Why are group 1 elements so low in density?
Because it is energetically easier for an atom (other than hydrogen) with only one outer electron to lose that electron, than to gain seven more, in order to have an inert gas configuration, that electron has a high probability of wandering off. In bulk samples, that causes metallic conduction. Having that lone, loosely-attached electron increases the effective atomic radius.

Hydrogen is an interesting special case. At the ionization potential is more than twice that of lithium, for example, and $\ce{H2}$ has a covalent, rather than metallic, bond. The gas is diatomic, with very little attraction between molecules, so its density is low. However, at sufficient pressure, it is believed that hydrogen would behave as a metal.