#### Answer

For an isolated atom, there exists discrete electron energy states (arranged into shells and subshells). Each state may be occupied by, at most, two electrons, which must have opposite spins.
On the other hand, an electron band structure is found for solid materials. Within each band exists closely spaced yet discrete electron states, each of which may be occupied by, at most, two electrons, having opposite spins. The number of electron states in each band will equal the total number of corresponding states contributed by all of the atoms in the solid.