Here's why:

* Atomic Mass: Oxygen has an atomic mass of 16 atomic mass units (amu), while nitrogen has an atomic mass of 14 amu.

* Molecular Mass: Oxygen exists as a diatomic molecule (O2), so its molecular mass is 32 amu (16 x 2). Nitrogen also exists as a diatomic molecule (N2), with a molecular mass of 28 amu (14 x 2).

Since oxygen has a higher molecular mass than nitrogen, it is denser and therefore heavier.