Here's why:

* Molecular weight: Chlorine gas (Cl₂) has a molecular weight of approximately 71 g/mol.

* Air composition: Air is primarily composed of nitrogen (N₂) and oxygen (O₂), with molecular weights of 28 g/mol and 32 g/mol respectively.

Since chlorine has a higher molecular weight than the main components of air, it is denser and therefore heavier.