CHE 104 Lecture 11: Lecture Notes 11

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6 Feb 2017

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February 3rd, 2017
Dr. Townsend
Lecture #11
Bonding in Metals and Semiconductors: Band Theory
Molecular orbital theory (MO) was introduced in chapter 9 to rationalize
covalent bonding in molecules
Dictionary Definition: molecular orbital (MO) theory is a method for
determining molecular structure in which electrons are not assigned
to individual bonds between atoms, but are treated as moving
under the influence of the nuclei in the whole molecule
Molecular orbital theory can also be used to describe metallic bonding
Metals can be thought of as a “supermolecule”
Metallic bonding is described as delocalized
The electrons are associated with all the atoms in the crystal and
not with specific bonded atoms
Valence electrons are free to move around the entire atom because
they are no localized (aka delocalized)
The theory of the metallic bonding is call BAND THEORY
Recall that if we combine a 2s orbital on one atom with 2s orbital on
another atom, we get two molecular orbitals, a 𝝈*2s bonding orbital
𝝈*2s antibonding orbital
If we combine a large number (n) of atomic orbitals we get n
molecular orbitals
n/2 bonding orbitals and n/2 antibonding orbitals
These will form a continuous band of energy levels lower in energy
and a continuous band higher in energy than the original orbitals
The lowest energy for a system occurs with all electrons in orbitals
with the lowest possible energy
● The lower
the energy the more stable
The Fermi band is the highest filled level at 0 K
Look at diagram and the top level that is filled with atoms is
the fermi band
In order for a material to conduct electricity, electrons must move
from the lower band (called the valence band) into the higher band
(called the conduction band)
Once conduction band, the electrons can move toward the positive
pole of the electric field and the resulting hole in the valence band
can shift towards the negative pole
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