Engineering Chemistry S Datta

SOLID STATE CHEMISTRY 495
At temperatures above the absolute zero, a few of the electrons may be promoted to
energy levels higher than the actual occupied region in bands. The energy distribution of these
electrons follows roughly the Maxwell–Boltzmann law. These electrons are much less in number
than the valence electrons and these are responsible to the heat capacity of the metal and
hence the contribution of electrons to the specific heat of metals is thus small. This conclusion
is in harmony with the Dulong’s and Petit’s law.
Valence and conduction bands
The outermost electrons of an atom i.e., electrons residing in the outermost shell of an
atom are called valence electrons having the highest energy. It is these electrons which are
mostly affected when a number of atoms are brought very close together as during the formation
of a solid.
The band of energy occupied by the valence electrons is called the valence band and is
the highest occupied band. It may be filled by electrons completely or partially.
The next higher permitted energy band is called the empty band or conduction band.
In fact, it may be defined as the lowest unfilled energy band.
In conduction band, electrons can move freely and therefore are known as conduction
electrons. The gap between these two bands, namely valence band and conduction band, is
known as forbidden energy gap.
Band Energy (eV)
0
–1
–8
–12
–16
Conduction band
Valence band
Empty or
partially filled
Fully or
partially filled
Completely filled
inner bands
Fig. 22.20
Now if a valence electron absorbs enough energy, it jumps across the forbidden energy
gap and enters the conduction band. An electron in the conduction band can jump more readily
to an adjacent conduction band than it can jump back to its original position. But, however, if
the conduction electron radiates too much energy, it will suddenly reappear in the valence
band again. It may be noted that the covalent forces of the crystal lattice have their source in
the valence band. So when an electron is raised from the valence band, a covalent bond is
broken and positively charged hole is formed. This hole can travel to an adjacent atom by
taking an electron from that atom, which involves the breaking an existing covalent bond and
then re-establishing a covalent bond by filling up the hole. It has to be kept in mind that holes
are filled by electrons which move from adjacent atoms without passing through the forbidden
energy gap. This is shown in Fig. 22.21 below.