energy band formed due to the valence orbitals is called valence band (VB) and that formed due to the unoccupied orbitals to which electrons can jump when energised is called the conduction band (CB). The energy gap between the valence band and the conduction band is called forbidden energy gap ( E g ). Electrons cannot exist in the forbidden energy gap. A simple pictorial representation of the valence band and conduction band is shown in Figure .
(a). E V represents the maximum energy of the valence band and E C represents minimum energy of the conduction band. The forbidden energy gap, E g = E C – E v . We know that the Coulomb force of attraction between the orbiting electron and the nucleus is inversely proportional to the distance between them.
Therefore, the electrons in the orbitals closer to the nucleus are strongly bound to it. Hence, the electrons closer to nucleus require a lot of energy to be excited. The electrons in the valence band are loosely bound to the nucleus and can be easily excited to become free electrons. .
. Energy band diagram of solids In an isolated atom, the electronic energy levels are widely separated and are far apart and the energy of the electron is decided by the orbit in which it revolves around the nucleus. However, in the case of a solid, the atoms are closely spaced and hence the electrons in the outermost energy levels of nearby atoms influence each other. This changes the nature of the electron motion in a solid from that in an isolated atom to a large extent.
The valence electrons in an atom are responsible for the bonding nature. Let us consider an atom with one electron in the outermost orbit. It means that the number of valence electrons is one. When two such atoms are brought close to each other, the valence orbitals are split up into two.
Similarly the unoccupied orbitals of each atom will also split up into two. The electrons have the choice of choosing any one of the orbitals as the