When the electronic configuration of a transition metal element X in +3 oxidation state is $[Ar]3d^5$, what is its atomic number?

26

The correct answer is Option (2) → 26

In transition metals, electrons are removed first from the $4s$ orbital and then from the $3d$ orbital during ion formation.

The given configuration is for $\mathbf{X^{3+}}$:

$X^{3+} = [Ar] \, 3d^{5}$

To find the atomic number, we determine the configuration of the neutral atom $X$.

During ion formation:

• First, electrons are removed from $4s$
• Then from $3d$

So while forming $X^{3+}$:

Removal of $3$ electrons occurs as:

• $2$ electrons from $4s$
• $1$ electron from $3d$

Hence, the neutral atom must have been:

$[Ar] \, 3d^{6} \, 4s^{2}$

This configuration belongs to Iron $(Fe)$

Atomic number of $Fe = 26$