Give an explanation of energy band,valence band,and conduction band in a crystal.

(N/A) Some solids have a crystalline structure,which means there is a systematic arrangement of atoms or molecules in them.
When atoms are arranged close to each other,they interact with neighboring atoms,and as a result,the energy levels of the electrons are modified.
The energy levels of inner shell electrons are not significantly affected,but the energy levels of the outer shell electrons (valence electrons) are changed because these electrons are shared by more than one atom in the crystal.
Instead of the widely separated energy levels found in an isolated atom,the electrons in a crystal have closely spaced energy levels. Such a collection of closely spaced energy levels is called an energy band.
The energy band that includes the energy levels of the valence electrons is called the valence band.
The energy band above the valence band is called the conduction band.
Usually,the valence band contains valence electrons,while the conduction band is empty at $0 \ K$.
There is a gap between the conduction band and the valence band. The energy difference of this forbidden region is called the band gap energy $(E_{g})$.
In metals (conductors),the conduction band and valence band overlap,allowing electrons to move easily.
In insulators,there is a large band gap,meaning electrons in the valence band remain bound,and no free electrons are available in the conduction band.
In semiconductors,the band gap is relatively small. If electrons in the valence band receive sufficient external energy to cross the band gap,they enter the conduction band,allowing for electrical conduction.