If bromine gas is allowed to enter a gas jar containing a vacuum,

the red-brown colour instantly fills the jar

because there are no air molecules to collide with.

This shows that the bromine molecules are moving very fast. When bromine is added to a gas jar filled with air, the red-brown gas is seen to slowly diffuse through the air.

The many collisions between the bromine molecules

and the air molecules slow down the rate

at which the red-brown bromine gas fills the jar.