band theory

(noun)

In a solid, those ranges of energy that an electron is allowed to have.

Related Terms

Examples of band theory in the following topics:

  • Conductors

    • In describing conductors using the concept of band theory, it is best to focus on conductors that conduct electricity using mobile electrons.
    • According to band theory, a conductor is simply a material that has its valence band and conduction band overlapping, allowing electrons to flow through the material with minimal applied voltage.
    • Band theory models the behavior of electrons in solids by postulating the existence of energy bands.
    • Bands may also be viewed as the large-scale limit of molecular orbital theory.
    • Apply the concept of band theory to explain the behavior of conductors.

  • Semiconductors

    • Most of the states with low energy (closer to the nucleus) are occupied, up to a particular band called the valence band.
    • In semiconductors, only a few electrons exist in the conduction band just above the valence band, and an insulator has almost no free electrons.
    • In semiconductors, the band gap is small, allowing electrons to populate the conduction band.
    • As the energy in the system increases, electrons leave the valence band and enter the conduction band.
    • Compare N-type and P-type semi-conductors, distinguishing them from semi-conductors and insulators using band theory.

  • Coloring Agents

    • This approach is described by the ligand field theory (LFT) and the molecular orbital theory (MO).
    • Ligand field theory, introduced in 1935 and built from molecular orbital theory, can handle a broader range of complexes.
    • The chemical applications of group theory can aid in the understanding of crystal or ligand field theory, by allowing simple, symmetry-based solutions to the formal equations.
    • Most transitions that are related to colored metal complexes are either d–d transitions or charge transfer bands.
    • A charge transfer band entails promotion of an electron from a metal-based orbital into an empty ligand-based orbital (Metal-to-Ligand Charge Transfer or MLCT).

  • Doping: Connectivity of Semiconductors

    • In the atomic lattice of a substance, there is a set of filled atomic energy "bands" with a full complement of electrons, and a set of higher energy unfilled "bands" which have no electrons.
    • The highest energy band contains valence electrons available for chemical reactions.
    • The conduction band is the band above the valence band.
    • Electrons in the conduction band are free to move about in the lattice and can conduct current.
    • In order for a substance to conduct electricity, its valence electrons must cross the band gap, which is the energy gap between the valence band and conduction band.

  • Properties of Waves and Light

    • In actuality, however, the pattern changes to one with a series of alternating light and dark bands.
    • Young's experiment, performed in the early 1800's, played a vital part in the acceptance of the wave theory of light, superseding the corpuscular theory of light proposed by Isaac Newton, which had been the accepted model of light propagation in the 17th and 18th centuries.

  • Vibrational Spectroscopy

    • Absorption bands associated with C=O bond stretching are usually very strong because a large change in the dipole takes place in that mode.
    • The general regions of the infrared spectrum in which various kinds of vibrational bands are observed are outlined in the following chart.
    • Note that the blue colored sections above the dashed line refer to stretching vibrations, and the green colored band below the line encompasses bending vibrations.
    • The complexity of infrared spectra in the 1450 to 600 cm-1 region makes it difficult to assign all the absorption bands, and because of the unique patterns found there, it is often called the fingerprint region.
    • Absorption bands in the 4000 to 1450 cm-1 region are usually due to stretching vibrations of diatomic units, and this is sometimes called the group frequency region.

  • Group Frequencies

    • Standard abbreviations (str = strong, wk = weak, brd = broad & shp = sharp) are used to describe the absorption bands.

  • Color

    • Transition metal complexes are often colored due to either d-d or change band electron transitions induced by the absorption of light.
    • Most transitions that are related to colored metal complexes are either d–d transitions or charge band transfer.

  • Transition State Theory

    • Transition state theory (TST) describes a hypothetical "transition state" that occurs in the space between the reactants and the products in a chemical reaction.
    • TST is used to describe how a chemical reaction occurs, and it is based upon collision theory.
    • TST is also referred to as "activated-complex theory," "absolute-rate theory," and "theory of absolute reaction rates."
    • This third postulate acts as a kind of qualifier for something we have already explored in our discussion on collision theory.
    • Transition state theory is most useful in the field of biochemistry, where it is often used to model reactions catalyzed by enzymes in the body.

  • Kinetic Molecular Theory and Gas Laws

    • Kinetic Molecular Theory explains the macroscopic properties of gases and can be used to understand and explain the gas laws.
    • The following are the basic assumptions of the Kinetic Molecular Theory:
    • Uses the kinetic theory of gases to explain properties of gases (expandability, compressibility, etc. )
    • Reviews kinetic energy and phases of matter, and explains the kinetic-molecular theory of gases.
    • Express the five basic assumptions of the Kinetic Molecular Theory of Gases.