Band Theory (ch. 11) Flashcards
What is band theory good at modelling?
Bonding solids
In band theory, we combine the atomic orbitals of the atoms within a solid crystal to form orbitals that….
Not localized on individual atoms, but delocalized over the entire molecule
How does the band occur?
When there is an atom with N number of electrons, there is infinitely small spacings between the the molecular orbitals so they are no longer discrete energy levels, but rather a band of energy levels
In an atom with N number of electrons and N number of molecular orbitals what does the band look like?
N/2 is made of bonding molecular orbitals that contain N e-
N/2 is made of molecular antibonding orbitals which are completely empty
Why do we call the occupied molecular orbitals the valence band and the unoccupied orbitals the conduction band?
Because electrons become more mobile when they make a transition from the highest occupied molecular orbital into a higher energy empty molecular orbital
Why are metals good conductors according to this theory?
Because the highest occupied molecular orbitals lie in the middle of the band and there is an extremely small energy difference between the highest occupied and the the first unoccupied. Therefore a valence electrons can easily move to the conduction band
In metals the valence band and conduct band are energetically _____
Continuous
Define band gap
an energy gap that exists between the valence band and the conduction band of semiconductors and insulators
In semiconductors and insulators an _____ exists between the valence band and the conduction band
Energy gap/band gap
Conductors have a ____ band gap.
Nonexistent
Semiconductors have a _____ band gap
Small
Insulators have a _____ band gap
Large
Define band theory
a model for bonding in atomic solids that comes from molecular orbital theory in which atomic orbitals combine and become delocalized over the entire crystal
