Question

Which one of the following species is stable in aqueous solution:
A. $MnO_{4}^{2-}$
B. $MnO_{4}^{3-}$
C. $C{{r}^{2+}}$
D. $C{{u}^{+}}$

Answer 1

The bigger the atom, and the further the electrons from the nucleus, the more polarizable it is. The more polarizable the atom, the more steady the anion., the more electronegative a particle, the more steady the anion.

Complete step by step answer:
A. Manganese in its +6 oxidation state is only stable in strongly alkaline solutions (pH equal to 13 and upwards) as the - $MnO_{4}^{2-}$ ion, but at such high pH it can be achieved very easily. $MnO_{4}^{2-}$ is stable in aqueous solution as $+6$ oxidation state of $Mn$ is stable.
B. $MnO_{4}^{3-}$ is unstable in aqueous solution as $+5$ oxidation state of $Mn$ is unstable.
C. $C{{r}^{2+}}$ is more stable than $C{{r}^{3+}}$. $C{{r}^{2+}}$ is $3d^4$ and can attain half filled stability easily whereas $C{{r}^{3+}}$ is $3d^3$. Hence, not more than $MnO_{4}^{2-}$
D. In an aqueous medium, $C{{u}^{2+}}$ is more stable than $C{{u}^{+}}$. This is because although energy is required to remove one electron from $C{{u}^{+}}$ to $C{{u}^{2+}}$, high hydration energy of $C{{u}^{2+}}$ compensates for it. Therefore, $C{{u}^{+}}$ ion in an aqueous solution is unstable.

**Hence, the correct option is A. $MnO_{4}^{2-}$

Additional Information: **
In fluid, change metal particles exist as perplexing particles. Since $H_2O$ is a frail field ligand, electrons keep them unpaired beyond what many would consider possible.

Note:
In aqueous solution the water particles legitimately connected to the metal particle are said to have a place with the main coordination circle, otherwise called the first, or essential, solvation shell. The connection between a water atom and the metal particle is a dative covalent bond.