8 molecular structure & covalent bonding theories
TRANSCRIPT
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8Molecular
Structure & Covalent
Bonding Theories
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Chapter Goals
1. A Preview of the Chapter
2. Valence Shell Electron Pair Repulsion (VSEPR) Theory
3. Polar Molecules:The Influence of Molecular Geometry
4. Valence Bond (VB) Theory
Molecular Shapes and Bonding
5. Linear Electronic Geometry: AB2 Species
6. Trigonal Planar Electronic Geometry: AB3 Species
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Chapter Goals
Molecular Shapes and Bonding
7. Tetrahedral Electronic Geometry: AB4 Species
8. Tetrahedral Electronic Geometry: AB3U Species
9. Tetrahedral Electronic Geometry: AB2U2 Species
10. Tetrahedral Electronic Geometry – ABU3 Species
11. Trigonal Bipyramidal Geometry
12. Octahedral Geometry
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Chapter Goals
Molecular Shapes and Bonding
13. Compounds Containing Double Bonds
14. Compounds Containing Triple Bonds
15. A Summary of Electronic and Molecular Geometries
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Stereochemistry
• Stereochemistry is the study of the three dimensional shapes of molecules.
• Some questions to examine in this chapter are:
1. Why are we interested in shapes?
2. What role does molecular shape play in life?
3. How do we determine molecular shapes?
4. How do we predict molecular shapes?
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Two Simple Theories of Covalent Bonding• Valence Shell Electron Pair Repulsion
Theory– Commonly designated as VSEPR– Principal originator
• R. J. Gillespie in the 1950’s
• Valence Bond Theory– Involves the use of hybridized atomic orbitals– Principal originator
• L. Pauling in the 1930’s & 40’s
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Overview of Chapter
• The same basic approach will be used in every example of molecular structure prediction:
1. Draw the correct Lewis dot structure.– Identify the central atom.– Designate the bonding pairs and lone pairs of
electrons on central atom.
2. Count the regions of high electron density on the central atom.– Include both bonding and lone pairs in the counting.
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Overview of Chapter
3. Determine the electronic geometry around the central atom.
– VSEPR is a guide to the geometry.
4. Determine the molecular geometry around the central atom.
– Ignore the lone pairs of electrons.
5. Adjust molecular geometry for effect of any lone pairs.
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Overview of Chapter
6. Determine the hybrid orbitals on central atom.
7. Repeat procedure if there is more than one central atom in molecule.
8. Determine molecular polarity from entire molecular geometry using electronegativity differences.
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Overview of Chapter
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VSEPR Theory
• Regions of high electron density around the central atom are arranged as far apart as possible to minimize repulsions.
• There are five basic molecular shapes based on the number of regions of high electron density around the central atom.
• Several modifications of these five basic shapes will also be examined.
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VSEPR Theory
• Two regions of high electron density around the central atom.
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VSEPR Theory
• Three regions of high electron density around the central atom.
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VSEPR Theory
• Four regions of high electron density around the central atom.
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VSEPR Theory
• Five regions of high electron density around the central atom.
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VSEPR Theory
• Six regions of high electron density around the central atom.
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VSEPR Theory
Frequently, we will describe two geometries foreach molecule. • Electronic geometry is determined by the
locations of regions of high electron density around the central atom(s).
• Molecular geometry is determined by the arrangement of atoms around the central atom(s).– Electron pairs are not used in the molecular
geometry determination just the positions of the atoms in the molecule are used.
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VSEPR Theory
• An example of a molecule that has the same electronic and molecular geometries is methane - CH4.
• Electronic and molecular geometries are tetrahedral.
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VSEPR Theory
• An example of a molecule that has different electronic and molecular geometries is water - H2O.
• Electronic geometry is tetrahedral.
• Molecular geometry is bent or angular.
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VSEPR Theory
• Lone pairs of electrons (unshared pairs) require more volume than shared pairs.– Consequently, there is an ordering of
repulsions of electrons around central atom.
• Criteria for the ordering of the repulsions:
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VSEPR Theory
1. Lone pair to lone pair is the strongest repulsion.2. Lone pair to bonding pair is intermediate
repulsion.3. Bonding pair to bonding pair is weakest
repulsion.
