Lewis%20Dot%20Structures

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Lewis Dot Structures

• Developed by G. N. Lewis to serve as a way to
  describe bonding in polyatomic systems.

• Central idea the most stable arrangement of
  electrons is one in which all atoms have a
  noble gas configuration.

• Example NaCl versus NaCl-

Na Ne3s1
Cl Ne3s23p5
Na Ne
Cl- Ne3s23p6 Ar
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LDS Mechanics

• Atoms are represented by atomic symbols
  surrounded by valence electrons.

Lone Pair (6 x)

• Electron pairs between atoms indicate bond
  formation.

Bonding Pair
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LDS Mechanics (cont.)

• Three steps for basic Lewis structures

• Sum the valence electrons for all atoms to
  determine
• total number of electrons.
• Use pairs of electrons to form a bond between
  each
• pair of atoms (bonding pairs).
• 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-
16 e- left
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LDS Mechanics (cont.)

• An example CH4

8 e-
0 e- left
Done!
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LDS Mechanics (cont.)

• An example CO2

16 e-
12 e- left
Octet Violation
0 e- left
CO double bond more than one pair of electrons is
shared
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Lewis Diagram for CO2
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LDS Mechanics (cont.)

• An example NO

10 e-
8 e- left

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Whiteboarding!

• In a group of 3 try the following Lewis
  Structures!

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CCl4 NF3
H2O H2Se
NH3 OF2
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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

Both structures are correct!
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Resonance Structures (cont.)

• In this example, O3 has two resonance structures

• 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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Resonance Structures for CO3-2
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Formal Charge

• Formal Charge Compare the nuclear charge (Z)
  to the number of electrons (dividing bonding
  electron pairs by 2). Difference is known as the
  formal charge.

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

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.

• We will deal with two classes of violation here
• Sub-octet systems
• Valence shell expansion

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Beyond the Octet Rule (cont.)

• Some atoms (Be and B in particular) undergo
  bonding, but will form stable molecules that do
  not fulfill the octet rule.

• Experiments demonstrate that the B-F bond
  strength is consistent with single bonds only.

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Beyond the Octet Rule (cont.)

• For third-row elements (Period 3), the
  energetic proximity of the d orbitals allows for
  the participation of these orbitals in bonding.

• When this occurs, more than 8 electrons can
  surround a third-row element.
• Example ClF3 (a 28 e- system)

F obey octet rule
Cl has 10e-
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Summary

• Remember the following
• C, N, O, and F 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
  shell expansion to exceed the octet rule.

• In the end, you have to practice..a lot!