The dominant term is the second ionisation enthalpy
which shows unusually high values for Cr and Cu where M
+ ions
have the d
5 and d
10 configurations respectively The value for Zn is
correspondingly low as the ionisation causes the removal of one 4s
electron which results in the formation of stable d
10 configuration The trend in the third ionisation enthalpies is not complicated by
the 4s orbital factor and shows the greater difficulty of removing an
electron from the d
5 (Mn
2+) and d
10 (Zn
2+) ions In general, the third
ionisation enthalpies are quite high