The electronic configuration of Co3+ metal ion is........

The correct answer is Option (2) → 3d⁶4s⁰

In the first transition series the 4s orbital is filled before the 3d orbitals. But when the atom forms a cation, the electrons are removed first from 4s, because 4s is the outermost shell (n = 4). Only after 4s is empty are electrons removed from 3d.

Cobalt (Z = 27) has the configuration [Ar] 3d⁷ 4s². To form \( \ce{Co^{3+}} \), three electrons are removed:

    \( \displaystyle \ce{Co}:\ [\mathrm{Ar}]\,3d^{7}\,4s^{2} \)

    \( \displaystyle \ce{Co^2+}:\ [\mathrm{Ar}]\,3d^{7}\,4s^{0} \quad \text{(both 4s electrons removed)} \)

    \( \displaystyle \ce{Co^3+}:\ [\mathrm{Ar}]\,3d^{6}\,4s^{0} \quad \text{(one 3d electron removed)} \)

So the electronic configuration of \( \ce{Co^{3+}} \) is [Ar] 3d⁶ 4s⁰.