Why is first ionization enthalpy of chromium lower than that of Zn ?

Why is first ionization enthalpy of chromium lower than that of Zn ?

Ans.

The first ionization enthalpy of chromium (Cr) is lower than that of zinc (Zn) due to the electronic configuration and the stability associated with half-filled and fully-filled orbitals.

Chromium has the electronic configuration [Ar] 3d^5 4s^1, while zinc has the electronic configuration [Ar] 3d^10 4s^2.

In the case of chromium, removing one electron would result in a configuration with a half-filled $3d$ subshell, which is a relatively stable configuration due to the exchange energy associated with electron-electron repulsions. The removal of an electron from chromium leaves it with the configuration [Ar] 3d^5, which is more stable than the configuration before ionization.

On the other hand, zinc has a completely filled $3d$ subshell. Removing an electron from zinc would disrupt this fully filled subshell, resulting in a less stable configuration. The removal of an electron from zinc leaves it with the configuration [Ar] 3d^10 4s^1, which is less stable compared to its initial configuration.

Due to the stability associated with the half-filled $3d$ subshell in chromium, it requires less energy to remove an electron compared to zinc. Therefore, the first ionization enthalpy of chromium is lower than that of zinc.