The lowest common oxidation state of these metals is +2 To
form the M
2+ ions from the gaseous atoms, the sum of the first and
second ionisation enthalpy is required in addition to the enthalpy of
atomisation 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