• Mnemonic for repulsion strengthslp/lp > lp/bp > bp/bp
• Lone pair to lone pair repulsion is why bond angles in water are less than 109.5o.
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Polar Molecules: The Influence of Molecular Geometry• Molecular geometry affects molecular
polarity.– Due to the effect of the bond dipoles and how
they either cancel or reinforce each other.
A B A
linear molecule nonpolar
A B A
angular molecule
polar
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Polar Molecules: The Influence of Molecular Geometry• Polar Molecules must meet two
requirements:– One polar bond or one lone pair of electrons
on central atom.– Neither bonds nor lone pairs can be
symmetrically arranged that their polarities cancel.
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Valence Bond (VB) Theory
• Covalent bonds are formed by the overlap of atomic orbitals.
• Atomic orbitals on the central atom can mix and exchange their character with other atoms in a molecule.– Process is called hybridization.
• Hybrids are common:– Pink flowers – Mules
• Hybrid Orbitals have the same shapes as predicted by VSEPR.
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Valence Bond (VB) Theory
Regions of High Electron
Density
Electronic Geometry
Hybridization
2 Linear sp
3 Trigonal planar
sp2
4 Tetrahedral sp3
5 Trigonal bipyramidal
sp3d
6 Octahedral sp3d2
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Molecular Shapes and Bonding
• In the next sections we will use the following terminology:A = central atom
B = bonding pairs around central atom
U = lone pairs around central atom
• For example:AB3U designates that there are 3 bonding pairs and 1 lone pair around the central atom.
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Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
• Some examples of molecules with this geometry are: – BeCl2, BeBr2, BeI2, HgCl2, CdCl2
• All of these examples are linear, nonpolar molecules.
• Important exceptions occur when the two substituents are not the same!– BeClBr or BeIBr will be linear and polar!
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Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
Electronic Structures
1s 2s 2pBe
3s 3p
Cl [Ne]
Lewis Formulas
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Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
Dot Formula
180o - linear
BeCl Cl··
····
····
··BeCl Cl
····
····
····
····
Electronic Geometry
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Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
Molecular Geometry
bondspolar very
3.5 51. 3.5 ativitiesElectroneg
Cl - Be- Cl
0.22.032143421
180o-linear
BeCl Cl····
Polarity
moleculenonpolar
symmetric are dipoles bond
Cl---Be---Cl
→←
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Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization)
1s 2s 2pBe
1s sp hybrid 2p
3s 3p
Cl [Ne]
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Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
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Trigonal Planar Electronic Geometry: AB3 Species(No Lone Pairs of Electrons on A)
• Some examples of molecules with this geometry are: – BF3, BCl3
• All of these examples are trigonal planar, nonpolar molecules.
• Important exceptions occur when the three substituents are not the same!– BF2Cl or BCI2Br will be trigonal planar and
polar!
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Trigonal Planar Electronic Geometry: AB3 Species(No Lone Pairs of Electrons on A)
Electronic Structures Lewis Formulas
1s 2s 2p
B
3s 3p
Cl [Ne]
B··.
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Trigonal Planar Electronic Geometry: AB3 Species(No Lone Pairs of Electrons on A)
Dot Formula
··
B
Cl
Cl Cl··
····
··
···· ··
····
·· ··
B··
··
··
120-trigonal planar
Electronic Geometry
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Trigonal Planar Electronic Geometry: AB3 Species(No Lone Pairs of Electrons on A)
Molecular Geometry
BClCl
Cl120o-trigonal planar
bondspolar ery v
3.0 1.5 ativitiesElectroneg
Cl - B
1.543421
Polarity
BClCl
Cl
bond dipoles are symmetric nonpolar molecule
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Trigonal Planar Electronic Geometry: AB3 Species(No Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization)
1s 2s 2p
B 1s sp2 hybrid
3s 3p
Cl [Ne]
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Trigonal Planar Electronic Geometry: AB3 Species(No Lone Pairs of Electrons on A)
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Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
• Some examples of molecules with this geometry are: – CH4, CF4, CCl4, SiH4, SiF4
• All of these examples are tetrahedral, nonpolar molecules.
