An atom consists of a proton and a neutron inside the nucleus and the electron orbiting round the nucleus. In an isolated atom, the moving electron carries kinetic and potential energy due to the electric field developed between the electrons and protons. In metal, electrons are loosely bounded in their outermost shell. The outermost shell in solid is known as valence shell and the electrons and the electron revolving round the valence shell in solid are called valence electrons.
Energy level: The definite energy carried by an electron in the orbit of isolated atom is known as energy level.
Energy band: There are more than one atom in a substance due to which energy level are combined and overlapped, resulting the formation of multiple energy level called energy band.
Valance band: The energy possessed by valance electron is called valance band.
Conduction band: The energy band possessed large number of free electron is called conduction band.
Forbidden Band: The valance band is separated by a certain gap. The gap left between the valance band and conduction band is known as forbidden band.