Chemical Bonding: Valence Bond

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Chemical BondingValence Bond Molecular
Orbital TheoriesChapter 9 Section 4 through
6of Brady Senese 5TH ed)

• Dr. C. Yau
• Fall 2009

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VSEPR Theory
You had previously learned how the Valence Shell
Electron Pair Repulsion Theory (VSEPR Theory)
helps us predict the molecular geometry of a
species from examining its Lewis structure. It
utilizes the concept of repulsion amongst the
charge clouds of the central atoms. It does not
explain how a bond is formed and how bonding
relates to the s, p, d orbitals.
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Valence Bond Theory (VB Theory)
The VB Theory explains bonding as an overlap of
valence orbitals. H2 bonds are due to the overlap
of their 1s valence orbitals.
A B
A. Two separate H atoms B. H2 molecule with
covalent bond due to overlap of the 1s orbitals.
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Valence Bond Theory

• Consider H2S. The valence electrons of S
• are 3s2 3p4 ?? ?? ? ?
• 3s 3p
• and the orbitals would be overlapping with the 1s
  orbitals from the two H atoms.

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Difficulties With VB Theory

• Most experimental bond angles do not support
  those predicted by mere atomic orbital overlap.
• For example C 1s22s22p2 and H 1s1
• Experimental bond angles in methane CH4 are
  109.5 and all are the same
• p orbitals are 90 apart, and not all valence e-
  in C are in the p orbitals
• How can multiple bonds form?

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Hybridization in VB Theory

• Atomic orbitals are mixed to allow formation of
  bonds that have realistic bond angles
• The new shapes that result are called hybrid
  orbitals
• Review charge clouds Hybridization Bond
  Angles
• 2 sp 180o
• 3 sp2 120o
• 4 sp3 109.5o
• 5 sp3d 120o 90o
• 6 sp3d2 90o 180o

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How are hybrid orbitals formed?

• Valence orbitals are ___ ____ ____ ____
• s p
• If we take s and all three p, then four sp3
  hybrid
• If we take the s and two p, then three sp2 and
  one p left over. sp2 p
• If we take s and one p, then two sp and one p
  left over. hybrid sp p p
• The un-hybridized p (pure p) are used in double
  and triple bonds.
• Note that orbitals is conserved.

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How are hybrid orbitals formed?

• When we run out of p orbitals, we start using d
  orbitals, but this would not be possible for
  elements smaller than Ne (Period 2 and smaller)
  because they do not have d-orbitals.
• Thus we have sp3d hybrid orbitals and sp3d2
  hybrid orbitals (but only for elements beyond
  Period 2).

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Bonding in CH4

• The 4 hybrid orbitals are evenly distributed
  around the C
• The H s-orbitals overlap the sp3 hybrid orbitals
  to form the bonds.

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Bonding Types

• Two types of bonds result from orbital overlap
• sigma (?) bonds
• from head-on overlap
• lie along the bond axis
• account for the first bond
• pi (?)bonds
• pi bonds are perpendicular to bond axis
• account for the second and third bonds in a
  multiple bond

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Sigma and Pi Bonding

• Please refer to lecture notes on ? and ? bonding
  of C2H4 and C2H2.
• Here is what you should be able to do
• Given the structural formula of a compound, be
  able to specify the hybridization of each atom,
  state the bond angles and determine the of
  sigma and pi bonds.

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KNOW THIS WELL!

• X X single bond ? bond
• X X double bond ? bond ? bond
• X X triple bond ? bond two ? bonds

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1 2 3 4

• Always start with hybridization.
• Give the hybridization of each C and each O.
• Give the bond angles.
• How many ? bonds are there?
• How many ? bonds are there?
• Ans. C1 sp C2 sp C3 sp2 C4 sp3 O
  sp3
• C1-C2-C3 180o
• C2-C3-O 120o
• C2-C3O 120o
• O-C4-H 109.5o

9 ? bonds 3 ? bonds
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Molecular Orbital Theory (MO Theory)

• VSEPR and VB theories do not explain everything.
• MO Theory considers the molecule as an entity
  rather than just a collection of atoms.
• Instead of considering atomic orbitals or hybrid
  orbitals, the MO theory considers molecular
  orbitals and molecular energy levels.

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MO Theory

• When two atomic orbitals overlap, their wave
  functions interact by constructive and
  destructive interference to form bonding and
  antibonding orbitals.
• Electrons in bonding molecular orbitals stabilize
  the molecule.
• Electrons in antibonding molecular orbitals
  de-stabilize the molecule.
• The Bond Order is bonds, and calculated thus
• Bond order
• ½ ( bonding electrons - antibonding
  electrons).

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MO Energy Diagram of H2

• Two 1s orbitals of H atoms combine to form
• two molecular orbitals
• ?1s (bonding) and ?1s (antibonding).
• H2 has 2 electrons, both in bonding MO orbitals.
• bond order 2/2 1 (a single bond)

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MO Theory

• See handout for application of MO theory on
  diatomic molecules such as N2, O2 etc.
• Know how to fill electrons into the MO energy
  diagram and calculate bond order.