What is Energy band?, what is valence band, conduction band & forbidden energy gap?

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In gaseous substances, the arrangement of molecules is not close. In liquids, the molecular arrangement is moderate.But in solid, the molecules are so closely arranged,that the electrons in the atoms of molecules tend to move into the orbitals of neighboring atoms.Hence the electron orbitals overlap when the atom come together. Due to the intermixing of atoms in solid,instead of single energy levels formed. These set of energy levels,which are closely packed are called as Energy Band.

Valence Band- 
The electrons in the outhermost shell are known as valence electrons. These valence electrons contain a series of energy levels and form an energy band known as a Valence Band.
The valence band has the highest occupied energy.

Conduction Band-
The valence electrons are not tightly held to the nucleus.Due to which a few of these valence electrons leave the outhermost orbit even at room temperature & become free electrons. The free electrons conduct current in conductor and known as Conduction Electrons. The conduction band is one that contains conduction electrons & has the lowest occupied energy levels.

Forbidden Energy Gap-
The gap between valence band & conduction band is referred to as forbidden gap. As the name suggests, the forbidden gap don't have any energy & no electron stay in this band. If the forbidden energy gap is greater, then the valence band electrons are tightly bound or firmly attached to the nucleus.We require some amount of external energy that is equal to the Forbidden Energy Gap.





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