Lecture 7: Chemical bonding
Lecture 7 Topics
1. Basic bonding background information
• Types of chemical bonds
• Valence electrons & Lewis structures
• The ‘Octet rule’
Brown, chapter 8
8.1
2. Ionic bonding
• Lattice energy
• Charge & size affect lattice energy
• Transition metal ions
8.2
3. Covalent bonding
• Lewis structures & unequal sharing
• Unequal sharing & electronegativity
• Bond polarity
• Guidelines for Lewis Structures
• Dipole moment & overall molecular polarity
• Multiple bonds & formal charge
8.3
4. Resonance structures
8.6
1.
8.7
Exceptions to the octet rule
8.4
8.5
Resonance: alternate arrangement of electrons
Molecules may have multiple resonance structures.
Resonance hybrids are more realistic.
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.309-10
Resonance structures…
… are Lewis structures that are equivalent except for e- placement.
ozone
(O3)
1.54Å
..
:O
..
..
O 1.34Å
..
O
..
..
O
..
resonance structures
..
O
..
O
.. :
..
O
..
..
O
..
O
.. :
1.40Å
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.309-10
Example: resonance structure
Which has shorter S - O bonds, SO3 or SO3-2 ?
SO3 = 6 + (6x3) = 24 ve-
:O
.. :
: O:
..
: O:
..
: O:
S
S
S
:O
.. :
:O :
:O
.. :
:O
.. :
For these 3 equivalent
resonance structures all
bonds also have equal
length. But it’s 1/3 shorter
than a single bond.
:O:
SO3-2 = 6 + (6x3) + 2 = 26 ve.. .. ..
:O
.. S O:
..
For this structure all bonds
|
have equal length – that of
:O:
..
a single bond (long).
p.311
Example: resonance structures & hybrid
HCO2-1 has resonance structures.
• How many resonance structures?
• Draw each resonance structure.
• Draw the resonance hybrid. ..
H
: O:
: O:
: O:
C
C
C
:O
.. :
H
:O:
H
:O:
p.311