Here's why:

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

* Molecular Mass: Oxygen molecules (O2) are made up of two oxygen atoms, so their molecular mass is 2 * 16 amu = 32 amu. Nitrogen molecules (N2) are made up of two nitrogen atoms, so their molecular mass is 2 * 14 amu = 28 amu.

Therefore, an oxygen molecule is heavier than a nitrogen molecule.