In solid-state physics, band theory accounts for the structure and properties of solid materials. Band theory proposes that the available energy states for electrons in solids are not discrete, as in the case of free atoms, but are merged into continuous, or wide, bands. This model is very successful for understanding the difference between conductors, insulators, and semiconductors.

The continuous energy bands are referred to as energy bands, and they are visualized as consisting of overlapping molecular orbitals. For example, sodium has a total of 11 electrons distributed as follows by increasing order of orbital energy: two electrons each in the 1s and 2s orbitals, six electrons in the 2p orbitals, and one electron in the 3s orbital. The next orbital (3p) with 3 suborbitals is empty. To make a sodium crystalline solid, sodium atoms are packed together in a configuration, in which the...