STRUCTURE OF ATOM AND ENERGY BANDS.

Atomic Particles. Atoms consist of three basic particles: protons, electrons, and neutrons. The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged).



Each band is formed due to the splitting of one or more atomic energy levels. Therefore, the minimum number of states in a band equals twice the number of atoms in the material. ... This situation occurs in materials consisting of atoms, which contain only one valence electron per atom.

In solid-state physics, the electronic band structure(or simply band structure) of a solid describes the range of energies an electron within the solid may have (called energy bands, allowed bands, or simply bands) and ranges of energy that it may not have (called band gaps or forbidden bands).

An atom is consisting of a nucleus and electrons orbiting the nucleus.
The electrons cannot orbit the nucleus at any distance in the atomic space surrounding the nucleus, but only certain, very specific orbits are allowed, and only exist in specific discrete levels. These energies are called energy levels. A large number of atoms gather to form a crystal, and interacts in a solid material, then the energy levels became so closely spaced that they form bands. This is the energy band.
Metals, semiconductors and insulators are distinguished from each others by their band structures. Their band structures are shown in the figure below.