In chemistry, effusion is the process where individual particles flow through a hole so tiny they must go one at a time. Gases effuse, the rate at which they do so is dependent on their atomic radius. Gases composed of large particles effuse slower than gases composed of small particles. This is why a balloon filled full of the small diatomic molecule hydrogen will deflate, if left alone for a reasonable amount of time, faster than one of the larger diatomic molecule oxygen.
See also diffusion.