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The electrons of an atom occupy certain energy levels when the atom is far from other atoms. When a large number of atoms are assembled to form a conducting metallic solid, the energy levels of the valence electrons are displaced in position to form a conduction band, as discussed in Section 28-4. The valence electrons occupy energy levels within the conduction band and are relatively free to drift from atom to atom within the metal. In accordance with a fundamental physical principle, known as the Pauli exclusion principle, only 1 electron may occupy a particular energy level. The conduction electrons are not all in the state of lowest energy, for only 1 electron can occupy that level. The conduction electrons tend to fill the bottom region of the conduction band.