Question

Which one of the following oxides of manganese is amphoteric?
A.$MnO$
B.$Mn{O_2}$
C.$M{n_2}{O_3}$
D.$M{n_2}{O_7}$

Answer 1

Metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Many metals (such as zinc, tin, lead, etc.) form amphoteric oxides or hydroxides.
Metallic oxides are generally basic whereas non-metallic oxides are acidic in nature. There are some metallic oxides that show both acidic and basic nature. The elements which form amphoteric oxides have characteristics of both metals and nonmetals.

Complete answer: $MnO$ has the lowest oxidation state among all oxides of $Mn$, hence it is basic in nature. Upon treatment with acids, it converts to the corresponding manganese (II) salt and water.
$Mn{O_2}$ is amphoteric oxide. It reacts with acids as well as bases.
With acid: $Mn{O_2} + 4HCl \to MnC{l_2} + C{l_2} + {H_2}O$
With base: $2Mn{O_2} + 4KOH + {O_2} \to 2{K_2}Mn{O_4} + 2{H_2}O$
$M{n_2}{O_3}$ is weakly acidic and mildly basic and can somewhat be classified as amphoteric, but this state is unstable and usually reverts to the $+ 2$ state.
$M{n_2}{O_7}$ has the highest oxidation state among all oxides of $Mn$, hence it is acidic in nature. Upon treatment with bases, it converts to the corresponding manganese (II) salt and water.
Hence, oxide of manganese which is amphoteric in nature is $Mn{O_2}$.
Hence, Option (B) is correct.

Note:
Transition metals are well known for having more than one stable oxidation state.
In its compounds, manganese exhibits oxidation states from $+ 2$ to $+ 7$.
Here, oxidation state of $Mn$ are as follows-
$MnO - M{n^{2 + }}$
$Mn{O_2} - M{n^{4 + }}$
$M{n_2}{O_3} - M{n^{3 + }}$
$M{n_2}{O_7} - M{n^{7 + }}$