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

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A periodic table element box for potassium shows atomic number 19, symbol K, atomic weight 39.0983, electron configuration [Ar]4s1, and name potassium. Icons indicate it is an alkali metal, solid at room temperature, body-centered cubic crystal structure, and has strongly basic higher-valence oxides. A key explains the symbols for alkali metals, solid state, body-centered cubic structure, and strong basicity. — potassium Properties of potassium.

The alkali metals, including lithium, sodium, and potassium, possess low densities primarily due to their loose crystallographic arrangement and weak bond energies.1

These elements are characterized by a body-centered cubic structure with large interatomic distances, which results in a less dense packing of atoms compared to other metallic crystals. Additionally, the large size of an alkali metal atom contributes to this low density, as each atom contains only one, weakly bound electron in a large outer s-type orbital. These structural factors collectively ensure that lithium, sodium, and potassium remain less dense than water.1