Question

Why second ionization enthalpies of chromium and copper are exceptionally higher than those of their neighbouring elements?

Hint:

Breaking a $d^5$ or $d^{10}$ stable core always results in a massive spike in ionization energy.

Answer 1

Step 1: Ground-state configurations of Cr and Cu.
Chromium: $[Ar]~3d^5~4s^1$ (half-filled $3d$). Copper: $[Ar]~3d^{10}~4s^1$ (fully filled $3d$). Both have a single $4s$ electron rather than two.
Step 2: Formation of $M^+$ ions (first ionisation).
Removing the single $4s$ electron gives $Cr^+$ with $3d^5$ (half-filled) and $Cu^+$ with $3d^{10}$ (fully filled). Both resulting ions have exceptionally stable d-subshell configurations.
Step 3: Stability of half-filled and fully filled subshells.
Half-filled ($d^5$) and fully filled ($d^{10}$) subshells possess maximum exchange energy and perfect spherical symmetry. These configurations are highly resistant to loss of another electron.
Step 4: Consequence for second IE.
Removing the second electron from $Cr^+$ ($3d^5$) or $Cu^+$ ($3d^{10}$) completely disrupts this extra stability, requiring anomalously large energy. This is why the second ionisation enthalpies of Cr and Cu are exceptionally higher than those of their neighbours.