96 Mo; atomic

weight

See also:

• 96 Mo; atomic weight MOLYBDENUM [symbol (0=16)]

MOLYBDENUM [

See also:

• SYMBOL (Gr. o-uµ(3oXov, a sign)

symbol (0=16)]  a metallic chemical element . The name is derived from Gr. poXui36os, lead, and was originally employed to denote many substances containing or resembling lead; ultimately the term was applied to graphite and to molybdenum sulphide . The difference between these two latter substances was first pointed out by Cronstedt, and in 1778 C . Scheele prepared molybdic acid from the sulphide . Molybdenum occurs in nature chiefly as the minerals molybdenite (MoS2) and wulfenite (PbMoO4), and more rarely as molybdic ochre (MoO3) and ilsemannite; it also occurs in many iron ores . The metal may be obtained by heating the trioxide with carbon in the electric furnace (H . Moissan, Comptes rendus, 1893, 116, p . 1225), or by the Goldschmidt method (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 .

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

Molybdenum sesquioxide, Mo203i a black

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

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

dialysis of a strong hydrochloric acid solution of sodium molybdate . Molybdenum trioxide, like 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 phosphorus and arsenic pentoxides) yielding very complex salts . The ordinary ammonium molybdate, used as a test reagent for phosphates, is a salt of composition (NH4)10MO12041; it has been examined physicochemically by J . 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 green and finally to a brown colour . . Molybdenum combines with the halogen elements in varying pro-portions, forming with chlorine a di-, tr1-, tetra- and penta-chloride, and similar compounds with bromine and 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

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 .

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 carrier . It is soluble in absolute alcohol and in ether . Molybdenum disulphide, MoS2, is found as the mineral molybdenite, and may be prepared by heating the trioxide with sulphur or sulphuretted hydrogen . It is a black crystalline powder, resembling graphite in appearance . It is readily oxidized by nitric acid, and when strohgly heated in a current of hydrogen is reduced to the metallic 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

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 . Berzelius (Pogg . Ann., 1826, 8, p . 23), by converting lead molybdate into lead nitrate, obtained the value 95.2 ;while J . B . A . Dumas (Ann., 186o, 113, p .

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

Pollard (Zeit. anorg . Chem., 1895, 8, p . 434) using this u cond method obtained the value 96.28; whilst E . F . Smith and P . 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 .