Question

Which one of the following ions is the most stable in aqueous solution?

[At. no.; Ti = 22, V - 23, Cr = 24; Mn = 25]

• A. $C{r}^{3+}$
• B. $V^{3+}$
• C. $T{i}^{3+}$
• D. $M{n}^{3+}$

Answer 1

The correct option is A $C{r}^{3+}$

In aqueous solution the configuration of the ions are following:

$C{r}^{3+}\left(21\right)=3d^{3}4s^0\Rightarrow {t_{2g}}^3{e}_g^0$

$V^{3+}\left(20\right)=3d^{2}4s^0\Rightarrow {t_{2g}}^2{e}_g^0$

$T{i}^{3+}\left(19\right)=3d^{1}4s^0\Rightarrow {t_{2g}}^1{e}_g^0$

$M{n}^{3+}\left(22\right)=3d^{4}4s^0\Rightarrow {t_{2g}}^4{e}_g^0$
We know that half-filled and fully filled orbitals are more stable than partially filled orbitals. Hence among the given ions the only ion which occupies half-filled $t_{2g}$ orbitals is Chromium (III) ion.

$Note:$ Although $M{n}^{3+}$ ion has half-filled $t_{2g}$ orbital but there is one more electron filled in $e_g$ orbital which reduces the stability.

Hence option A is the correct answer.