Page 13 - 12-phy-17 PHYSICS OF SOLIDS

17. Physics of Solids eLearn.Punjab

The electrons in the outermost shell of an atom are called valence electrons and the energy band

occupying these electrons is known as valence band. It is obviously the highest occupied band. It

may be either completely filled or partially filled with electrons and can never be empty. The band
above the valence band is called conduction band. In conduction band, electrons move freely and
conduct electric current through solids. That is why the electrons occupying this band are known

as conductive electrons or free electrons. Any electron leaving the valence band is accommodated

by this band. It may be either empty or partially filled with electrons. The bands below the valence
band are normally completely filled and as such play no part in
Do You Know?
the conduction process. Thus, while discussing the electrical

Glass is also known as solid liquid conductivity we will consider only the valence and conduction
because its molecules are irregularly
arranged as in a liquid but fixed in bands.
their relative positions.
Insulators

Insulators are those materials in which valence electrons are bound
very tightly to their atoms and are not free. In terms of energy
bands, it means that an insulator, as shown in Fig. 17.6 has

(a) an empty conduction band (no free electrons)

(b) a full valence band
(c) a large energy gap (several eV) between them

Conductors

Conductors are those which have plenty of free electrons for
electrical conduction. In terms of energy bands, conductors are

those materials in

which valence and
conduction bands
largely overlap each Fig. 17.6

other (Fig. 17.7).

There is no physical
distinction between the two bands which ensures the
availability of a large number of free electrons.

Fig. 17.7

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