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23.5 Metallic Bonding
MO theory helps us explain the bonding in metals. Recall that n AOs
are used to make n MOs.
Electrons delocalized (distributed) over several atoms
Overlap of valence atomic orbitals
Extend over the whole structure
# MOs large
continuous band
The energy differences between orbitals are small, so promotion of
electrons requires little energy: electrons readily move through the
metal.
Empty orbitals are close in energy
Electrons promoted by electrical,
thermal excitations
MO energy levels for Nan molecules
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The following picture shows the electron populations of the
composite s-d bands for three different transition metals as you go
across a period:
The melting points of chromium and zinc are 1907°C and 420°C,
respectively. Use band theory to account for the difference
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The following pictures represent the electron population of the
composite s-d band for three metals—Ag, Mo, and Y:
Mo
Y
Ag
The following pictures represent the electron population of the
composite s-d band for three metals—Hf, Pt, and Re:
Hf
Pt
Re
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12.1 Metals and Semiconductors
Materials may be classified according to their band structure.
Metals
Good electrical conductors.
Valence electrons
partially fill band
Semiconductors
Band structure has an energy gap separating totally filled bands and
empty bands.
Eg = band gap energy
VB = valence band
CB = conduction band
Insulators
Eg ~ 350 kJ /mol or larger
Ceramic
Hard brittle
Bands of MO energy levels for (a) a metallic conductor, (b) an
electrical insulator, and (c) a semiconductor.
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