• Important exceptions occur when the four substituents are not the same!– CF3Cl or CH2CI2 will be tetrahedral and polar!
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Electronic Structures
C..
. .
H .
2s 2p
C [He]
1s
H
Lewis Formulas
C..
. .
Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
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Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
Dot Formula
C
H
H
H H......
.. C
....
..
..
tetrahedral109.5o bond angles
Electronic Geometry
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Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
Molecular Geometry
CH H
H
H
tetrahedralbondspolar slightly
2.1 2.5 ativitiesElectroneg
H- C
0.443421
Polarity
symmetric dipolesnonpolar molecule
CH H
H
H
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Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization) 2s 2p
C [He] four sp3 hybrid orbitals
C [He]
1s H
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Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
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Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
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Example of Molecules with More Than One Central Atom • Alkanes are hydrocarbons with the general
formula CnH2n+2.
– CH4 - methane
– C2H6 or (H3C-CH3) - ethane
– C3H8 or (H3C-CH2-CH3) - propane
• The C atoms are located at the center of a tetrahedron.– Each alkane is a chain of interlocking tetrahedra.– Sufficient H atoms to form a total of four bonds for
each C.
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Example of Molecules with More Than One Central Atom
C HH
H
H
C C
H
HH
H
HH
CH4
C2H6
CC
C
H
HH
H HH
HH
C3H8
HCHHH
HCHHHHCHHH
HCHHH
HCHHH
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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
• Some examples of molecules with this geometry are: – NH3, NF3, PH3, PCl3, AsH3
• These molecules are our first examples of central atoms with lone pairs of electrons.– Thus, the electronic and molecular geometries are
different.– All three substituents are the same but molecule is
polar.
• NH3 and NF3 are trigonal pyramidal, polar molecules.
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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Electronic Structures
N..
..
.
F..
.... .
H .
Lewis Formulas
2s 2p
N [He] 2s 2p
F [He] 1s
H
N..
..
.
F..
.... .
N..
..
.
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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Dot Formulas
NH H
H..
.... ..
.. ....NF F
F..
..
..
....
....
.
.....
tetrahedral
N
..
....
..
Electronic Geometry
NH H
H..
.... ..
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NF F
F
..bond dipolesoppose effect of lone pair
asymmetrical dipoles polar moleculeμ=0.2D
NH H
H
..
asymmetrical dipoles polar moleculeμ=1.5D
bond dipoles reinforce effect
of lone pair
Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Molecular Geometry
1 lone pair
pyramidal
NF F
F
..
1 lone pair
pyramidal
NH H
H
..
bondspolar ry ve
4.0 3.0 ativitiesElectroneg
F-N
bondspolar y ver
2.1 3.0 ativitiesElectroneg
H-N
1.0
0.9
321
321
Polarity1 lone pair
pyramidal
NH H
H
..
bondspolar y ver
2.1 3.0 ativitiesElectroneg
H-N
0.9321
NH H
H
..
asymmetrical dipoles polar moleculeμ=1.5D
bond dipoles reinforce effect
of lone pair
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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Valence Bond Theory (Hybridization)
2s 2p
N [He]
four sp3 hybrids
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Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of Electrons on A)
• Some examples of molecules with this geometry are: – H2O, OF2, H2S
• These molecules are our first examples of central atoms with two lone pairs of electrons.– Thus, the electronic and molecular geometries are
different.– Both substituents are the same but molecule is polar.
• Molecules are angular, bent, or V-shaped and polar.
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Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of Electrons on A)
Electronic Structures
O··
··.
.
H .
Lewis Formulas
2s 2p
O [He]
1s
H
O··
··.
.
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Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of Electrons on A)
Molecular Geometry
OH
H
··
··
2 lone pairs
bent, angularor V-shaped
bondspolar y ver
2.1 3.5 ativitiesElectroneg
H- O
1.443421
Polarity
OH
H
··
··
bond dipolesreinforce lonepairs
asymetric dipolesvery polar moleculeμ≈1.7D
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Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization)
2s 2pO [He]
four sp3 hybrids
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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)
• Some examples of molecules with this geometry are: – HF, HCl, HBr, HI, FCl, IBr
• These molecules are examples of central atoms with three lone pairs of electrons.– Again, the electronic and molecular geometries are
different.
