Valence bond theory

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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.

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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.

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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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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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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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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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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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Chemical Bonding Theory

• Valence bond theory
• PCl5 sp3d

• SF6 sp3d2

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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.
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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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