Paragraph III
Lithium forms monoxide only when heated in oxygen. Sodium forms monoxide and peroxide in excess of oxygen. Other alkali metals form superoxide with oxygen ie, `MO_2` The abnormal behaviour of lithium is due to small size. The larger size of nearer alkali metals also decides the role in formation of superoxides. The three ions are related to each other as follows:
`underset("Oxide ion")(O^(2-)) overset(1//2O_2) rarr underset("Peroxide ion")(O_2^(2-)) overset(O_2) rarr underset("Superoxide ion")(2O_2^(-))`
All the three ions abstract proton from water.

Options

A. M cannot be Li and Na
B. M connot be Cs and Rb
C. M cannot be Li and Rb
D. none of these

Answer

A

Explanation

Li and Na do not form superoxide.