Handout Lecture22

7/27/2019 Handout Lecture22

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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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An example: CO2

16 e-

12 e- left

O CO

OCO

OCO 0 e- leftOctet Violation

OCOCO double bond


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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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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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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!

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