What explains why most metals expand when heated?

• A. The metal becomes heavier.
• B. The metal's chemical composition changes.
• C. The lattice contracts.
• D. As heat is absorbed, the electrons move faster and expand their orbits in the molecules of the metal.

Answer: (D)

When a metal is heated, its atoms gain kinetic energy and vibrate more vigorously around their lattice positions. The forces between neighboring atoms act like springs, and because the interatomic potential is not perfectly symmetric, larger vibrational amplitudes push the average distance between atoms outward. In other words, the lattice expands as the average spacing increases with temperature. The delocalized electrons help hold the lattice together, but the macroscopic expansion mainly comes from the ions moving farther apart due to stronger vibrations and the anharmonic nature of the bonding, not from electrons simply moving to larger orbits.