1. Chapter 8 Covalent Bonding 8.3 Bonding Theories
8.1 Molecular Compounds
8.2 The Nature of Covalent Bonding
8.3 Bonding Theories
8.4 Polar Bonds and Molecules
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2. What is the problem with a flat 2-dimensional map?
CHEMISTRY & YOU
What is the problem with a flat 2-dimensional map?
It does not show an accurate picture of what is really going on or what something truly looks like.
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3. VSEPR Theory What do scientists use the VSEPR theory for? VSEPR Theory
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4. VSEPR Theory
To explain the three-dimensional shape of molecules, scientists use Valence-Shell Electron-Pair Repulsion theory (VSEPR theory).
VSEPR theory states that the repulsion between electron pairs causes molecular shapes to adjust so that the valence electron-pairs stay as far apart as possible.
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5. (electron dot structure)
VSEPR Theory
The structural formula and electron dot structure of methane (CH4) show the molecule as if it were flat and two-dimensional.
They fail to reflect the true three-dimensional shape of the molecule.
Methane
(electron dot structure)
(structural formula)
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6. All of the H–C–H angles are 109.5°, the tetrahedral angle.
VSEPR Theory
In methane, the hydrogen atoms are at the four corners of a geometric shape called a tetrahedron.
All of the H–C–H angles are 109.5°, the tetrahedral angle.
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7. In ammonia (NH3) one of the pairs of electrons are unshared.
VSEPR Theory
Unshared (lone) pairs of electrons are also important in predicting the shapes of molecules.
Ammonia (NH3)
In ammonia (NH3) one of the pairs of electrons are unshared.
Nonbinding pairs repel electrons more than shared pairs.
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8. VSEPR Theory
Because no atom needs these electrons; they are held closer than bonding pairs.
The lone pair pushes the bonding pairs closer together changing the angle and shape.
The H—N—H bond angle is only 107°, rather than the tetrahedral angle of 109.5°.
Unshared electron
pair
107°
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9. Thus, the water molecule is bent but flat
VSEPR Theory
Water’s two lone pairs of electrons push the bonding pairs father around the central oxygen.
Thus, the water molecule is bent but flat
With two unshared pairs, the H—O—H bond angle is compressed to about 105°.
Unshared pairs
105°
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10. Thus, CO2 is a linear molecule.
VSEPR Theory
The carbon in a carbon dioxide molecule has no unshared electron pairs.
The double bonds joining the oxygens to the carbon are farthest apart and the O=C=O bond angle is 180°.
Thus, CO2 is a linear molecule.
Carbon dioxide (CO2)
No unshared
electron pairs
on carbon
180°
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11. Here are some common molecular shapes.
VSEPR Theory
Here are some common molecular shapes.
Linear
Trigonal planar
Bent
Pyramidal
Tetrahedral
Trigonal
bipyramidal
Octahedral
Square
planar
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12. What causes valence electron-pairs to stay as far apart as possible?
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13. What causes valence electron-pairs to stay as far apart as possible?
The repulsion between electron pairs due to their negative charges causes valence electron-pairs to stay as far apart as possible.
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14. Molecular Orbitals How are atomic and molecular orbitals related?
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15. Molecular Orbitals
Just as an atomic orbital belongs to a particular atom, a molecular orbital belongs to a molecule as a whole.
A molecular orbital that can be occupied by two electrons of a covalent bond is called a bonding orbital.
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16. Molecular Orbitals
When two atoms combine, their atomic orbitals overlap to produce molecular orbitals, or orbitals that apply to the entire molecule.
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17. Molecular Orbitals
Covalent bonding results from an imbalance between the attractions and repulsions of the nuclei and electrons involved.
The nuclei and electrons attract each other.
Nuclei repel other nuclei.
Electrons repel other electrons.
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18. The result is a stable diatomic molecule.
Molecular Orbitals
In a bonding molecular orbital, the attractions between the nuclei and the electrons are stronger than the repulsions.
The balance of all the interactions between the atoms is thus tipped in favor of holding the atoms together.
The result is a stable diatomic molecule.
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19. Its symbol is the Greek letter sigma (σ).
Molecular Orbitals
Sigma Bonds
When a molecular orbital forms that is symmetrical around the axis connecting two atomic nuclei, a sigma bond molecular orbital is formed.
Its symbol is the Greek letter sigma (σ).
s atomic orbital
Bond axis
Sigma-bonding molecular orbital
 represents the nucleus
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20. Molecular Orbitals
Sigma Bonds
Just like s orbitals, p orbitals can also overlap to form molecular orbitals.
When two fluorine atoms combine, the p orbitals overlap end to end to produce a bonding molecular orbital.
The result is a sigma bond by the p atomic orbitals.
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21. Molecular Orbitals
Sigma Bonds
There is a high probability of finding a pair of electrons between the positively charged nuclei of a sigma bond.
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22. Molecular Orbitals
Pi Bonds
The side-by-side overlap of p orbitals produces what is called a pi bond molecular orbital.
Its symbol is the Greek letter pi ().
p atomic orbital
Pi-bonding molecular orbital
 represents the nucleus
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23. Molecular Orbitals
Pi Bonds
In a pi bond, the bonding electrons are most likely to be found in sausage-shaped regions above and below the bond axis of the bonded atoms.
Because orbitals in pi bonding overlap less than in sigma bonding, pi bonds tend to be weaker than sigma bonds.
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24. In these drawings, show where an electron is most likely to be found?
Sigma Pi
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25. In these drawings, show where an electron is most likely to be found?
Sigma Pi
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26. Hybrid Orbitals What is orbital hybridization? Hybrid Orbitals
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27. Hybrid Orbitals
Orbital hybridization is the combining of two different types of atomic orbitals.
In hybridization, several atomic orbitals mix to form the same total number of equivalent hybrid orbitals.
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28. Hybridization of Orbitals
Hybrid Orbitals
Hybridization of Orbitals
Recall that the carbon atom promotes one of the 2s electrons to a 2p orbital to give it four bonding sites.
This creates four sp3 hybrid orbitals.
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29. S F F Hybridization of Orbitals
Hybrid Orbitals
Hybridization of Orbitals
Sulfur has 6 valence electrons leaving room for 2 flourine atoms to combine creating a SF2 molecule.
So why do we get SF6? S F F
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30. Hybrid Orbitals
Hybridization of Orbitals
Hybridization of orbitals helps explain why some atoms can violate the octet rule.
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31. How many hybridized orbitals form when one 2s orbital is hybridized with two 2p orbitals?
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32. How many hybridized orbitals form when one 2s orbital is hybridized with two 2p orbitals?
Three sp2 orbitals form.
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33. Orbital hybridization provides for the expansion of the octet rule.
Key Concepts
Just as an atomic orbital belongs to a particular atom, a molecular orbital belongs to a molecule as a whole.
In order to explain the three-dimensional shape of molecules, scientists use the valence-shell electron-pair repulsion theory (VSEPR theory).
Orbital hybridization provides for the expansion of the octet rule.
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34. END OF 8.3
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