• Molecules are linear and polar when the two atoms are different.– Cl2, Br2, I2 are nonpolar.
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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)
Dot Formula
H F··
····
··
tetrahedral
FH
··
····
Electronic Geometry
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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)
Molecular Geometry
linear
FH
··
····
3 lone pairs
PolarityHF is a polar molecule.
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Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization)
2s 2pF [He]
four sp3 hybrids
tetrahedral
FH
··
····
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Some examples of molecules with this geometry are: PF5, AsF5, PCl5, etc.
• These molecules are examples of central atoms with five bonding pairs of electrons.The electronic and molecular geometries are the same.
• Molecules are trigonal bipyramidal and nonpolar when all five substituents are the same.If the five substituents are not the same polarpolar
molecules can result, AsF4Cl is an example.
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Electronic Structures
As··
...
F···· .··
Lewis Formulas
4s 4p
As [Ar] 3d10
2s 2p
F [He]
As··
...
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Dot Formula
··
As
F
F
FF
F
··
··
··
····
··
····
····
····
··
··
··
··
··
·· ··trigonal bipyramidal
As
··
··
······
Electronic Geometry
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Molecular Geometry
trigonal bipyramid
AsF
F
F
F
F
··
··
··
··
·· ··
··
··
····
··
····
····
bondspolar ry ve
4.0 2.1 ativitiesElectroneg
F- As
1.943421
Polarity
symmetric dipoles cancel nonpolar molecule
AsF
F
F
F
F
··
··
··
··
·· ··
··
··
····
··
····
····
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Valence Bond Theory (Hybridization)
4s 4p 4dAs [Ar] 3d10
five sp3 d hybrids 4d
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
• If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new shapes.
1. One lone pair - Seesaw shape2. Two lone pairs - T-shape3. Three lone pairs – linear
• The lone pairs occupy equatorial positions because they are 120o from two bonding pairs and 90o from the other two bonding pairs.
– Results in decreased repulsions compared to lone pair in axial position.
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
• AB4U molecules have:
1. trigonal bipyramid electronic geometry
2. seesaw shaped molecular geometry
3. and are polar
• One example of an AB4U molecule is
SF4
1. Hybridization of S atom is sp3d.
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Molecular Geometry
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
• AB3U2 molecules have:
1. trigonal bipyramid electronic geometry
2. T-shaped molecular geometry
3. and are polar
1. One example of an AB3U2 molecule is
IF3
• Hybridization of I atom is sp3d.
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Molecular Geometry
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
• AB2U3 molecules have:
1.trigonal bipyramid electronic geometry
2.linear molecular geometry
3.and are nonpolar
1.One example of an AB3U2 molecule is
XeF2
• Hybridization of Xe atom is sp3d.
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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Molecular Geometry
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• Some examples of molecules with this geometry are: – SF6, SeF6, SCl6, etc.
• These molecules are examples of central atoms with six bonding pairs of electrons.
• Molecules are octahedral and nonpolar when all six substituents are the same.– If the six substituents are not the same polar
molecules can result, SF5Cl is an example.
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Electronic Structures
··F·· .··
·· Se·· ..
Lewis Formulas
4s 4p
Se [Ar] 3d10
2s 2p
F [He]
·· Se·· ..
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Molecular Geometry
octahedral
SeF
F
F
F
F
F
bondspolar ery v
4.0 2.4 ativitiesElectroneg
F - Se
1.643421
Polarity
symmetric dipoles cancel nonpolar molecule
SeF
F
F
F
F
F
HCHHH
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Valence Bond Theory (Hybridization)
4s 4p 4d
Se [Ar] 3d10
six sp3 d2 hybrids 4d
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• If lone pairs are incorporated into the octahedral structure, there are two possible new shapes.
1. One lone pair - square pyramidal
2. Two lone pairs - square planar
• The lone pairs occupy axial positions because they are 90o from four bonding pairs.
– Results in decreased repulsions compared to lone pairs in equatorial positions.
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• AB5U molecules have:
1.octahedral electronic geometry
2.Square pyramidal molecular geometry
3.and are polar.
