The atoms in a gas can be treated as classical particles if their de Broglie wavelength is much smaller than the average separation between the particles. Compare the average de Broglie wavelength and the average separation between the atoms in a container of monatomic helium gas at 1.00 atm pressure and at room temperature (20 degrees C). At what temperature and pressure would you expect quantum effects to become important. I've calculated the de Broglie wavelength but I can't get the spacing between the atoms. Am I forgetting something from Chemistry? Can anyone give me a hint? I feel like I need a volume but clearly it isn't in the given info. Thanks.