CHEMICAL
BONDING
Set 4
Cocaine
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Credits
• Thank you to Mr. Neil
Rapp who provided the
bulk of this powerpoint
on his website
www.chemistrygeek.co
m
• Other information
comes from Zumdahl, Steven,
and Susan Zumdahl. Chemistry.
Boston: Houghton Mifflin, 2003.
Violations of the Octet Rule
Usually occurs with B and elements of
higher periods. Common exceptions
are: Be, B, P, S, and Xe.
Be: 4
B: 6
P: 8 OR 10
S: 8, 10, OR 12
Xe: 8, 10, OR 12
SF4
BF3
Lewis Structures: Comments about the Octet Rule
• The second-row elements C, N, O and F should always be
assumed to obey the octet rule.
• The second-row elements B and Be often have fewer than
eight electrons around them in their compounds. These
electron-deficient compounds are very reactive.
• The second-row elements never exceed the octet rule, since
their valence orbitals (2s and 2p) can accommodate only eight
electrons.
• Third-row and heavier elements often satisfy the octet rule
but can exceed the octet rule by using their empty valence d
orbitals.
• When writing the Lewis structure for a molecule, satisfy the
octet rule for the atoms first. If electrons remain after the
octet rule has been satisfied, then place them on the
elements having available d orbitals (elements in Period 3 or
beyond).
Resonance
• Resonance occurs when more than one valid
Lewis structure can be written for a particular
molecule.
• Example: Nitrate Ion
Resonance Explained
• If any one of these structures accurately represented
the bonding in the nitrate ion, there should be two
types of N-O bonds observed in the molecule: one
shorter bond (the double bond) and two identical
longer ones (the two single bonds).
• Experiments clearly show that the nitrate ion exhibits
only ONE bond length and bond strength between
those expected for a double and single bond.
• Resonance structures are the depictions of all possible
structures, but in reality, the molecule’s bonding is the
average of all three.
Structural Isomers
http://xsiborganic1011.wikispaces.com/TangYaoFlo
• Examine the three models of the three alkanes above to determine how
they are similar and how they are different.
•All three have 5 carbon atoms and 12 hydrogen atoms, so they all have
the molecular formula C5H12.
•BUT, you can see that the arrangements of the atoms results in three
different compounds: pentane, 2-methyl butane, and 2,2 –dimethyl
propane
Isomer Definitions
• Isomers are two or more compounds that have the same
molecular formula, but their atoms are bonded in different
arrangements.
• Structural isomers have different chemical and physical
properties despite having the same chemical formula.
• The b.p. of each of the three isomers shown in the previous
slide are as follows (in order as shown): 36°C, 28°C, 9°C
– This observation supports a main principle of chemistry:
structure determines properties.
– How does the trend in boiling points of C5H12 isomers
relate to their structures?
Number of Possible Isomers
• As the number of carbons in a hydrocarbon
increases, the number of possible structural
isomers increases.
– There are nine alkanes with the molecular formula
C7H16
– There are more than 300,000 structural isomers
with the formula C20H42
Odd-Electron Molecules
• Relatively few molecules formed from
nonmetals contain odd numbers of electrons.
• Two common examples are NO and NO2 (try
drawing them)
• Since the Lewis structure model is based on
electron pairs, it does not handle odd-electron
cases in a natural way. We usually just tack on
the odd electron onto the central atom.
Homework Questions
• 1) Which has the greater bond lengths: NO2- or
NO3- ? Explain. (HINT: You’ll have to draw the
Lewis structures first)
• 2) The following ions are best described with
resonance structures. Draw the resonance
structures:
a) NCOb) CNO• 3) Explain the terms resonance and its connection
to delocalized electrons. Use additional resources
if necessary.
• 4) Explain the difference between a resonance
structure and an isomer.