The electron configuration of the Fe$^{3+}$ ion (iron, atomic number 26) is
A[Ar] $3d^3 4s^2$
B[Ar] $3d^6$
C[Ar] $3d^5$CORRECT
D[Ar] $3d^6 4s^2$
JUPEBChemistryAtomic Structure and Periodicity
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Why the answer is C, and why the others tempt you.
Neutral Fe is [Ar] $3d^6 4s^2$. When transition metals ionise, the 4s electrons are lost first, then 3d: removing three electrons gives [Ar] $3d^5$. Removing electrons from 3d while leaving 4s occupied is the common error.
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