Valence Bond Theory

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Valence Bond Theory
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Valence Bond Theory

• Why do atoms form bonds with other atoms?
• To achieve a more stable (lower) energy state.
• Atoms are happiest (most stable) when their
  orbitals are full.
• They do not like being half full.

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Sharing Electrons

• Consider hydrogen
• Electron configuration
• Hydrogen would be much happier with another
  electron.
• If hydrogen bumped into another hydrogen, its
  electron configuration would be identical.

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Sharing Electrons

• The shared electrons automatically assume
  opposite spins.
• Thus, the atoms are sharing not just electrons,
  but orbitals.

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Consider a graph of intermolecular distance vs.
energy
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Sharing Electrons

• The 1s orbitals are overlapped to share
  electrons.
• The new combined orbital is called a sigma (s)
  bond.

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Sharing Electrons

• Now consider hydrofluoric acid, HF.
• Draw energy level diagrams for hydrogen and
  fluorine in their ground states.
• Propose a theory for how they form a bond.
• Draw a picture of the resulting bond.

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Sharing Electrons
n2
n1


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Sharing Electrons

• What orbitals are being shared?
• 1s and 2pz (or 2py, or 2px)
• What does this look like?

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Sharing Electrons


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Sharing Electrons


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Problem

• Draw energy level diagrams for NH3.
• Draw a diagram of what the molecule would look
  like with bonded orbitals
• More practice pg 232 1-4.

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Orbital Hybridization

• One of the most influential chemistry books ever
  written was The Nature of the Chemical Bond by
  Linus Pauling (1901-1994).
• Published in 1939, Pauling was awarded the Nobel
  prize in 1954. Pauling received a second Nobel
  prize for peace in 1962.

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Orbital Hybridization

• Pauling made tremendous contributions to the
  field of chemistry, and was an outspoken activist
  against war and nuclear weapons.

Pauling in 1987.
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Orbital Hybridization

• Consider the energy level diagram for carbon

• Carbon appears to have only 2 unpaired electrons,
  yet it is capable of forming 4 covalent bonds.

C
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Orbital Hybridization

• Carbon is able to form four bonds by hybridizing
  its s and p sublevels into an sp3 orbital.

sp3
The resulting orbitals consist of 1 s orbital,
and 3 p orbitals, so they are called sp3.
2p
2s
Carbon promotes an electron from its 2s orbital
to the empty 2p orbital
1s
C
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sp3 orbitals are shaped like balloons.
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sp3 orbitals form (s) sigma bonds with other
orbitals.
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Orbital Hybridization

• Other hybridizations are possible
• Boron may form 3 sigma (s) bonds with each sp2
  orbital.

The resulting orbitals consist of 1 s orbital,
and 2 p orbitals, so they are called sp2.
sp2
2p
2s
1s
B
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Orbital Hybridization

• What hybrid orbitals can Be form?
• Beryllium forms the hybrid orbital sp (the 1 is
  assumed).

The resulting orbitals consist of 1 s orbital,
and 1 p orbitals, so they are called sp.
sp
2p
2s
1s
Be
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Orbital Hybridization

• It should be remembered that these atoms only
  form hybrid orbitals for bonding.
• The shape of the hybrid orbital is also very
  important.
• sp3 tetrahedral, bond angle 109.5o
• sp2 trigonal planar, bond angle 120o
• sp linear, bond angle 180o
• Page 234 Summary table

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Problem

• Phosphorous can form 2 different molecules with
  chlorine PCl3 and PCl5.
• Nitrogen can only form NCl3.
• Can you explain why?
• Hint How does P differ from N in its electron
  configuration?
• Hint What does P have available to it that N
  does not?

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Orbital Hybridization

• What hybrid orbital(s) will N form?
• Nitrogen forms 4 sp3 orbitals, but only has 3
  unpaired orbitals with which to form covalent
  bonds.

sp3
2p
2s
1s
N
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Orbital Hybridization

• Can Phosphorous form hybrid orbitals?
• It must! How?

3d
3p
3s
Phosphorous forms 5 hybrid orbitals using the d
subshell.
2p
2s
The resulting hybridized orbitals consist of 1 s,
3 p, and 1 d, and are therefore called sp3d.
1s
P
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Practice

• Page 235 8 - 13

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Double Triple Bonds

• Consider the molecule ethane, C2H6.
• Each carbon forms 4 sp3 hybrid orbitals in order
  to bond to 3 hydrogens and to each other.
• Now consider ethene, C2H4.
• What is the Lewis diagram for C2H4?

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Double Triple Bonds

• Ethene forms a double bond between the carbons to
  satisfy the octet rule.
• The sp3 hybrid orbital cannot explain this bond
  formation.

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Double Triple Bonds

• The carbon atoms instead form 3 sp2 hybrid
  orbitals, retaining the normal 2pz orbital.

sp2
2pz
2p
2s
1s
C
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Double Triple Bonds

• 3 sp2 hybrid orbitals take the shape of trigonal
  planar.

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Double Triple Bonds

• 3 sp2 orbitals 1 2pz orbital

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Double Triple Bonds

• When two p orbitals overlap, this allows a new
  type of bond to form.
• A bond formed by two overlapping p orbitals is
  called a p (pi) bond.
• A p bond is weaker than a s bond, due to the
  greater distances involved.

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Double Triple Bonds

• When the sp2 orbitals form a s bond between
  carbons, the 2pz orbitals of each carbon overlap.

s
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Double Triple Bonds

• Now consider ethyne, or C2H2.
• The Lewis diagram predicts a triple bond between
  the carbons.
• Can you hypothesize what a triple bond consists
  of?

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Double Triple Bonds

• Carbon forms 2 sp hybrid orbitals.

sp
2p
2s
2py 2pz
1s
C
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Double Triple Bonds

• The resulting s bond between the sp orbitals
  brings both 2py and 2pz orbitals together to form
  two p bonds.

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Double Triple Bonds
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Homework

• Pg 235 8-12
• Pg 238 18-21
• Pg 239 26-28
• Good flash animation of hybrid orbitals
• http//ochem.jsd.claremont.edu/tutorials.htm
• Good summary of resonance and molecular orbitals
• http//www.chem.ufl.edu/itl/2045/lectures/lec_15.
  html