Mean Free Path

Even though room temperature air molecules have a large RMS speed v_RMS  500 m/sec, that does not mean that they move across a room in a fraction of a second. If you open a bottle of perfume at one end of a room, it takes a while for you to notice the smell at the other end of the room. This is because the molecules undergo an enormous number of collisions on their way across the room. The mean free path λ is the average distance that a molecule travels in between collisions. It is given by

where d is the average diameter of a molecule, and N/V is the average number of molecules per unit volume.