Gases have a thermodynamic tendency to mix, despite weight differences. The atmosphere isn't a layer cake of heavy gases below light gases. If it was, we wouldn

Gases have a thermodynamic tendency to mix, despite weight differences. The atmosphere isn't a layer cake of heavy gases below light gases. If it was, we wouldn't be able to breathe because the ~1% of argon (and all the heavier gases) would sit below the oxygen layer.