Metallic Bond Strength Increases Across Period 3

Steps:

• Identify trend: Melting points rise from Na (98°C) to Mg (650°C) to Al (660°C) due to stronger metallic bonds.
• Recall metallic bonding: Strength depends on number of delocalised electrons per ion and ion size/charge.
• Analyze from Na to Al: Atomic number increases, adding electrons to valence shell, forming +1, +2, +3 ions with 1, 2, 3 delocalised electrons.
• Conclude: More delocalised electrons enhance electrostatic attraction, explaining higher melting points.

Why A is correct:

• More delocalised electrons per metal ion increase the attraction between positive ions and electron sea, per metallic bonding theory, strengthening bonds and raising melting points.

Why the others are wrong:

• B: Ion charge increases (+1 to +3), but this alone doesn't account for the electron contribution to bond strength.
• C: Ion radius decreases due to higher effective nuclear charge, but the primary driver is electron density, not size alone.

Final answer: A