Ionization enthalpy is the energy required to remove the most loosely bound electron from the isolated gaseous atoms to produce a cation. Those of transition elements tend to increase from left to right in the periodic table as there is an increase in nuclear charge which accompanies the filling of the inner d orbitals. However, there are some exceptions, for example, Titanium (22) has first ionization enthalpy of 656kJ/mole whereas vanadium (23) has first ionization enthalpy of 650kJ/mole.
These irregular trends in the first ionization enthalpy of transition metals can be explained by considering the fact that the removal of one electron alters the relative energies of 4s and 3d orbitals. Hence, in general, we can conclude that the uni positive ions furnish dn configuration with no 4s electrons. Thus, there is a reorganization energy accompanying ionization with some gains in exchange energy as the number of electrons increases from the transference of s electrons into d orbitals. Let us analyse the general trend in the first row:
First ionization enthalpies of transition elements show general trend of increase from left to right. However, in case of chromium (24), the change in the value of first ionization enthalpy is lower as there is no change in d orbital configuration whereas, zinc (30) shows a higher value as it represents an ionization from 4s level.
The second ionization enthalpy for Cr and Cu are quite higher as the d5 and d10 configuration of M+ ions gets disrupted whereas, the value for Zn is correspondingly low as the ionization consists of the removal of an electron which allows the production of the stable d10 configuration.
The trend in third ionization enthalpy doesn’t involve 4s orbitals and thus there is a greater difficulty for the removal of an electron from Mn2+ (d5) and Zn2+ (d10) ions. Thus, we see a marked difference between third ionization enthalpies of iron (26) and manganese (25).
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