Stronger metallic bonding in lithium

Steps:

• Both lithium and sodium are alkali metals with one valence electron forming a "sea" of delocalized electrons.
• Metallic bonding strength depends on electrostatic attraction between positive ions and the electron sea.
• Lithium ions are smaller than sodium ions due to lower atomic radius in period 2 versus period 3.
• Smaller ions bring positive charges closer to electrons, increasing attraction and requiring more energy to melt.

Why D is correct:

• Metallic bond strength follows Coulomb's law: force increases as distance decreases, so lithium's smaller ions create stronger bonds than sodium's larger ones.

Why the others are wrong:

• A: Both have one valence electron, so equal valency electrons.
• B: Sodium is more reactive due to lower ionization energy, but reactivity doesn't directly affect melting point.
• C: Sodium's softness relates to weaker bonding, but this describes the effect, not the cause.

Final answer: D