Compare the density of the he atom with that of pure water, which is 1.0 g/cm3.

At room temperature, water is liquid in form. For helium, it is gas because it is the second lightest element in nature. So, you would expect the density of Helium to be much less than 1 g/cm³. Because gas is very light and occupies a lot of space, the density would result to a very small number.