Explanation:

* Metallic bonding: Both lithium and sodium are alkali metals, and their softness is due to their weak metallic bonding.

* Atomic size: Lithium has a smaller atomic radius than sodium. This means that the lithium atoms are closer together in the metallic lattice, resulting in weaker metallic bonds.

* Electropositivity: Lithium is more electropositive than sodium, meaning it has a greater tendency to lose electrons. This further weakens the metallic bonding in lithium.

Therefore, due to its smaller atomic size and higher electropositivity, lithium has weaker metallic bonds and is softer than sodium.