Nitrogen is the major constituent of Earth's atmosphere, so it is, in a way, exactly as heavy as air. Though nitrogen makes up the bulk of the atmosphere by volume, almost 21 percent of the air is composed of molecular oxygen. To determine nitrogen's weight relative to air, it is important to determine whether the nitrogen in the atmosphere is lighter or heavier than the oxygen.
Atom for atom, nitrogen is lighter than oxygen. Nitrogen has an atomic weight of just over 14, while oxygen has an atomic weight of almost 16. An admixture of oxygen and nitrogen should be heavier than nitrogen alone.
Atomic weight is not the final determinant; however, as the term "heavier than air" is usually taken to mean "more dense." If nitrogen has a higher density than oxygen, it should sink in the presence of oxygen regardless of its atomic or molecular mass. In this case, the specific gravity of molecular nitrogen is 0.9669, and the equivalent figure for oxygen is 1.1044. Dividing nitrogen's figure by that of oxygen yields a ratio of just over 0.875:1. Molecular oxygen is heavier and denser than nitrogen by itself, and pure nitrogen can be expected to float in its presence.