Among Al₂O₃, SiO₂, P₂O₃ and SO₂ the correct order of acid strength is

The formation of the oxide ion O^2-(g) requires first an exothermic and then an endothermic step as shown below

O_(g) + e^- = $$O_{(g)}^{-}$$ $$\Delta $$H^o = -142 kJmol^-1

$$O_{(g)}^{-}$$ + e^- = $$O_{(g)}^{2-}$$ $$\Delta $$H^o = 844 kJmol^-1

This because

Which one of the following is an amphoteric oxide?

According to the Periodic Law of elements, the variation in properties of elements is related to their