Question

If Hund's rule is not followed, magnetic moment if Fe${}^{2+}$, Mn${}^{\oplus }$ and Cr all having 24 electrons will be in order :

• A. Fe${}^{2+}$, Mn${}^{\oplus }$ < Cr
• B. Fe${}^{2+}$ = Cr$<$Mn${}^{\oplus }$
• C. Fe${}^{2+}$ = Mn${}^{\oplus }$ < Cr
• D. Mn${}^{2\oplus }$ = Cr < Fe${}^{2+}$

Answer 1

The correct option is B Fe${}^{2+}$ = Cr$<$Mn${}^{\oplus }$

According to Hund's rule electrons are distributed among the orbitals
of subshell in such a way as to give maximum number of unpaired electron
with parallel spin. i.e. in a subshell pairing of electron will not
start until and unless all the orbitals of that subshell will get one
electron each with same spin.
So $M{n}^{+}$ have 2 unpaired electron,
Cr have zero unpaired electron and
$F{e}^{+2}$ have zero unpaired electron and order is :
Fe${}^{2+}$ = Cr < Mn${}^{\oplus }$