mean free path The vacuum distillation section says that Li 6 has a greater mean free path than Li 7 However the mean free path of an ideal gas does not depend on the speed or mass of the particles where k is the Boltzmann constant T is temperature is collision cross section p is pressure Since Li 6 is lighter it most probably evaporates faster than Li 7 since equal kinetic energy results in larger speed for Li 6 more Li 6 atoms are fast enough to escape but it has nothing to do with mean free path Szaszicska from huwiki