Here's a breakdown:

* Atomic Structure: Helium has only two protons and two neutrons in its nucleus, while nitrogen has seven protons and seven neutrons. This means a helium atom has significantly less mass than a nitrogen atom.

* Density: Density is mass per unit volume. Since helium atoms are much smaller and lighter, they occupy a smaller volume for the same amount of mass, resulting in a lower density.

* Molecular Weight: Helium exists as a single atom (He), while nitrogen forms a diatomic molecule (N₂). Although a nitrogen molecule has two atoms, each atom is heavier than a helium atom. The combined effect makes nitrogen much heavier than helium.

This difference in weight is why helium balloons float in air. Helium is less dense than air, which is mostly composed of nitrogen and oxygen. The buoyant force pushing the helium balloon upwards is greater than the force of gravity pulling it downwards.