When the electronic configuration of a transition metal element X in +3 oxidation state is \( [Ar]3d^5 \), what is its atomic number?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 26
When a transition metal atom forms a cation, it loses its 4s electrons first and then 3d electrons. So the neutral atom is found by adding back the electrons that were lost, first to the 4s orbital and then to 3d.
\( \ce{X^{3+}} \) has \( \mathrm{[Ar]\,3d^{5}} \) and has lost three electrons. Adding back two to the 4s orbital and one to the 3d subshell:
\( \displaystyle \text{X}: \mathrm{[Ar]\,3d^{6}\,4s^{2}} \)
\( \displaystyle Z = 18 + 6 + 2 = 26 \)
So X is iron, with atomic number 26: Fe is \( \mathrm{[Ar]\,3d^{6}\,4s^{2}} \), and \( \ce{Fe^3+} \) is \( \mathrm{[Ar]\,3d^{5}} \), a stable half-filled subshell.