Why do most metals expand when they are heated?

• A. Because their weight decreases.
• B. Because their electrons gain mass.
• C. As heat is absorbed, the electrons move faster and expand their orbits in the molecules of the metal.
• D. Because external pressure increases.

Answer: (C)

When metals heat up, energy is transferred to the atoms, and they start vibrating more vigorously. In a solid, these increased vibrations effectively push atoms slightly farther apart on average, so the lattice expands. This happens because the forces between atoms are not perfectly symmetric as distance changes; the potential energy curve is such that larger vibrations lead to a larger average spacing. In metals, there’s also the idea of delocalized electrons that don’t constrain the lattice enough to prevent this expansion, so the overall effect is an increase in size with temperature.

The option describes heating as making electrons move faster and expand their orbitals, which is a simplified way of capturing that increased energy leads to more space being occupied around atoms and, therefore, a larger overall structure. The other ideas—weight changing, electrons gaining mass, or external pressure increasing—don’t explain why the metal actually gets larger when heated.