Question

explain why lithium on being heated in air mainly forms the oxide and not peroxide

Answer 1

This is because of the small size of the lithium because stability of the peroxides increases as the size of the alkali metals goes on increasing.

Answer 2

Well, Lithium on being heated in air mainly forms the oxide and not peroxide because we Know Lithium has Atomic Number 3, this means that it has 1 electron in outermost shell and The Oxygen has Atomic no. 8 and it can gain two electrons therefore it's valency is 2.
When Reacting Both of them the Valencies of Both Elements will be Crossed and Changed to make up a chemical Reaction.
This is as Follows:-
Lithium--> 1
Oxygen ---> 2 (Valency).

Crossed to form
Li2O ( Lithium oxide) ....
Remember that Electrons of Oxygen has been written with Lithium Respectively...
Thus it can't be possible to form a Compound of Lithium Peroxide(LiO2) while Reaction of Lithium And Oxygen..

Therefore we have came to conclusion of this question..

I hope it Helped You...
Thanks for asking the question :-)