Molecules are in constant chaotic motion moving in straight lines between collisions.
The physical dimensions of the molecules occupy a very small fraction of the total system volume available. All molecules are traveling at the same speed. The average kinetic energy of the molecules is directly proportional to the temperature of the sample and independent of their mass. An empirical equation which attempts to account for deviations of gases from ideal behavior is the van der Waal equation of state. Parameter b accounts for the physical sizes of the molecules themselves.