handout lecture22
TRANSCRIPT
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Lecture 22: Lewis Dot Structures
Reading: Zumdahl 13.9-13.12
Outline
Lewis Dot Structure Basics
Resonance
Those annoying exceptions
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Localized Bond Models
Consider our energy diagram for H2 bonding:
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Localized Model Limitations It is important to keep in mind that the models we
are discussing are just that..models.
We are operating under the assumption that when
forming bonds, atoms share electrons using
atomic orbitals.
Electrons involved in bonding: bonding pairs.
Electrons not involved in bonding: lone pairs.
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Lewis Dot Structures (cont.)
Developed by G. N. Lewis to serve as away to describe bonding in polyatomicsystems.
Central idea: the most stable arrangement of
electrons is one in which all atoms have a noble
gas configuration.
Example: NaCl versus Na+Cl-
Na: [Ne]3s1 Cl: [Ne]3s23p5
Na+: [Ne] Cl-: [Ne]3s23p6 = [Ar]
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LDS Mechanics Atoms are represented
by atomic symbols
surrounded by valenceelectrons.
F C
Electron pairs between
atoms indicate bondformation.F F
Bonding Pair
Lone Pair (6 x)
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LDS Mechanics (cont.) Three steps for basic Lewis structures:
1. Sum the valence electrons for all atoms to determine
total number of electrons.
2. Use pairs of electrons to form a bond between each
pair of atoms (bonding pairs).
3. Arrange remaining electrons around atoms (lone pairs)
to satisfy the octet rule (duet rule for hydrogen).
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LDS Mechanics (cont.) An example: Cl2O
20 e-O Cl
Cl
OCl Cl 16 e- left
OCl Cl
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LDS Mechanics (cont.)
An example: CH4
8 e-
0 e- left
H CH HH
H C
H
H
H
Done!
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LDS Mechanics (cont.)
An example: CO2
16 e-
12 e- left
O CO
OCO
OCO 0 e- leftOctet Violation
OCOCO double bond
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LDS Mechanics (cont.)
An example: NO+
10 e-ON
+
ON
+
ON 8 e- left
ON+
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Resonance Structures
We have assumed up to this point that there is one
correct Lewis structure.
There are systems for which more than one Lewis
structure is possible:
Different atomic linkages: Structural Isomers Same atomic linkages, different bonding: Resonance
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Resonance Structures (cont.)
The classic example: O3.
O O O
O O O
O O O
Both structures are correct!
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Resonance Structures (cont.)
In this example, O3 has two resonance structures:
O O O
Conceptually, we think of the bonding being an
average of these two structures.
Electrons are delocalized between the oxygens
such that on average the bond strength is
equivalent to 1.5 O-O bonds.
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Structural Isomers
What if different sets of atomic linkages can be
used to construct correct LDSs:
OCl Cl ClCl O
OCl Cl ClCl O
Both are correct, but which is more correct?
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Formal Charge
Formal Charge: Compare the nuclear charge (+Z)to the number of electrons (dividing bondingelectron pairs by 2). Difference is known as theformal charge.
OCl Cl ClCl O
#e- 7 6 7 7 6 7
Z+ 7 6 7 7 7 6
Formal C. 0 0 0 0 +1 -1
Structure with less F. C. is more correct.
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Formal Charge
Example: CO2
OCO O O C O O C
e- 6 4 6 6 4 6 7 4 5
Z+ 6 4 6 6 6 4 6 6 4
FC 0 0 0 0 +2 -2 -1 +2 -1
More Correct
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Beyond the Octet Rule
There are numerous exceptions to the octet rule.
Well deal with three classes of violation here:
Sub-octet systems
Valence shell expansion
Odd-electron systems
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Beyond the Octet Rule (cont.)
Some atoms (Be and B in particular) undergobonding, but will form stable molecules that donot fulfill the octet rule.
FBF
F
FBF
F
Experiments demonstrate that the B-F bondstrength is consistent with single bonds only.
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Beyond the Octet Rule (cont.)
For third-row elements (Period 3), the energeticproximity of the d orbitals allows for theparticipation of these orbitals in bonding.
When this occurs, more than 8 electrons cansurround a third-row element.
Example: ClF3 (a 28 e- system)
FClF
F F obey octet rule
Cl has 10e-
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Beyond the Octet Rule (cont.)
Finally, one can encounter odd electron systemswhere full pairs will not exist.
ClO O
Example: Chlorine Dioxide.
Unpaired electron
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Summary
Remember the following:
C, N, O, and F almost always obey the octet rule.
B and Be are often sub-octet Second row (Period 2) elements never exceed the octet
rule
Third Row elements and beyond can use valence shellexpansion to exceed the octet rule.
In the end, you have to practice..a lot!