CHM 120 CHAPTER 11: Theories of Covalent Bonding

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CHM 120CHAPTER 11 Theories of Covalent
Bonding

• Dr. Floyd Beckford
• Lyon College

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VALENCE BOND THEORY

• Complementary to the VSEPR it describes
• how bonds are formed
• Basic idea covalent bonds are the result of
• electron pairing through overlap of atomic
• orbitals
• - the greater the overlap, the stronger
• the bond

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• Hybridization
• Involves the combination of two different
• sets of atomic orbitals to form one equivalent
• set
• Consider the case of methane, CH4

• Requires four degenerate orbitals

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• Hybridize the 2s and 2p orbitals
• Generates 4 equivalent, degenerate hybrid
• atomic orbitals

• sp3 orbitals overlap with 1s orbital on the H
  atom

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• Hybridization is a general process
• 1 s 1 p ? 2 sp orbitals
• 1 s 3 p 1 d ? 5 sp3d orbitals
• Each set of hybrid orbitals is arranged with
• a specific geometry
• a sp3 atom ? tetrahedral geometry
• a sp3d atom ? trigonal bipyramidal
• a sp3d2 atom ? octahedral

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• Some elements form multiple bonds
• A sigma, ?, bond results from head-on
• overlap of atomic orbitals
• - have electron density centered about
• the molecular axis
• A pi, ?, bond results from side-on overlap
• of atomic orbitals
• - have electron density above and below
• the molecular axis

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MOLECULAR ORBITAL THEORY

• Probably the best of the bonding models
• In the MO THEORY the combination of
• atomic orbitals on different atoms form
• molecular orbitals so that electrons in them
• belong to the molecule as a whole

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• Description of atomic orbitals applies to MOs
• - a MO can have only a specific energy
• - a MO can hold only two electrons
• - the Aufbau principle holds
• Atomic wave functions combine in two ways
• in-phase and out-of-phase
• Two AO overlap in-phase ? a bonding
• molecular orbital is formed

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• Energy of the bonding MO is always lower
• than the original AOs from which it is formed
• When two AO overlap out-of-phase ? an
• anti-bonding molecular orbital results
• - energy of the antibonding MO is always
• higher than the parent AOs

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• When two 1s orbitals overlap a sigma
• molecular orbital is formed
• - the bonding MO designated ?
• - the antibonding MO designated ?
• Electronic arrangement of MOs can be
• depicted by MO diagrams
• Consider the formation of H2 and He2

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• The nature of the bonding in a molecule may
• be determined by looking at the bond order
• Bond order
• ( of bonding e- - of antibonding e-)/2
• p orbitals may also overlap
• 1. Head-on overlap ? ? bonds
• 2. Side-on overlap ? ? bonds

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2px 2px ? ?2p (?2p) 2py (or z) 2py (or
z) ? ?2p (?2p) e.g. O2 and N2
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• A blend of the MO and VB theories gives
• rise to the concept of delocalization
• e.g. CO32-
• VB theory
• - each C is sp2 hybridized
• - the unhybridized p orbital overlaps with
• a p orbital on O atom to form a pi bond
• - leads to three resonance forms

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• MO theory
• - bonding involves simultaneous overlap of
• the unhybridized C p orbital with the p
• orbital on all three oxygen atoms
• - overlap forms a delocalized ? system
• - electrons in the ? system belongs to
• the molecule