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

Hint: 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.