Valence Bond Theory and Hybridization

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Valence Bond Theory and Hybridization
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• Linus Pauling
• Author Nature of the chemical bond
• Received Nobel prize in 1954 for his work
• Introduced concept of orbital hybridization

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Valence bond theory the basic ideaTwo
half-filled orbitals overlap to form a covalent
bond. The electrons in this new probability
density are then shared by both atoms (equally
attracted to both nuclei). Arrange themselves to
have maximum overlap of their half filled
orbitals, producing a bonding orbital of lowest
energy!
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Two s orbitals overlapping
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all form covalent bonds- all overlapping
orbitals must be half-filled - the new
probability distribution formed can only have a
max of two electrons
Other overlaps
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Types of covalent bonds

• Orbitals can overlap in two main ways creating
  two different types of covalent bonds
• Sigma bonds Pi bonds

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The s bond
Electron density is between the nuclei of the
overlapping atoms
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Single bondss bonds
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The p bond
Electron densities are above and below the nuclei
of the bonding atoms
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Only p orbitals can form pi bonds!
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Double Bondone p bond one s bond
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Triple bond two p bond one s bond
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The benzene ring
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Promotion Hybridization

• Certain atoms can change their electron
  configuration in order to bond and form a wide
  variety of compounds
• This change in electron configuration takes
  place in two steps Promotion of an electron to a
  higher energy orbital hybridization or blending
  of orbitals creating a new type of orbital for
  bonding

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Promotion

• Most of the time atoms exist in their ground
  state but, in certain cases the instant before
  bonding promotion takes place allowing more
  bonding spaces

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HybridizationThe merging of orbitals

• Merging orbitals must all be half filled
• No orbitals are lost due to merging if you
  blend one s orbital and one p orbital you will
  end up with TWO hybrid orbitals!

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HybridizationContinued

• Mix at least 2 nonequivalent atomic orbitals (eg.
  s and p). Hybrid orbitals have different shapes
  from original atomic orbitals
• Covalent bonds are formed by
• Overlap of hybrid orbitals with atomic orbitals
• Overlap of hybrid orbitals with other hybrid
  orbitals

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Example Hybridization in carbon to form methane
(CH4)
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Types Names of hybrid orbitals

• The type of hybrid orbital depends upon the
  orbitals which have been blended

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sp2 Hybridization in BF3

• ?_ _ _
• ? ? 2p1 Unhybridized Boron
• 2s2
• For BF3, 3 hybrid orbitals are needed, so 3
  atomic orbitals are required as follows (s p
  p) sp2
• ?_ ?_ ?_
• sp2 sp2 sp2 Hybridized Boron

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3 sp2 orbitals needed to form 3 sigma bonds
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sp Hybridization in BeCl2

• _ _ _
• ? ? 2p Unhybridized Be
• 2s2
• For BeCl2, 2 hybrid orbitals are needed, so 2
  atomic orbitals are required as follows (s p )
  sp
• ?_ ?_
• sp sp Hybridized Be

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• Once hybridization has occurred hybridized
  orbitals are ready to bond just like regular
  orbitals
• Bonding in hybridized orbitals

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Because of their shape, hybrid orbitals can
only undergo sigma bonding.
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Shapes Hybrids a little trick 0)
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Hybridization Involving d Orbitals
3s 3p 3d
3s 3p
3d
unhybridized P atom P Ne3s23p3
vacant d orbitals
degenerate orbitals (all EQUAL)
Trigonal bipyramidal
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Sigma and Pi Bonds

• Single Bond 1 sigma bond
• Double Bond 1 sigma bond and 1 pi bond
• Triple Bond 1 sigma bond and 2 pi bonds