Describe in brief the Formation of energy bands in solids and hence explain how it helps to classify the solid in to conductors and insulators.

Energy Bands in Solids

According to K conig-Penney model it has been observed that in solids energy bands exist instead of single energy levels. The formation lf energy bands in solids can also be visualized from the following experiment. In an isolated atom, the electrons are tightly bound and have discrete sharp energy levels. When the wave functions of electrons in different atoms overlap the energy levels corresponding to these wave functions split resulting two levels. The splitting depends on the extent of overlapping or the distance between the atoms. If more and mere atoms are brought closer, more and more closely spaced levels are formed. In a solid of N atoms each main energy level splits into a band of N closely spaced energy levels. The levels are so close that they form energy band. The energy separation E between successive levels in an allowed band is of the order of e V. The width of the energy band depends on the extent of overlapping of orbital. Hence the outer band is very wide. As we go to inner energy levels or orbits the width of the energy bands decreases. The energy bands are separated by gaps called forbidden energy gap. The splitting of energy levels and the energy bands at equilibrium interatomic distance are shown in the figure. The energy band containing valence electron is called valence band. The energy band above the valence band is called conduction band. The electrons forming the covalent bands between the atoms are supposed to be present in the covalent band. The excited electrons, which are removed from the atoms and move freely in the crystal, are called free electrons or conduction electrons. They are supposed to be present in the conduction band. The electrical properties of solids depend on the condition of valence band, conduction band and the forbidding energy gap in between them. The inner energy bands have no significance.