Covalent Bonding

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Covalent Bonding

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Covalent Bonding Models

• Lewis Dot Structures
• Wave Functions
• Valence Bond Theory
• Resonance
• Formal Charge
• Hybridization
• Molecular Orbital Theory

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Lewis Dot Diagrams
Each dot represents an electron in the outer
(valence) shell in one of the atoms.
Each atom attempts to attain a stable outer
shell electronic configuration
by sharing electrons with other atoms.
(typically a noble gas electronic configuration)
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Lewis Dot Diagrams
Each atom sharing an electron can count that
electron towards attaining its stable electronic
configuration.
Each pair of electrons which are shared forms a
bond between those atoms.
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Lewis Dot Diagrams

• Bonding electrons may come from both atoms or
  they may only come from one atom.
• Normally two electrons form each bond.
• Octet rule

• d-orbitals can be used to expand beyond the
  octet. (18 electron rule)
• Lowest energy structure
• most strongest bonds, geometry

What is the basis of the octet rule?
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Types of Bonds Formed
Lighter members of a family often form multiple
bonds
Heavier members favor additional single bonds.
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Heavier Members of a Familytend to form single
bonds
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Wave Functions
each electron in an atom or molecule can be
described by a wave function
?a
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Wave Functions
wave functions are mathematical functions (cosine
sine) that meet certain boundary conditions
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Solution to the Schrödinger Wave Equation for the
Hydrogen Atom
the minimum corresponds to a mathematical
solution to the wave function and the energy of
the atom
E1s -1.31 MJmol-1
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What Happens WhenTwo Hydrogen Atoms Come Together
E1s -1.31 MJmol-1
E1s -1.31 MJmol-1
EH2 -0.458 MJmol-1
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Valence Bond Theory
?molecule ?A1 ?B2
wave functions for multiple electrons can
be combined, differentiated and minimized
electron 2 on atom B
electron 1 on atom A
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Valence Bond Theory
?A1?B2
(?A1 ?B2) (?A2 ?B1)
(?A1 ?B2) (?A2 ?B1) ?(?A1 ?A2) ?(?B1 ?B2)
By including additional combinations of wave
functions a better approximation to the actual
wave function of the molecule is obtained.
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Resonance
(?A1 ?B2) (?A2 ?B1) ?(?A1 ?A2) ?(?B1 ?B2)
sometimes the electrons are equally shared
sometimes the electrons are unequally shared
covalent contribution
ionic contribution
H H ?? H- H ?? H H-
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Resonance
All atoms obey their bonding rules if possible.
Same number of unpaired electrons in contributing
structures.
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Resonance
Maximum number of bonds.
Formal charges are minimized.
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Resonance
Same central atom hybridization.
Same molecular structure.
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Resonance
For some skeleton and dot structures there is not
an unique, correct answer.
Carbonate ion has three resonant configurations.
Individually, none of these structures represents
the actual structure.
But the average of these structures is a
good representation of the ion.
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Resonance
What is the carbon-oxygen bond order?
(112) / 3 1.33
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Resonance
Where is the negative charge located in each
contributing structure?
On the oxygen atoms with a single bond.
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Formal Charge
Assume bonding electrons belong 1/2 to each
bonding atom.
The formal charge is the net charge on each atom.
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Formal Charge
unshared electrons
formal charge
e donated -
-
formal charge of oxygen
6 valence electrons
4/2 shared electrons
4 unshared electrons

-
-
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Formal Charges of AtomsMethanol
Formal charge valence electrons - electrons
shared/2 - electrons not shared
Atoms with low formal charges generally indicate
the correct bonding pattern of chemically stable
molecules.
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Incorrect Diagrams for Methanol
What are the formal charges for each atom in the
following structures?
Validity Check The sum of the formal charges
always equals the charge on the molecule or ion.
Validity Check The sum of the formal charges
always equals the charge on the molecule or ion.
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Resonance
What is the average formal charge on each atom in
the carbonate ion?
carbon 0 oxygen -2/3
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Thiocyanate Ion
Skeleton diagrams
S 6 valence e
C 4 valence e
N 5 valence e
charge 1 e
Only 8 electrons shown (4 pairs)
total 16 valence e
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Thiocyanate Ion
Possible Lewis Dot structures
-
C
What carbon-sulfur and carbon-nitrogen bond
orders are predicted for this ion?
Formal charges favor which of these structures?
C-S 1.5 C-N 2.5
(nitrogen is slightly more electronegative than
sulfur 3.1, 2.4)
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Resonance Structures

• Factors to consider when picking the best
  resonance structures

• Zero or low formal charges are the best.
• Formal charges are consistent with the
  electronegativity of the elements involved.
• Alternating positive and negative formal charges
  are better than highly polarized patterns.

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Sulfate Ion
Skeleton diagram
Lewis Dot diagram
S 6 valence e
O 24 valence e
charge 2 e
total 32 valence e
16 e pairs
Is sulfate ion a good oxidizing agent? Is it
explosive?
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Rationalizing the Inertness of theSulfate Ion

• The octet rule (8 electrons around an atom) is
  based on use of one s-orbital and three
  p-orbitals.
• If d-orbitals are included, the octet rule is
  broken.
• Expanding the valence results in illegal Lewis
  Dot structures, but may explain the inert
  chemistry of the sulfate ion.

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Formal Charges of AtomsSulfate Ion
Where are the two negative charges located?
How many different resonance configurations can
be drawn?
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Resonance Forms of Sulfate Ion
What is the sulfur and oxygen bond order?
What is the average formal charge on each atom?
Why is sulfate ion relatively inert?
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Formal Charges of AtomsPerchlorate Ion
-
Where are the two negative charges located?
How many different resonance configurations can
be drawn?
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Resonance Forms of Perchlorate Ion
What is the sulfur and oxygen bond order?
What is the average formal charge on each atom?
Which orbitals of chlorine are used to form the
pi bonds?
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Cyanide Ion
Which end of cyanide ion is negative?
Cyanide ion reacts with iron(III) ion to make the
hexacyanoferrate(III) ion.
Does the iron(III) ion bond to carbon or nitrogen?
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Hexacyanoferrate(III) ion
Carbon is the negative end of the cyanide ion.
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