A solid is a quantum system because the behavior of its electrons and atoms is governed by quantum mechanics, not classical physics.
• Atoms in a solid form a periodic lattice, and electrons in this lattice occupy quantized energy states.
• Instead of discrete atomic orbitals, electrons form energy bands (valence band, conduction band).
• The allowed and forbidden states (band gaps) explain whether a solid is a metal, semiconductor, or insulator.
• Many solid properties — electrical conductivity, magnetism, heat capacity, superconductivity — cannot be explained without quantum mechanics.