Q.Why first ionization enthalpy of chromium is lower than that of zinc?

Step 1. Write out both neutral configurations: Cr is [Ar]3d⁵4s¹ (only one 4s electron, an Aufbau exception); Zn is [Ar]3d¹⁰4s² (both 4s and 3d completely filled).

Step 2. Consider what happens on removing the first electron from each. For Cr, removing its single 4s electron leaves [Ar]3d⁵ -- the exactly half-filled, extra-stable configuration. Because the atom is essentially 'gaining' stability by losing this electron, the electron is comparatively easy to remove, giving chromium a LOW first ionization enthalpy relative to its immediate neighbours.

Step 3. For Zn, removing one of its two 4s electrons leaves [Ar]3d¹⁰4s¹ -- breaking apart the fully-filled, extra-stable 4s²3d¹⁰ arrangement that the neutral zinc atom enjoys. Because this removal DISRUPTS an already-stable configuration rather than creating one, more energy is required, giving zinc a comparatively HIGH first ionization enthalpy. …

Compare what configuration results from removing one electron from each metal -- Cr reaches a MORE stable configuration (assisting ionization, lowering IE), Zn mo …

• Explaining the difference purely via 'rising nuclear charge across the period' without addressing the specific electronic-configuration-stability reasoning that the question is really testing. …