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Q.
The number of $d$-electrons in $Fe ^{2+}(Z-26)$ is not equal to the number of electrons in which one of the following?
Structure of Atom
Solution:
Electronic configuration of $Fe ^{2+}$ is [Ar] $3 d^{6} 4 s^{0}$
$\therefore$ Number of $d$-electrons $=6$
$Mg =1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2}(6 s \text {-electrons) }$
It matches with the the $6 d$-electrons of $Fe ^{2+}$
$Cl =1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{5} \text { (11p-electrons) }$
It does not match with the $6 d$-electrons of $Fe ^{2+}$.
$Cr =[ Ar ] 3 d^{6} 4 s^{2}(6 d \text {-electrons })$
It matches with the $6 d$-electrons of $Fe ^{2+}$.
$Ne =1 s^{2} 2 s^{2} 2 p^{6} \text { (6p-electrons) }$
It matches with the $6 d$-electrons of $Fe ^{2+}$.
Hence, $Cl$ has $11 p$-electrons which does not matches in number with $6 d$-electrons of $Fe ^{2+}$.