What happens to the ionization enthalpy among the $3\mathrm{d}$ transition series?

Ionization enthalpy:

• The energy needed to remove one electron from the outermost shell of an element is called ionization enthalpy.
• It depends upon the atomic radius and symmetry of electronic configuration.

The trend of ionization enthalpy among the $3\mathrm{d}$ transition series:

• In the first transition series, the first and second ionization enthalpy values increase slowly on moving across the period.
• As the nuclear charge rises, the interaction between the nucleus and valence electrons increases, and as a result, the ionization enthalpy increases.
• The trend in third ionization enthalpy doesn’t involve $4\mathrm{s}$ orbitals. The irregular trend in the $3\mathrm{d}$ series are as follows:

1. $\mathrm{Sc}$ has the lowest value of $\mathrm{I}$ ionization enthalpy i.e. $633.1\mathrm{kJ}{\mathrm{mol}}^{-1}$ while $\mathrm{Zn}$ has the highest $\mathrm{I}$ ionization enthalpy i.e. $906.4\mathrm{kJ}{\mathrm{mol}}^{-1}$.
2. $\mathrm{Cu}$ has the highest while $\mathrm{Sc}$ has the lowest $\mathrm{II}$ ionization enthalpy.
3. $\mathrm{Zn}$ has the highest $\mathrm{III}$ ionization enthalpy.
4. Ionization Enthalpy of 3d elements from Sc to Zn is given below