- ✓$Fe$
- B$Ni$
- C$Co$
- D$Mn$
$\quad \quad \quad \quad _{25} \mathrm{Mn} \quad \quad \quad _{26} \mathrm{Fe} \quad \quad \quad _{27} \mathrm{Co} \quad \quad \quad \quad _{28}Ni$
$\mathrm{M}= \quad [\mathrm{Ar}] 3 \mathrm{d}^{3} 4 \mathrm{s}^{2} \quad[\mathrm{Ar}] 3 \mathrm{d}^{6} 4 \mathrm{s}^{2} \quad[\mathrm{Ar}] 3 \mathrm{d}^{7} 4 \mathrm{s}^{2} \quad[\mathrm{Ar}] 3 \mathrm{d}^{2} 4 \mathrm{s}^{2}$
$\mathrm{M}^{2+}=[\mathrm{Ar}] 3 \mathrm{d}^{3} 4 \mathrm{s}^{0} \quad[\mathrm{Ar}] 3 \mathrm{d}^{6} 4 \mathrm{s}^{0} \quad[\mathrm{Ar}] 3 \mathrm{d}^{7} 4 \mathrm{s}^{0} \quad[\mathrm{Ar}] 3 \mathrm{d}^{2} 4 \mathrm{s}^{6}$
So third ionisation energy is minimum for $Fe$
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$C{r_2}O_7^{2 - } + F{e^{2 + }} + {C_2}O_4^{2 - } \to C{r^{3 + }} + F{e^{3 + }} + C{O_2}$ (Unbalanced)