Question

When there is excessive oxygen why sodium forms peroxide and others forms superoxide in alkali metals why?

Answer 1

The answer is in the question itself. It is because there is excess oxygen

Metallic sodium reacts with oxygen at 200–350°C to produce sodium oxide Na2O;-

4 Na + O2 → 2 Na2O

If enough excess oxygen is present, sodium oxide reacts with oxygen to produce sodium peroxide at temperatures of 350–450°C ;-

2 Na2O + O2 ←→ 2 Na2O2

If sodium peroxide is reacted with an even larger excess of oxygen at high pressure, the result is sodium superoxide (NaO2);-

Na2O2 + O2 → 2 NaO2.. there cannot be any why for this case. Sorry.