198 SCIENCE PROGRESS 



the conductivity increases enormously and becomes metallic, 

 practically the whole of the current being now carried by the 

 freely moving negative electrons. 



The behaviour of solutions of metals in ammonia is similar 

 to that of solutions of salts in water. Thus metallic calcium 

 reacts with potassium chloride in ammonia solution according 

 to the equation, 2KCI + Ca = CaClg + 2K and bismuth 

 chloride with sodium thus : BiCls + 6Na = sNaCl + NagBi. 

 These reactions emphasise the salt-hke behaviour of metals in 

 ammonia solution. 



Compounds of the alkali metals with the heavy metals of 

 the fourth, fifth, and sixth groups are very soluble in ammonia, 

 giving solutions which are characteristically coloured. These 

 solutions behave as if the electropositive element is present as 

 cation, and the more electronegative element as a more or less 

 complex anion. A solution of lead and sodium in hquid 

 ammonia gives a complex anion containing lead and sodium 

 which is deposited on the anode on passing an electric current. 

 Sodium forms several compounds with tellurium which give 

 complex ions in ammonia solution. NagTe is insoluble in 

 ammonia, but NagTcg and Na2Te4 are soluble, giving deep 

 violet and deep red solutions respectively. The anion in these 

 cases appears to be Te ~ " associated with additional tellurium, 

 Te Te - - and Tcg Te " ". This view of the constitution of 

 the complex tellurium ion is in harmony with the accepted 

 theory of the constitution of the complex sulphide ions, 

 S.S--, S2S--, S3S--, S4S--, S5S — . The chemical 

 behaviour of metallic compounds in solution indicates that 

 they are true electrolytes. Thus the lead compound vvith 

 sodium reacts with a cadmium salt according to the equation, 

 NagPb^ + Cd(N03)2 = 2NaN03 + CdPb,, the compound CdPb« 

 being precipitated. The corresponding NH4Pb^ is unstable, 

 decomposing into ammonia, hydrogen and metallic lead. An 

 interesting case occurs when the electronegative ion of a 

 metal is precipitated by means of an electropositive ion of 

 the same metal. Thus sodium plumbide on the addition 

 of lead nitrate precipitates lead according to the equation, 

 Na^Pbg H- 2Pb (N03)2 = PbgPbg + 4NaN03. The precipitated 

 lead may be regarded as lead plumbide. These reactions 

 suggest that the metals themselves occur as compounds in which 

 some of the atoms are charged positively, and some negatively. 

 The formation of such compounds in the solid state would 

 hinder the free movement of the valence electrons and hence 

 diminish the conductivity. The low equivalent conductivity 

 of the less electropositive elements suggests that they are not 

 homogeneous, but contain positively and negatively charged 

 ions, between which some of the valence electrons are tightly 



