Among the 3d transition series, the ionization enthalpy increases gradually within the period, but the relative increase is not sharp.
In the first transition series, the first and second ionization enthalpy values increase slowly from left to right with certain irregularities. As the nuclear charge increases, the attraction between the nucleus and outer electron increases, due to which the ionization enthalpy increases. However, the electron is added to (n-1) d subshell, the screening effect increases. Thus, the increase in the nuclear charge is opposed by the increase in the screening effect. Hence, the ionization enthalpy values slowly increase. The irregular trend in 3d series is because the removal of electrons alters the relative energies of 4s and 3d orbitals.
- Sc has the lowest while Zn has the highest first ionization enthalpy.
- Cu has the highest while Sc has the lowest second ionization enthalpy.
- Zn has the highest third ionization enthalpy.
- Fe has lower third ionization enthalpy relative to Mn.