Why is oxygen heavier than air?

Oxygen is denser

Explanation:

Oxygen is denser than both air and nitrogen, at all temperatures and pressures, but only slightly. Since they don't separate from each other, we generally don't worry which is lighter or heavier. The difference in the density of nitrogen and oxygen gas comes from their molecular weight, which is small (4 g/mol)

the mantle

explanation:

earth is divided into three layers: the crust or the outermost layer on which we live; the underlying, middle layer - the mantle - and the core (the innermost layer of earth).