews the compositions of the best marked of these acids, and exhibits the relations of composition between them and their corresponding oxides.

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