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Why do alkali metals have relatively low melting points?

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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 have relatively low melting points because of the large interatomic distances in their crystal structures and the weak bond energies associated with such loose atomic arrays.1

As a group, alkali metals possess a looser crystallographic arrangement than any other metallic crystals. These physical characteristics—which also result in low densities, low heats of fusion, and small volume changes upon melting—are a direct consequence of the presence of only one, weakly bound valence electron in a large outer orbital.1

While the melting points of these elements are low compared to other nongaseous groups in the periodic table, they vary within the group, ranging from 179 °C for lithium to 28.5 °C for cesium. Potassium, for instance, has a melting point of 63.28 °C.12