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MOLYBDENUM [symbol, Mo; atomic weight...

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Originally appearing in Volume V18, Page 682 of the 1911 Encyclopedia Britannica.
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MOLYBDENUM [See also:symbol, Mo; atomic See also:weight, 96 (0=16)] a metallic chemical See also:element. The name is derived from Gr. poXui36os, See also:lead, and was originally employed to denote many substances containing or resembling lead; ultimately the See also:term was applied to See also:graphite and to molybdenum sulphide. The difference between these two latter substances was first pointed out by Cronstedt, and in 1778 C. See also:Scheele prepared molybdic See also:acid from the sulphide. Molybdenum occurs in nature chiefly as the minerals See also:molybdenite (MoS2) and See also:wulfenite (PbMoO4), and more rarely as molybdic ochre (MoO3) and ilsemannite; it also occurs in many See also:iron ores. The See also:metal may be obtained by See also:heating the trioxide with See also:carbon in the electric See also:furnace (H. See also:Moissan, Comptes rendus, 1893, 116, p. 1225), or by the See also:Goldschmidt method (See also:Rosenheim and Braun, Zeit. anorg. Chem., 1905, p. 311) or by dissociating the tetra- and pentachloride in a graphite crucible with an electric current below 1330 (J. N. Pring and W.

See also:

Fielding, Jour. Chem. See also:Soc., 1909, 95, p. 1497). It forms a See also:grey coloured See also:powder of specific gravity 9.oi; it is malleable, and not as hard as See also:glass. It is rapidly oxidized on heating to a temperature of 5oo°-600° C., and also when fused with See also:nitre or See also:potassium chlorate. It is soluble in dilute nitric acid, and in concentrated sulphuric acid; in the latter See also:case with the formation of a See also:blue See also:solution which on heating becomes colourless, molybdenum trioxide being formed with the liberation of See also:sulphur dioxide. Molybdenum combines with See also:oxygen to See also:form many oxides, the most important of which are : the monoxide, MoO.n (See also:H2O), the sesquioxide, Mo203, the dioxide, MoO2, and the trioxide, MOOS• Molybdenum monoxide, MoO.n(H2O), is a See also:black powder obtained when the dichloride is boiled with concentrated potash solution. According to W. Muthmann and W. Nagel (Ber., 1898, 31, p. 2009), this See also:oxide does not exist, the reaction leading to the formation of an hydroxide according to the See also:equation: Mo3C14(OH)s + 4KHO + 3H2O = 3Mo(OH)3+4KBr{-3H.

Molybdenum sesquioxide, Mo203i a black See also:

mass insoluble in acids, is formed by heating the corresponding hydroxide in vacuo, or by digesting the trioxide with See also:zinc and hydrochloric acid. Molybdenum dioxide, MoO2, is formed by heating See also:sodium trimolybdate, Na2Mo3Ols, to redness in a current of See also:hydrogen (L. Svanberg and H. See also:Struve, Jour. prak. Chem., 1848, 44, p. 301), or by See also:long See also:fusion of a mixture of ammonium molybdate, potassium carbonate, and See also:boron trioxide (W. Muthmann, See also:Ann., 1887, 238, p. 114). It forms quadratic prisms, having a See also:violet reflex and insoluble in boiling hydrochloric acid. Molybdenum trioxide, MoO3, is prepared by oxidizing the metal or the sulphide by heating them in See also:air, or with nitric acid. It, is a See also:white powder, which turns See also:pale yellow on heating, and melts at a red See also:heat. It sublimes in small rhombic tables or needles, and is slightly soluble in See also:cold See also:water, the solution possessing an acid reaction.

Several hydrated forms of the oxide are known, and a colloidal variety may be obtained by the See also:

dialysis of a strong hydrochloric acid solution of sodium molybdate. Molybdenum trioxide, like See also:chromium trioxide, is an acidic oxide, and forms salts known as molybdates. The normal molybdates show a tendency to pass into polymolybdates. The molybdates are also capable of combining with other oxides (such as See also:phosphorus and See also:arsenic pentoxides) yielding very complex salts. The See also:ordinary ammonium molybdate, used as a test reagent for See also:phosphates, is a See also:salt of See also:composition (NH4)10MO12041; it has been examined physicochemically by J. See also:Sand and F. Eisenlohr (Abst. J.C.S., 1907, 1i. pp. 178, 179). The molybdates may be recognized by the fact that they give a white precipitate on the addition of hydrochloric or nitric acids to their solutions, and that with reducing agents (zinc and sulphuric acid) they give generally a blue coloration which turns to a See also:green and finally to a See also:brown See also:colour. . Molybdenum combines with the halogen elements in varying See also:pro-portions, forming with See also:chlorine a di-, tr1-, tetra- and penta-chloride, and similar compounds with See also:bromine and See also:iodine. Molybdenum dichloride (MoC12)3 or C14Mo3C12 (chlormolybdenum chloride), is prepared (together with some tetrachloride) by heating the trichlonde in a stream of carbon dioxide (C.

W. Blomstrand, Jour. f. prak.Chem.,1857, 71, p. 449 ; 1861, 82, p. 433). It is a yellow amorphous powder which is soluble in dilute alkalis, the solution on acidification giving an hydroxide, C14Mo3(OH)2, which is soluble in nitric acid, and does not give a reaction with See also:

silver nitrate. The molecular weight determinations of W. Muthmann and W. Nagel (Ber., 1898, 31, p. 2009) show the salt to possess the composition Mo3C16. Molybdenum trichioride, MoC13, is obtained when the pentachloride is heated to a temperature of about 250° C. in a current of hydrogen. It forms red crusts, is insoluble in cold water, but is decomposed by boiling water. It is easily soluble in hot nitric acid.

Molybdenum pentachloride, MoCl5,. is obtained when molybdenum is gently heated in dry chlorine (L. P. Liechti and B. See also:

Kempe, Ann., 1873, 169, p. 345). It is a dark-coloured crystalline solid which melts at 194° C. and boils at 268° C. It fumes in moist air and deliquesces gradually. It is occasionally used as a chlorine See also:carrier. It is soluble in See also:absolute See also:alcohol and in See also:ether. Molybdenum disulphide, MoS2, is found as the See also:mineral molybdenite, and may be prepared by heating the trioxide with sulphur or sulphuretted hydrogen. It is a black crystalline powder, resembling graphite in See also:appearance. It is readily oxidized by nitric acid, and when strohgly heated in a current of hydrogen is reduced to the metallic See also:condition.

Molybdenum trisulphide, MoS,, is obtained by saturating a solution of an alkaline molybdate with sulphuretted hydrogen and adding a mineral acid. It is a brown powder which on heating in air loses sulphur and leaves a See also:

residue of the disulphide. A tetrasulphide, MoS4, has also been described. Many varying values have been given for the atomic weight of molybdenum. J. J. See also:Berzelius (Pogg. Ann., 1826, 8, p. 23), by converting lead molybdate into lead nitrate, obtained the value 95.2 ;while J. B. A. See also:Dumas (Ann., 186o, 113, p.

32), by converting the trioxide Into the metal, obtained the value 95.65. K. Seubert and W. See also:

Pollard (Zeit. anorg. Chem., 1895, 8, p. 434) using this u cond method obtained the value 96.28; whilst E. F. See also:Smith and P. See also:Maas (Zeit. anorg. Chem., 1894, 5, p. 280), by heating pure sodium molybdate in hydrochloric acid and estimating the amount of sodium chloride formed, obtained the value 96.087.

End of Article: MOLYBDENUM [symbol, Mo; atomic weight, 96 (0=16)]

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