Question

Which of the following is the correct electron configuration for the ion \(\text{Fe}^{3+}\)?

• A. [Ar] 3d\(^5\)
• B. [Ar] 3d\(^6\)
• C. [Ar] 4s\(^2\) 3d\(^3\)
• D. [Ar] 3d\(^8\)

Hint:

When an atom forms a positive ion, electrons are removed starting from the outermost orbitals, typically 4s before 3d.

Answer 1

The Correct Option is A

Electron Configuration of Fe³⁺ 

Step 1: Neutral Fe Atom

Atomic number of Fe = 26
So, electron configuration of Fe is:

\( \text{Fe}: 1s^2\, 2s^2\, 2p^6\, 3s^2\, 3p^6\, 4s^2\, 3d^6 \)

Step 2: Removing Electrons to Form Fe³⁺

When forming cations, electrons are first removed from the outermost shell — that is, from the 4s orbital before the 3d orbital.

• Remove 2 electrons from 4s → \( 4s^0 \)
• Remove 1 electron from 3d → \( 3d^5 \)

Final electron configuration of Fe³⁺ is:

\( 1s^2\, 2s^2\, 2p^6\, 3s^2\, 3p^6\, 3d^5 \)

or in short form: \( [\text{Ar}]\, 3d^5 \)

✅ Correct Answer:

\( \boxed{[\text{Ar}]\, 3d^5} \)