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Valence bond theory

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This compound is known but is extremely reactive. ... way to explain this is to postulate that an excited electronic state of carbon forms ... – PowerPoint PPT presentation

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Title: Valence bond theory


1
Chemical Bonding Theory
  • Valence bond theory
  • The bonding in carbon because the valence shell
    electron configuration is 2s22px12py1, we would
    expect the simplest compound between C and H
    would be CH2.
  • This compound is known but is extremely reactive.
  • CH4 is the compound between H and C with one atom
    of C per molecule.
  • One way to explain this is to postulate that an
    excited electronic state of carbon forms
  • Hybrid orbitals are formed by mixing the 2s
    orbital with the three 2p orbitals. These four
    new orbitals are degenerate
  • The hybrid sp3 orbitals can form four CH bonds.
    These bonds point to the corners of a regular
    tetrahedron.

2
Chemical Bonding Theory
  • Valence bond theory
  • The bonding in carbon
  • One advantage of this scheme is that four bonds
    are formed between C and H instead of two bonds.
    Bond formation is exothermic and produces a more
    stable state for carbon and hydrogen. This
    process more than compensates for the energy
    required to form the hybrid orbitals.
  • The four new sp3 orbitals are one fourth s and
    three-fourths p in character and are fatter
    than a p orbital.
  • Each sp3 orbital has a nodal plane containing the
    nucleus. The lobes are not symmetrical in
    size like a p orbital.

3
sp3 hybrid orbitals a. A single sp3 hybrid
orbital showing the two regions of electron
density. b. The four sp3 hybrid orbitals are
directed at the corners of a tetrahedron.
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5
Chemical Bonding Theory
  • Valence bond theory
  • H2O revisited if the O is hybridized sp3,
  • There are 2 electron pairs in two sp3 orbitals
    and two unpaired electrons in the other two sp3
    orbitals
  • This allows for the formation of two bonds
    between H and O
  • The H-O-H bond angle is predicted to be 109.5o,
    but its found to be 104.5o
  • The decrease of 5.0o is due to non-bonded
    electron pair - bonded electron pair repulsions
    from the two pair of non-bonded electrons.
  • This is easier to explain than the 14.5o increase
    from the earlier model not involving orbital
    hybridization.

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7
Chemical Bonding Theory
  • Valence bond theory
  • NH3 revisited if the N is hybridized sp3,,
  • There is one electron pair in one of the sp3
    orbitals and three unpaired electrons in the
    other three sp3 orbitals.
  • This allows for the formation of three bond
    between H and N
  • The H-N-H bond angle is predicted to be 109.5o,
    but its found to be 107o
  • The decrease is only 2.5o caused by repulsion
    between the non-bonded electron pair and the
    bonding pairs of electrons.
  • NH3 is a good base indicating the non-bonded
    electron pair is available for donation to
    acids. This would be difficult if this pair were
    in an s orbital on N.

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9
Chemical Bonding Theory
  • Valence bond theory
  • BF3 Only three electron pair bonds are formed.
  • The orbital hybridization scheme produces three
    electrons in three equivalent sp2 orbitals.
  • Overlap between each sp2 orbital and a p orbital
    in F with one unpaired electron produces three
    electron pair bonds.
  • The three sp2 orbitals point to the corners of a
    planar triangle.

For clarity, the non-bonding electrons on F are
not shown Note, there is a left over,
unhybridized p orbital on B. When BF3 reacts
with NH3, the NH3 provides the electrons for a
coordinate- covalent bond. In this case, B will
rehybidize to four sp3 orbitals
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11
Chemical Bonding Theory
  • Valence bond theory
  • BeCl2 Only two electron pair bonds are formed.
  • The orbital hybridization scheme produces three
    electrons in two equivalent sp orbitals.
  • Overlap between each sp orbital and a p orbital
    in Cl with one unpaired electron produces three
    electron pair bonds.
  • The three sp orbitals point in a strait line.

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13
Chemical Bonding Theory
  • Valence bond theory
  • PCl5 sp3d
  • SF6 sp3d2

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15
Chemical Bonding Theory
  • Single bonds in the valence bond theory, single
    bonds are made up of atomic orbitals that are
    cylindrically symmetric about the line joining
    the bonded atoms.
  • Such bonds are called sigma - s - bonds.
  • Overlap of 2 s orbitals in H2
  • Overlap of an s and a p orbital in HF
  • Overlap of 2 p orbitals in F2
  • Overlap of an s or p orbital with an spy hybrid
    orbital - BeCl2, CH4, PCl5, etc.
  • Multiple bonds the second bond involves overlap
    of two p orbitals on different atoms that are
    perpendicular to the internuclear axis

The internuclear axis contains a nodal
plane Electron density is above and below the
nodal plane These bonds are called pi - p - bonds
Single bonds in valence bond theory are s
bonds. Double bonds in valence bond theory are
one s bond and one p bond. Triple bonds in
valence bond theory are one s bond and two p
bonds.
16
Chemical Bonding Theory
Multiple bond examples Ethylene
The 2 s bond from each C to H involve overlap of
C sp2 and H s orbitals The single s bond between
each C involves overlap of C sp2 orbitals The
single p bond between each C involve p overlap of
C unhybridized p orbitals
2 lobes of p bond
The p bond locks this molecule into a planar
structure all 6 atoms are in the same plane.
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Chemical Bonding Theory
  • Multiple bonds example acetylene

The s bonds between C and H involve overlap of C
sp and H s orbitals The s bond between C and C
involve overlap of C sp orbitals The p bonds
between each C involve overlap of two pairs of
unhybridized p orbitals.
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