MO Theory
MO Theory
1.1 Molecular Orbital Theory
1.2 Failure of Hybrid Orbital Theory
 Failures of valence bond/hybrid orbital theory – H2S
and O2
H
H-S-H = 90o
MO Theory
1.3 Failure of Hybrid Orbital Theory
S
H
MO Theory
1.4 Molecular Orbital Theory
 The basic idea
 Pure s and p atomic orbitals (AOs) combine to
produce molecular orbitals (MOs) that “spread
spread out
out”
over several atoms or even the entire molecule
 Start with atoms at known bond distances
 Allow AOs with the correct energy,
energy size
size, shape and
orientation to overlap to create MOs
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MO Theory
MO Theory
1.5 Molecular Orbital Theory
 Basic principles of MO theory
 Total number of molecular orbitals = total number of
atomic orbitals contributed by the atoms
AOs combine to form bonding and antibonding
MOs
 Bonding MOs are lower energy than the parent AOs
 Antibonding MOs are higher energy than the parent
AOs
 Electrons
El t
are assigned
i
d tto th
the MOs
MO ffollowing
ll i aufbau
fb
principle, Hund’s rule and Pauli exclusion principle
 AOs combine most effectively to form MOs when
the AOs are of similar energy (size) and orientation
MO Theory
2.1 Dihydrogen
 Combine the 1s AOs
 Bond order =
MO Theory
2.2 Helium
 Why is it He instead of He2?
2.3 Other Molecules
 Li2
 Bond order of He2 =
2
MO Theory
MO Theory
2.4 Other Molecules and Ions
2.5 Other Ions
 Be2
 He2+
 Li2+
 Be2+
MO Theory
MO Theory
3.1 Shapes of MOs
 -bonding
3.2 MOs from p-orbitals
 -bonding
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MO Theory
MO Theory
3.3 MOs from p-orbitals
3.4 Shapes and Energies of the MOs
 -bonding
MO Theory
MO Theory
3.5 MO Diagram
3.6 MO Diagrams for N2, O2, F2
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MO Theory
MO Theory
3.7 Simple Diatomics
4.1 Other Molecules and Ions
O
MO Theory
4.2 Other Molecules and Ions
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