• One example of an AB5U molecule is
IF5
• Hybridization of I atom is sp3d2.
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Molecular Geometry
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80
Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
• AB4U2 molecules have:
1.octahedral electronic geometry
2.square planar molecular geometry
3.and are nonpolar.
1.One example of an AB4U2 molecule is
XeF4
• Hybridization of Xe atom is sp3d2.
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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Molecular Geometry
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Compounds Containing Double Bonds
• Ethene or ethylene, C2H4, is the simplest organic compound containing a double bond.
Lewis dot formulaN = 2(8) + 4(2) = 24A = 2(4) + 4(1) = 12 S = 12
• Compound must have a double bond to obey octet rule.
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Compounds Containing Double Bonds
Lewis Dot Formula
CC
H
HH
H
C CH
H
H
H····
·· ·· ··
··orC C
H
H
H
H····
·· ·· ··
··
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Compounds Containing Double Bonds• VSEPR Theory suggests that the C atoms
are at center of trigonal planes.
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Compounds Containing Double Bonds• VSEPR Theory suggests that the C atoms
are at center of trigonal planes.
C C
H
HH
H
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Compounds Containing Double Bonds
Valence Bond Theory (Hybridization)
C atom has four electrons.
Three electrons from each C atom are in sp2 hybrids.
One electron in each C atom remains in an unhybridized p orbital
2s 2p three sp2 hybrids 2p
C
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Compounds Containing Double Bonds• An sp2 hybridized C atom has this shape.
Remember there will be one electron in each of the three sp2 lobes and one in the p orbital.
Top View Side View
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Compounds Containing Double Bonds
• Two sp2 hybridized C atoms plus p orbitals in proper orientation to form C=C double bond.
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Compounds Containing Double Bonds
• The portion of the double bond formed from the head-on overlap of the sp2 hybrids is designated as a bond.
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Compounds Containing Double Bonds
• The other portion of the double bond, resulting from the side-on overlap of the p orbitals, is designated as a bond.
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Compounds Containing Double Bonds
• Thus a C=C bond looks like this and is made of two parts, one and one bond.
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Compounds Containing Triple Bonds• Ethyne or acetylene, C2H2, is the simplest triple
bond containing organic compound.
Lewis Dot FormulaN = 2(8) + 2(2) = 20
A = 2(4) + 2(1) =10
S = 10
• Compound must have a triple bond to obey octet rule.
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Compounds Containing Triple Bonds
Lewis Dot Formula
C C HHCH HC·· ·· ···· ·· orCH HC·· ·· ···· ··
VSEPR Theory suggests regions of high electron density are 180o apart.
H C C H
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Compounds Containing Triple Bonds
Valence Bond Theory (Hybridization)
Carbon has 4 electrons.
Two of the electrons are in sp hybrids.
Two electrons remain in unhybridized p orbitals.
2s 2p two sp hybrids 2p
C [He]
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Compounds Containing Triple Bonds
A bond results from the head-on overlap of two sp hybrid orbitals.
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Compounds Containing Triple Bonds
• The unhybridized p orbitals form two bonds. Note that a triple bond consists of one and
two bonds.
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Compounds Containing Triple Bonds• The final result is a bond that looks like this.
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Summary of Electronic & Molecular Geometries
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Synthesis Question• The basic shapes that we have discussed in Chapter
8 are present in essentially all molecules. Shown below is the chemical structure of vitamin B6 phosphate. What is the shape and hybridization of each of the indicated atoms in vitamin B6 phosphate?
N+
H
CH3
OH
COH
CH2
OP
O
O
O
1
2
4
5
3
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Synthesis Question
trigonal planar sp2
bent or angular sp3
tetrahedral sp3
trigonal planar sp2
trigonal planar sp2
N+
H
CH3
OH
COH
CH2
OP
O
O
O
1
2
4
5
3
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Group Question
• Shown below is the structure of penicillin-G. What is the shape and hybridization of each of the indicated atoms in penicillin-G?
CH
C N
CH S
CH
C
CH3
CH3
OH
OO
NHO
CH2CCC
CC C
HH
H
HH
1
23 4
5 6
7
8910
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8Molecular
Structure & Covalent
Bonding Theories