The triads. The element iron plays a special part in the chemical family, страница 3

The potential for the change from cobaltous to cobaltic ion is —1.84 volts. This is strong enough to oxidize water with the formation of oxygen. There are not many examples of cobaltic ions; CoF₃ and Co₂(SO₄)₃ • 18H₂O have been formed, but they decompose in aqueous solution to give cobaltous ions. When the cobaltic ion unites with other ions to form com­plexes, it can be stabilized. Examples of this are Co(CN),r³ and Co(NH₃)J³.

 Nickel. Nickel, like cobalt, is difficult to extract in the pure state from its mineral source. The name comes from the Swedish word, kopparnickel, which means false copper. It is found in the form of a sulfide mixed with sulfides of iron, or as a silicate mixed with magnesium. It is normally purified electrolytically; but there is an unusual method available employ­ing a volatile compound, called nickel carbonyl, Ni(CO)₄, that nickel forms with carbon monoxide at room temperature. When the vapor of this substance is heated to 200° C, it decom­poses into nickel and carbon monoxide. Of the three members of this iron triad, nickel is the least subject to oxidation; for this reason, nickel-plating of iron produces resistance to corro­sion. It also increases the strength of iron when present in small amount. A great deal of nickel is also used in coins. In practically all nickel compounds, the Ni atoms have lost only the two s electrons in the fourth level and are in the +2

oxidation state. The oxide in this state is NiO, nickelous oxide, which is black in color The nickelous complex ions are quite stable in air with respect to oxidation; they can be either square-planar or octahedral in shape: Ni(NH.,)+² and Ni(NH₃)+²; and other ions are found with tetrahedral coordination: Ni(CO)₄. Solutions of nickelous ion are usually pale green, but in the presence bf complexing agents the color changes; for example, in the presence of NH₃ nickelous solutions are blue. Nickelous oxide combines with oxygen to give a substance with the rock salt structure that has a larger percentage of oxygen than the formula, NiO, indicates; it is believed that this is the result of a complex solid solution in which some of the nickel ions are ¹ present as nickelic ions and some nickelous ions are missing from their lattice positions. Nickel has a boiling point almost the same as that of iron and a melting point slightly lower; its ionization potential is the lowest of the three members ¹ of the triad, 7.6 volts, and its density is the highest, 8.9 gm/cm³. It also has the lowest oxidation potential %r the change   from    the   metal   to   the   nickelous ion, 0.25 volt.