Diffusion has to do with movement and the concentration of the molecules in the substance. Therefore, when the molecules in a material begin in a higher concentration, and then they start moving, they are beginning the diffusion process. Then it moves to a lower intensity. This would be when diffusion takes place. Some factors affect the rate of diffusion.
Temperature and particle or molecule size may affect the price of diffusion. When you compare the gases, you can determine their rate of diffusion based on the temperature and type of concentration. However, some gases have a higher rate of diffusions than others, even when they are at the same temperature. Of these gases, Helium or He has a faster rate of diffusion.