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. $Cr^{3+} $
• B. $V^{3+} $
• C. $Ti^{3+} $
• D. $Mn^{3+} $

Answer 1

Answer: (A)

In aqueous solution the configuration of the ions are following:
$C r^{3+}(21)=3 d^{3} 4 s^{0} \Rightarrow t_{2 g}{ }^{3} e_{g}^{0}$
$V^{3+}(20)=3 d^{2} 4 s^{0} \Rightarrow t_{2 g}{ }^{2} e_{g}^{0}$
$T i^{3+}(19)=3 d^{1} 4 s^{0} \Rightarrow t_{2 g}{ }^{1} e_{g}^{0}$
$M n^{3+}(22)=3 d^{4} 4 s^{0} \Rightarrow t_{2 g}{ }^{4} e_{g}^{0}$
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 halffilled $t_{2 g}$ orbitals is Chromium (III) ion.
Note : Although $M n^{3+}$ ion has half-filled $t_{2 g}$ orbital but there is one more electron filled in $e_{g}$ orbital which reduces the stability.