Here's why:

* Molar Mass: Bromine gas (Br₂) has a molar mass of approximately 160 g/mol. Air, composed mostly of nitrogen (N₂) and oxygen (O₂), has an average molar mass of about 29 g/mol.

* Density and Molar Mass: Density is directly related to molar mass. Since bromine has a significantly higher molar mass than air, it will be denser.

Important Note: Bromine is a reddish-brown liquid at room temperature. It vaporizes into a red-brown gas, but it's still denser than air.