Which transition element ion M^3+ has electronic configuration 3d^54s…

The correct answer is Option (3) → Fe³⁺

A transition metal atom loses its 4s electrons first and then 3d electrons, so a doubly or triply charged ion has no 4s electrons. An M³⁺ ion has (Z − 18 − 3) electrons outside the argon core, all in 3d. For 3d⁵4s⁰, Z − 21 = 5, so Z = 26, iron:

    \( \displaystyle \text{Fe }(Z = 26):\ \mathrm{[Ar]\,3d^6\,4s^2} \;\rightarrow\; \ce{Fe^3+}:\ \mathrm{[Ar]\,3d^5\,4s^0} \)

Why the other options are incorrect

• Option (1) Cr³⁺ — Incorrect. Cr (Z = 24) is [Ar] 3d⁵4s¹; removing three electrons leaves \( \ce{Cr^{3+}} \) as 3d³.

• Option (2) Mn³⁺ — Incorrect. Mn (Z = 25) is [Ar] 3d⁵4s²; \( \ce{Mn^{3+}} \) is 3d⁴. It is \( \ce{Mn^{2+}} \), not \( \ce{Mn^{3+}} \), that is 3d⁵.

• Option (4) Co³⁺ — Incorrect. Co (Z = 27) is [Ar] 3d⁷4s²; \( \ce{Co^{3+}} \) is 3d⁶.

NCERT (2026-27 edition):
Ionisation Enthalpies – “… The irregular trend in the first ionisation enthalpy of the metals of 3d series, though of little chemical significance, can be accounted for by considering that the removal of one electron alters the relative energies of 4s and 3d orbitals. You have learnt that when d-block elements form ions, ns electrons are lost before (n – 1) d electrons. … The doubly or more highly charged ions have dⁿ configurations with no 4s electrons. … However, the trend of steady increase in second and third ionisation enthalpy breaks for the formation of Mn²⁺ and Fe³⁺ respectively. In both the cases, ions have d⁵ configuration.”