Q.The first ionization enthalpy does not rise perfectly smoothly across the 3d series. Explain why manganese and zinc show comparatively high values while chromium's is lower than expected from a simple increasing trend.

The first ionization enthalpy across the 3d series rises only irregularly, because the stability of the electron configuration left behind after ionization matters just as much as the bare increase in nuclear charge across the row.

Manganese has the ground-state configuration $[\text{Ar}]3d^54s^2$. Removing its first electron (from the $4s$ orbital) leaves the ion $\text{Mn}^+$ with the configuration $3d^54s^1$, and removing a second would eventually expose the exceptionally stable, half-filled $3d^5$ configuration of $\text{Mn}^{2+}$. Because reaching that stable half-filled arrangement is itself energetically favourable, manganese's neutral atom "holds onto" its outer electrons somewhat more tightly than a simple extrapolation from its neighbours vanadium and chromium would suggest, giving it a noticeably higher first ionization enthalpy than either.

Zinc has the ground-state configuration $[\text{Ar}]3d^{10}4s^2$: both of its outer subshells are already completely filled, the most symmetric and lowest-repulsion arrangement possible for each. Removing an electron from zinc means disrupting one of these two already-maximally-stable closed subshells (specifically, breaking into the paired $4s^2$), which is energetically costly in exactly the same way that removing an electron from a noble gas's filled shell is costly. This is why zinc shows the single highest first ionization enthalpy anywhere in the first-row transition series (approximately $906\ \text{kJ mol}^{-1}$, distinctly above even its immediate neighbour copper). …

Write out the ground-state configurations of Mn and Zn, identify the stable (half-filled / fully-filled) subshell each ionization interacts with, and explain the resulting higher-than-trend ioniza …

Explaining the irregularity purely by 'nuclear charge' without reference to subshell stability; confusing chromium's low IE (already stable before ionization) with manganese's high IE …