Can anyone help me to find an approximation to the size of a He atom at very low temperature (10^(-9)K)? My attempt: At this very low temperature all electrons will be in the ground state, thus using the equipartition theorem: [tex] 1/2kT = h*c/lambda [/tex] now, for the zero point energy, lambda is 1/2 the diameter of the Helium atom. However, this approach does not yield the correct answer. I have had a look at several textbooks, but couldn't find any hints in there. Thanks a lot for your help!