sodium forms peroxide while lithium gives monoxide.why?

Since the alkali metals are the most electropositive (the least electronegative) of elements, they react with a great variety of nonmetals.
The peroxides (Li2O2and Na2O2) can be made by passing oxygen through a liquid-ammonia solution of the alkali metal, although sodium peroxide is made commercially by oxidation of sodium monoxide with oxygen.
Li2O2 being less stable than Li2O,therefore lithium peroxide formed decomposes into lithium oxide.
Lithium oxide is formed along with small amounts of lithium peroxide when lithium metal is burned in the air and combines with oxygen:4Li + O2 → 2Li2O.