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Which one of the following ions is the most stable in aqueous solution?
(At. No. Ti = 22, V = 23, Cr = 24, Mn = 25)
(At. No. Ti = 22, V = 23, Cr = 24, Mn = 25)
A
$V^{3+}$
B
$Ti^{3+}$
C
$Mn^{3+}$
D
$Cr^{3+}$
Detailed Solution
For chromium, the +3 oxidation state is the most stable in aqueous solution.
$Cr^{3+}$ has the configuration $3d^3$. In the octahedral field of water molecules these three electrons occupy the three $t_{2g}$ orbitals singly, giving a half-filled $t_{2g}^3$ level with a large crystal field stabilisation energy.
So $Cr^{3+}(aq)$ has no tendency to be oxidised or reduced.
$Ti^{3+}$ ($3d^1$) and $V^{3+}$ ($3d^2$) are reducing agents; they are easily oxidised to the +4 state in aqueous solution.
$Mn^{3+}$ ($3d^4$) is a strong oxidising agent; it readily changes to $Mn^{2+}$, which has the stable $3d^5$ configuration, and it also disproportionates in water.
So the most stable ion in aqueous solution is $Cr^{3+}$.
$Cr^{3+}$ has the configuration $3d^3$. In the octahedral field of water molecules these three electrons occupy the three $t_{2g}$ orbitals singly, giving a half-filled $t_{2g}^3$ level with a large crystal field stabilisation energy.
So $Cr^{3+}(aq)$ has no tendency to be oxidised or reduced.
$Ti^{3+}$ ($3d^1$) and $V^{3+}$ ($3d^2$) are reducing agents; they are easily oxidised to the +4 state in aqueous solution.
$Mn^{3+}$ ($3d^4$) is a strong oxidising agent; it readily changes to $Mn^{2+}$, which has the stable $3d^5$ configuration, and it also disproportionates in water.
So the most stable ion in aqueous solution is $Cr^{3+}$.