Post on 22-Feb-2016
description
Bonding – General Concepts
Shows how valence electrons are arranged among atoms in a molecule.Reflects central idea that stability of a compound relates to noble gas electron configuration.
Lewis Structures
CH
H
H
Cl
..
....
..
Completing a Lewis Structure -CH3Cl
Add up available valence electrons: C = 4, H = (3)(1), Cl = 7 Total = 14
Join peripheral atoms
to the central atom with electron pairs.
Complete octets on
atoms other than hydrogen with remaining electrons
Make carbon the central atom
..
..
..
Multiple Covalent Bonds:Double bonds
C C
H
H
H
H
Two pairs of shared electrons
C C
H
H
H
H
Ethene
Multiple Covalent Bonds:Triple bonds
C C HH
Three pairs of shared electronsEthyne
C C HH
ResonanceResonance is invoked when more than one valid Lewis structure can be written for a particular molecule.
The actual structure is an average of the resonance
structures.
H
H
H
H
HH
H
H
H
H
H
HBenzene, C6H6
The bond lengths in the ring are identical, and between those of single and double bonds.
Resonance Bond Length and Bond Energy
Resonance bonds are shorter and stronger than single bonds.
Resonance bonds are longer and weaker than double
bonds.
H
H
H
H
HH
H
H
H
H
H
H
Resonance in Ozone, O3
Neither structure is correct.
O O O
O O O
Oxygen bond lengths are identical, and intermediate to single and double bonds
Resonance in a carbonate ion:
Resonance in an acetate ion:
Resonance in Polyatomic Ions
Localized Electron ModelLewis structures are an application of the “Localized Electron Model”
L.E.M. says: Electron pairs can be thought of as “belonging” to pairs of atoms when bondingResonance points out a weakness in the Localized Electron Model.
Models Models are attempts to explain
how nature operates on the microscopic level based on experiences in the macroscopic world.Models can be physical as with this DNA modelModels can be mathematicalModels can be theoretical or philosophical
Fundamental Properties of Models
A model does not equal reality.Models are oversimplifications, and
are therefore often wrong.Models become more complicated
as they age.We must understand the underlying
assumptions in a model so that we don’t misuse it.
VSEPR • Valence Shell Electron
Pair Repulsion theory.• Most important factor in
determining geometry is relative repulsion between electron pairs.
Molecule adopts the shape that minimizes the electron pair repulsions.
MOLECULAR GEOMETRY
VSEPR – Valence Shell Electron Pair RepulsionX + E Overall Structure Forms
2 Linear AX2 AX3 Trigonal Planar AX3, AX2E4 Tetrahedral AX4, AX3E, AX2E2
5 Trigonal bipyramidal AX5, AX4E, AX3E2, AX2E3
6 Octahedral AX6, AX5E, AX4E2
A = central atomX = atoms bonded to A
E = nonbonding electron pairs on A
VSEPR: Linear
AX2 CO2
John A. SchreifelsChemistry 211
Chapter 10-16
Figure 9.2 Molecular Shapes 2,3,4 electron pairs.
John A. SchreifelsChemistry 211
Chapter 10-17
Figure 9.3 Molecular shapes 5, 6 electron pairs
• AX2 - Linear
• AX - Linear
• Draw CO2 and HF
AX3
AX2E
BF3
SnCl2
Triganol planar
Bent
VSEPR: Tetrahedral
AX4
AX3E
AX2E2
CCl4
PCl3
Cl2O
tetrahedral
Triangular Pyramidal
Bent
VSEPR: Trigonal Bi-pyramidal
AX5
AX4E
AX3E2
AX2E3
PCl5
SF4
ClF3
I3-
Triangular bipyramidal
See-saw
T-shaped
Linear
VSEPR: Octahedral
AX6
AX5E
AX4E2
SF6
ICl4-
BrF5
Octahedral
Square pyramidal
Square planar
Are the following molecules polar or nonpolar ? Which will dissolve in water?
• OF2 SO3 PCl3 SF6 NH3
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Formal charge (FC) in Lewis structures
Comparison of the formal number of valence electrons about an atom in the Lewis structure of a molecule and
comparison of FN with the number of valence electrons (VE) in the neutral atom.
Computation of FC:
FC = VE (neutral atom) - LE (atom in molecule) - 1/2BE (atom in molecule)
where
VE = the number of valence electrons in the neutral atomLE = the number of lone pair electrons on the atom in the moleculeBE = the number of bonding electrons on the atom in the molecule
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Example of formal charge computation: ozone, O3
O O O
Note: New charge for molecule = 0, sum of formal charges must = 0.
VE (atom) = 6 6 61/2 BE (molecule)= -2 -3 -1LE (molecule) = -4 -2 -6FC = 0 +1 -1
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HCN = atomic compositional structure
HCN possesses 10 VE = Lewis compositional structures
Two possible Lewis constitutional structures:
H-C-N or H-N-C
Both need to have 10 VE in their Lewis structureProblem: Try to achieve an acceptable Lewis structure (duet and octet rule followed) for
both.
Isomers: Same composition, two different constitutional Lewis structures
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HCN = atomic compositional structure
HCN: 10 VE = Lewis compositional structures
Two possible Lewis constitutional structures
H-C-N or H-N-C
Any acceptable Lewis structure for HCN needs to show 10 VE
Try to achieve an acceptable Lewis structure (duet and octet rules obeyed) for all isomeric structures.
H C N H N C
Two acceptable Lewis structures. Which is better?
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Use formal charges to decide on the stability of isomeric Lewis structures
VE (atom) 1 4 5 1 5 4
1/2 BE (molecule) -1 -4 -3 -1 -4 -3
UE (molecule) 0 0 -2 0 0 -2________________________________________________________FC on atom 0 0 0 0 +1 -1
H C N H N C
H C N H N C
Important: the net charge of composition HCN = 0, so the sum of the formal charges in any
acceptable Lewis structure must be = 0 also.