Why are lithium, sodium, and potassium less dense than water?

The low density of lithium, sodium, and potassium is a direct result of their atomic structure and the nature of their crystalline arrangement. These alkali metals possess large atomic sizes due to the presence of a single, weakly bound valence electron in a large outer s-type orbital.1

In their solid state, these elements form a body-centred cubic crystal structure. This arrangement is characterized by large interatomic distances and weak bond energies, creating a relatively loose packing of atoms compared to other metallic crystals. These factors—the large atomic size and the loose crystallographic array—are responsible for the low densities of these elements, which allow lithium, sodium, and potassium to be less dense than water.1