In the case of single isolated atom, there are single energy levels as shown. Each isolated has only a certain numbers of orbits available which represents energy levels for the electrons. The energy levels are numbered n=1,2… in increasing order of energy.

When atom bond together to form a solid, the orbit of an electron is influenced not only by charges in its own atom but by nuclei and electron of every atom in solid, therefore each electron occupies a different position inside the solid, no two electrons have exactly the same pattern of surrounding charges. As a result, the orbits of electron are different.

The simple energy level diagram in first fig. now modifies to that shown in second fig. There are millions of electrons belonging to the first orbit of atom in solid. Each of them has different energy, since there are millions of first orbit electrons, the closely spaced energy levels differing very slightly in energy from a cluster or band. Similarly, the second orbit & the higher orbit electrons also forms bands. Thus now we have first energy bond, second energy bond, third energy bond & so on.

These energy bands help us to classify the material into conductors, isolators & semiconductor as follows –

1) Conductor

Valence band (usually filled) & conduction band (usually empty) are overlapping. Thus there is no energy or forbidden gap. E.g. platinum, gold, silver, copper.

2) Insulator