# Why are group 1 elements so low in density?

I was studying the s-block elements and found that they extremely low in density. Lithium is said to be the least dense solid in the entire periodic table and their Cohesive Energies are also very low. Could anyone please explain in detail the reason for this?

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.