Ionization Enthalpy

Ionization enthalpy across the transition series is intermediate in magnitude between the low ionization enthalpies typical of s-block metals (which lose an electron readily) and the higher ionization enthalpies typical of p-block elements (which hold onto their electrons more tightly). Moving from left to right across a given transition series, ionization enthalpy increases, broadly as expected, because the nuclear charge is steadily rising as more protons and (d-sub-shell) electrons are added.

However, this increase is not perfectly regular or smoothly monotonic the way it is, for instance, across a typical s- or p-block period. The added electron at each step enters the (n-1)d orbital rather than the outermost ns orbital, and these inner d electrons act as a partial shield, reducing the effective nuclear charge actually experienced by the valence ns electrons that are being removed during ionization. Because the degree of shielding is not perfectly uniform (it depends on the precise d-sub-shell occupation at each element, including whether that occupation is approaching a half-filled or fully-filled, extra-stable configuration), the ionization enthalpy values show real element-to-element irregularities layered on top of the overall rising trend. …