Lanthanoids · Part 3

hardness increases with increasing atomic number, samarium being steel hard. Their melting points range between to K but samarium melts at K. They have typical metallic structure and are good conductors of heat and electricity. Density and other properties change smoothly except for Eu and Yb and occasionally for Sm and Tm. Many trivalent lanthanoid ions are coloured both in the solid state and in aqueous solutions. Colour of these ions may be attributed to the presence of f electrons. Neither La + nor Lu + ion shows any colour but the rest do so. However, absorption bands are narrow, probably because of the excitation within f level. The lanthanoid ions other than the f type (La + and Ce + ) and the f type (Yb + and Lu + ) are all paramagnetic. The first ionisation enthalpies of the lanthanoids are around kJ mol – , the second about kJ mol – comparable with those of calcium. A detailed discussion of the variation of the third ionisation enthalpies indicates that the exchange enthalpy considerations (as in d orbitals of the first transition series), appear to impart a certain degree of stability to empty, half-filled and completely filled orbitals f level. This is indicated from the abnormally low value of the third ionisation enthalpy of lanthanum, gadolinium and lutetium. In their chemical behaviour, in general, the earlier members of the series are quite reactive similar to calcium but, with increasing atomic number, they behave more like aluminium. Values for E o for the half-reaction: Ln + (aq) + 3e – ® Ln(s) . . General Characteristics LnC with C K N Ln heated with N with H O Ln O H with acids burns in O heated with S with halogens LnX Ln(OH) + H Ln S Ln