1.1) Energy bands in solids:

(i) In solids, the group of closely lying energy levels is known as energy band.

(ii) In solids the energy bands are analogous to energy levels in an atom.

(iii) In solids the atoms are arranged very close to each other. In these atoms there are discrete energy levels of electrons. For the formation of crystal these atoms come close together, then due to nucleus-nucleus, electron-electron and electron-nucleus interactions the discrete energy levels of atom distort and consequently each energy level spits into a large number of closely lying energy levels.

(iv) The number of split energy levels is proportional to the number of atoms interacting with each other. If two atoms interact then each energy level splits into two out of which one will be somewhat above and another will be somewhat below the main energy level. In solids the number of atoms is very large (» 1023). Hence each energy level splits into large number of closely lying energy levels. Being very close to each other these energy levels assume the shape of a band.

(v) In an energy band there are 1023 energy levels with energy difference of 10–23 ev.

(vi) Curve between energy and distance i.e. U-r curve

(a) When two atoms are interacting

(b) When 1023 atoms are mutually interacting

2) THREE TYPE OF ENERGY BANDS IN A SOLID VIZ.

(a) Valence energy band

(b) Conduction energy band

(c) Forbidden energy gap.

•   Difference between valence, forbidden and conduction energy bands.

(1) The conduction band is also known as first permitted energy band or first band.

(2)  As there are energy levels f electrons in an atom, similarly there are three specific energy bands for the electrons in the crystal formed by these atoms as shown in the figure

(3) Completely filled energy bands: The energy band, in which maximum possible number of electrons are present according to capacity is known as completely filled bank.

(4) Partially filled energy bands: The energy band, in which number of electrons present is less than the capacity of the band, is known as partially filled energy band.

(5) Electric conduction is possible only in those solids which have empty energy band or partially filled energy band.

(6) Energy gap or Band gap (Eg):

(a) The minimum energy which is necessary for shifting electrons from valence band to conduction band is defined as band gap (Eg)

(b) The forbidden energy gap between the valence band and the conduction band is known as band gap (Eg). i.e. Eg = Ec – Ev

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