atom radius
2 3 4 5
1 2 4 8

Figure 2.19. Gas diffusion model. In this model, 100 gas atoms are represented in two-dimensions as color-coded circles dispersed randomly in a closed, rectangular chamber with two equal halves separated by a place-marker dashed line (not a barrier after diffusion starts). Initially, all the atoms on the right are red, and the all atoms on the left are green. When the "Start Diffusion" button is clicked, the atoms will all begin to move at the same velocity, but each in a randomly-selected direction. In this model, atoms do not interact with one another, only with the walls. When an atom reaches a chamber wall, it will "bounce" off the wall, changing its direction symmetrically.

The sliders below the chamber reflect the proportions of the two atom types in each half. Over time, random motion of the atoms will tend to equalize the percentages (=concentration) of red atoms on the two sides. However, because of the small total number of atoms, the percentages will continue to fluxtuate over time. You can click the "Start Diffusion" button again to continue the atom motions.

To explore this model further, you may make other changes with the sliders (number of atoms, number of steps) and radio buttons (atom radius, atom velocity), click the "Submit Changes" button, then click the "Start Diffusion" button. Changing the velocity is what happens when the temperature rises. The atoms move farther in the same amount of time.

A more complete model would have a statistical distribution of atom velocities and include atom collisions, although adding these features do not significantly change the gas diffusion outcome. You can see a model with atom collisions and more information on the PhET Interactive Simulations website.