Density

A large object does not necessarily have a greater density than a small one. Density describes how much mass there is in each unit of volume. Comparing equal volumes makes this clearer: a bag full of metal has much more mass than a similar-sized bag full of feathers.

Density also increases down the group, when the gases are compared at the same temperature and pressure. Helium is the least dense noble gas. Remember that low density compared with liquids or solids does not mean every noble gas is less dense than air.