What is a Conduction Band?

Used in quantum mechanics, the term conduction band refers to an area of combined orbitals, or a band, for electrons in a molecule. Unlike the valence band, the conduction band rarely contains electrons. In excited states, electrons will move into the conduction band momentarily before releasing their energy and falling back into lower electron orbitals. Understanding the behavior of electrons with regard to the this band is helpful in understanding the way various substances behave. In quantum mechanics, the concept of the conduction band is addressed in band theory.

Atoms are arranged with protons — positively charged particles — and neutrons — neutral particles — clustered in the center. Electrons — tiny negatively charged molecules — orbit the central cluster, similar to the way the planets in the solar system orbit the sun. Like the planets, electrons have set orbits. Unlike the planets, however, electrons can move into a different orbit if they gain enough energy.

Generally, electrons are found in the lower orbitals of an atom. Electrons will always fill the lowest orbital first, only moving to the next when the first is filled. This natural placement is called the atom's ground state.

Valence electrons of one atom, or those usually found in the outermost band of the ground state orbitals, are capable of being shared with other atoms. In covalent bonds, valence electrons of multiple atoms share their orbitals. The original orbitals of the valence electrons blur together, creating a valence band in the molecule.

When electrons gain energy, or reach an excited state, they can move to higher orbitals, found in the conduction band. Electrons must have enough energy to jump over a non-electron area, or band gap, in order to reach the conduction band. Since electrons ultimately prefer being in ground state, once in the conduction band, they release energy in the form of light photons and fall back to their valence band orbitals. The total time an electron is in the conduction band is less than one second.

The ability of electrons to reach the conduction band determines the electrical conductivity of an object. Different substances have different band gap sizes, so some substances require less energy to move electrons between orbitals. For example, conductors have a small band gap, so electrons do not require much energy to jump this minimal gap and reach the conduction band. This is why conductors are ideal for conducting electricity. Conversely, insulators have a very large band gap, so they require significantly more energy for electrons to make the jump and hence do not conduct electricity well.