How come gases which are normally considered heavier than air are reaching the upper layers of our atmosphere?
In addition to the answers given in NI 265, the thing to remember is that gases unlike liquids or solids have no natural boundary. When a new gas is put in a container the individual molecules continue to move around at an average speed depending on the amount of energy in the gas (the hotter the gas, the greater the energy). The molecules move in random directions, colliding frequently with one another. This process of collisions equalizes the energies and the heat. The gases thus diffuse into each other. Any individual molecule of a heavier gas needs a greater energy to rise to a given height than a molecule of lighter gas. But the distribution of energy in a gas follows a statistical spread (the Maxwell-Boltzmann distribution), so within the mixture of gases there will always be some with enough energy to reach great heights.
