The irregular trend in the first ionization enthalpy is attributed to the alteration of relative energies between 4s and 3d orbitals upon the removal of one electron, affecting the shielding of 4s electrons by the increasing nuclear charge along the 3d series.
What accounts for the irregular trend in the first ionization enthalpy along the 3d series of transition metals?
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The irregular trend in the first ionization enthalpy along the 3d series of transition metals is attributed to the alteration of relative energies between 4s and 3d orbitals upon the removal of one electron. As electrons are added to the 3d orbitals, they shield the 4s electrons from the increasing nuclear charge more effectively than the outer shell electrons can shield each other. This differential shielding leads to a less rapid decrease in atomic radii and only a slight increase in ionization energies along the 3d series. The alteration in the 4s and 3d orbital energies contributes to the irregularity observed.