Ionization Enthalpy: Trends

Ionization enthalpy is the energy required to remove an electron from a gaseous atom or ion. Across a p-block period, this quantity rises in a fairly smooth, predictable way as nuclear charge increases; across the 3d transition series, by contrast, the rise is distinctly irregular, because the stability of the resulting $d^n$ or $d^ns^1$ configuration after ionization also matters, not just the bare nuclear charge.

The overall trend. In broad terms, the first ionization enthalpy does rise from scandium to zinc, reflecting the general increase in effective nuclear charge across the series (the same effective-nuclear-charge increase responsible for the radius contraction discussed in the previous section). Approximate first ionization enthalpies (in kJ mol$^{-1}$) illustrate the trend: $\text{Sc}\ 631 < \text{Ti}\ 658 \approx \text{V}\ 650 \approx \text{Cr}\ 653 < \text{Mn}\ 717 < \text{Fe}\ 762 \approx \text{Co}\ 760 > \text{Ni}\ 737 < \text{Cu}\ 745 < \text{Zn}\ 906$.

Manganese's comparatively high value. Manganese ($3d^54s^2$) shows a noticeably higher first ionization enthalpy than its immediate neighbours vanadium and chromium, because removing an electron from manganese's $4s^2$ leaves behind the exceptionally stable, half-filled $3d^5$ configuration; the extra stability of this half-filled subshell in the resulting ion is "paid for" by requiring more energy to form it, i.e. a higher ionization enthalpy for the parent atom.

Chromium's comparatively low value. Chromium is unusual in the opposite direction: because its ground-state atomic configuration is already the anomalous $3d^54s^1$ (discussed in the electronic-configuration section), removing chromium's single $4s$ electron is comparatively easy -- the atom already possesses the extra-stable half-filled $3d^5$ arrangement before ionization, so there is no additional exchange-energy gain to be won by losing the electron, and its first ionization enthalpy sits lower relative to the smooth trend a simple increasing-nuclear-charge picture would predict.

Zinc's high value. Zinc, with the completely filled configuration $3d^{10}4s^2$, shows the highest first ionization enthalpy of the series by a clear margin. Both the $3d^{10}$ and $4s^2$ subshells are already maximally stable (fully paired, fully filled), so removing an electron disrupts a genuinely stable closed-subshell arrangement, requiring comparatively more energy -- the same basic reason noble gases have very high ionization enthalpies, applied here to a filled d-subshell rather than a filled p-subshell. …