This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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Energy band is a continuous range of allowed and closely spaced energy levels for electrons in solids, e.g. crystalline materials. Individual atoms can have only certain quantized energy levels as shown on the left in Figure 3791. As atoms bond to form a solid, the wavefunctions of the individual electrons overlap with those of electrons confined to neighboring atoms, therefore, each energy level widens to accommodate the shifted levels of adjacent atoms. The energy bands can be though of as the collection of the individual energy levels of electrons surrounding each atom. Figure 3791 shows the difference of energies between an isolated Na atom and a Na solid.

Figure 3791. Difference of energies between an isolated Na atom and a Na solid.