The first ionisation enthalpies of the lanthanoids are around
600 kJ mol
–1, the second about 1200 kJ mol
–1 comparable with those
of calcium A detailed discussion of the variation of the third ionisation
enthalpies indicates that the exchange enthalpy considerations (as in
3d 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