1. Lecture 7: Chemical bonding
Lecture 7 Topics Brown, chapter 8
Basic bonding background information
Types of chemical bonds
Valence electrons & Lewis structures
The ‘Octet rule’
Ionic bonding
Lattice energy
Charge & size affect lattice energy
Transition metal ions
Covalent bonding
Lewis structures & unequal sharing
Unequal sharing & electronegativity
Bond polarity
Guidelines for Lewis Structures
Dipole moment & overall molecular polarity
Multiple bonds & formal charge
Resonance structures
Exceptions to the octet rule

2. Resonance: alternate arrangement of electrons
Molecules may have multiple resonance structures.
Resonance hybrids are more realistic.
The general process of advancing scientific knowledge by making experimental observations and by formulating hypotheses, theories, and laws.
It’s a systematic problems solving process AND it’s hands-on….. Experiments must be done, data generated, conclusions made.
This method is “iterative”; it requires looping back and starting over if needed. [Why do you think they call it REsearch?] Often years, decades or more of experiments are required to prove a theory.
While it’s possible to prove a hypothesis wrong, it’s actually NOT possible to absolutely prove a hypothesis correct as the outcome may have had a cause that the scientist hasn’t considered.

3. Resonance structures
Take a look at the geometry of this molecule. What suggests that the two “outer” oxygen atoms are behaving the same way?
Now try to draw the Lewis dot structure of this molecule, O3, or ozone. p

4. Resonance structures…
… are Lewis structures that are equivalent except for e- placement. .. .. ..
ozone (O3)
1.54Å O
1.34Å O O .. .. .. .. .. .. : O O O O : O : .. .. .. .. O .. ..
1.40Å
resonance structures
resonance hybrid
The actual structure is a blend of the two alternative structures shown
above, & is best represented by the resonance hybrid.
Note that the actual structure does not oscillate between 2 resonance structures.
Both structures exist - as both blue & yellow are present in green. p

5. Example: resonance structure
Which has shorter S - O bonds, SO3 or SO3-2 ?
SO3 = 6 + (6x3) = 24 ve-
For these 3 equivalent resonance structures all bonds also have equal length. But it’s 1/3 shorter than a single bond. .. .. : O : : O : : O : S S S : : O : : : : .. O : .. O : : O : O : : O .. ..
SO3-2 = 6 + (6x3) + 2 = 26 ve- .. .. ..
:O S O: |
:O: .. ..
For this structure all bonds have equal length – that of a single bond (long). ..
p.311

6. Example: resonance structures & hybrid
HCO2-1 has resonance structures.
How many resonance structures?
Draw each resonance structure.
Draw the resonance hybrid. .. : O : : O : : O : C C C H : O : H : O : : O : .. H
p.311