• # question_answer Sodium forms peroxide while lithium gives monoxide.

The size of $N{{a}^{+}}$ ion is bigger as compared to that $L{{i}^{+}}$ ion and, therefore, the magnitude positive charge around it is comparatively less. It is not in a position to provide stability to oxide ion $({{O}^{2-}})$  with high electron density. However, it can hold relatively large peroxide ion${{(O-O)}^{2-}}$ more easily since the negative charge on it is quite dispersed. $L{{i}^{+}}$ion, on the other hand, can  accommodate oxide ion as it has a greater positive charge density around it.