Which of the following is the correct ground-state electron configuration for the Co^{3+} ion?

• A. [Ar] 3d^7
• B. [Ar] 4s^2 3d^5
• C. [Ar] 3d^6
• D. [Ar] 4s^1 3d^6

Answer: (C)

Verified step by step guidance

1. Step 1: Identify the atomic number of cobalt (Co), which is 27, meaning a neutral Co atom has 27 electrons.
2. Step 2: Write the ground-state electron configuration for neutral Co: \([\mathrm{Ar}]\,4s^2\,3d^7\).
3. Step 3: Understand that when forming the \(\mathrm{Co}^{3+}\) ion, three electrons are removed from the neutral atom. Electrons are removed first from the outermost shell, which is the 4s orbital, followed by the 3d orbitals.
4. Step 4: Remove two electrons from the 4s orbital and one electron from the 3d orbital to account for the \$3+\( charge, resulting in the configuration \)[\mathrm{Ar}]\,3d^6$.
5. Step 5: Confirm that the correct ground-state electron configuration for \(\mathrm{Co}^{3+}\) is \([\mathrm{Ar}]\,3d^6\), as electrons are lost from 4s before 3d orbitals.