An atom has electron, protons and neutrons. According to the Bohr’s atomic model theory nucleus of an atom contains protons and neutrons. Neutrons has no charge and protons have positively charged electrons moves around the nucleus in different orbits far away from this nucleus.
A configuration is shown of silicon. Silicon contains 14 protons and 14 electrons. First shell contains 2 electrons, second shell contains 8 electron and third shell contains 4 electrons. In nucleus 14 protons contain so the atom is electrically neutral with 14 electrons and 14 protons. At third shell there is 4 electrons called valance electrons. Valence electrons carries chemical and physical properties of the material.
In figure (a) the nucleus and different orbits and in figure (b) different energy states are shown. Electrons can only revolve in the prescribed first, second, third orbits and so on shown in the figure. As there are no orbit between r1 and r2 and r3, so no electron can stay there. That means there are forbidden regions for electrons.
In case of a free atom each electron possess definite energy in each orbit. This energy levels are separated. In a solid there are huge number of atoms closely packed in a given volume (about 1029 atom per cubic meter) in a periodic pattern. Energy levels in an individual atom are formed because of the interaction between positively charged nucleus and an electron. In case of a solid, a given electron in the same orbit is under the influence of large number of electrons presents in the nearby atoms. Since a crystal consists of a large number of energy levels to deal with. Each atomic level will split in about 1029 levels. Because of such large number of levels in a small range of energy (say a few eV), one may consider these energy levels to be continuously distributed within a range of energy.
The energy levels as a result become widened and form what is called a continuous band.
Energy band: Electrons of an isolated atom have discrete energy levels. Since a crystal consist of large number of atoms we have an enormously large number of levels to deal with. Each atomic level will split into an enormous number of levels in a small range of energy range. One may regard these energy levels to be continuously distributed within a range of energy. The energy levels become widened and form a band which is called a continuous energy band.
Valence band: The band of energy occupied by the valence electrons is called the valence band and is obviously the highest occupied band.
It may be completely filled or partially filled with electrons but can never be empty.
Conduction band: Unoccupied sub shells or orbitals contain some definite energy levels in each isolated atom. These levels also split in crystal forming another band or group called conduction band. It may either be empty or partially filled with electrons. In fact it may be defined as the lowest unfilled energy band. In conduction band electrons can move freely and hence are known as conduction electrons. Electrons in this band take part in the passage of electricity.
Forbidden energy gap or forbidden gap: Valence band is separated from the conduction band with an energy gap. This gap contains no energy level for an electron. This gap is called as forbidden energy gap or forbidden gap. Characteristics of electrical behaviors of elements and compounds can be explained in the light of energy of electrons in the valence band and the conduction band. Actually the role of valence band and conduction band is very important for electrical behavior of different substances.