Download Part One: Molecular Geometry and Directional Bonding A

CHAPTER TEN: MOLECULAR GEOMETRY AND CHEMICAL BONDING THEORY
Part One: Molecular Geometry and Directional Bonding
A. Introduction to Covalent Bonding Theories.
B.
1.
Lewis structures allowed us to account for valence electrons and bonds formed in
molecules.
2.
Provided no insight into molecular geometry = 3-D shape of a molecule as determined
by the relative positions of the nuclei.
3.
Hydrogen atom Atomic Orbitals showed us the organization of electrons in atoms, but
not molecules.
4.
Thus, we need:
a.
Theory of electrons in molecules - i.e., replace atomic orbitals with molecular
orbitals.
b.
Theory of geometry of molecules.
5.
Valence Shell Electron Pair Repulsion model (VSEPR) provides a predictive tool for
geometries.
6.
Valence Bond Theory (VB) provides a more detailed picture of what happens to
electron wave functions (orbitals) in molecules.
7.
Molecular Orbital Theory (MO) = the most detailed and accurate theory of electrons
in molecules, in which orbitals may spread over the entire molecule. (Less intuitive
than VB theory) HyperChem uses MO theory.
Valence Bond Theory. (Linus Pauling - 1930’s) (Section 10.3)
1.
Chapter 10
Case Study - CH4.
a.
Known to be tetrahedral.
b.
Must have 4 identical bonds, but how?
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Chapter 10
c.
Before bonding:
d.
How does C form 4 identical bonds?
e.
Incoming H• atoms cause a distortion of the atomic orbitals around Carbon,
called hybridization.
f.
(2s atomic orbital) + 3 (2p atomic orbitals) mix together to produce 4 equal sp3
hybrid orbitals arranged in a tetrahedron:
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g.
2.
Chapter 10
Now 4 equal bonds can form:
Other hybridizations (mixing of AO’s of central atom) are also possible to produce
other geometries.
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C.
VSEPR Model. (Section 10.1)
-makes use of VB theory (hybrid orbitals) to fully predict geometry.
1.
First write Lewis structure and identify a central atom:
2.
Count the number of regions of high electron density (RHED) around the central
atom:
3.
VSEPR says these will repel each other forming angles in which they are as far apart
us possible.
4.
Note that some RHED will be lone pairs (unshared pairs) and some will be bonding
pairs (shared pairs).
5.
Total pairs, that is, total number of RHED determines the electronic geometry around
central atom. See Table 8-1.
Chapter 10
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6.
Now place the bonded atoms (B) around the central atom (A) as well as the unshared pairs
(U).
Chapter 10
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a.
CH4 case, is of type AB4. (no U’s)
With no unshared pairs:
molecular geometry = electron geometry
b.
NH3 case, is of type AB3U.
With unshared pairs:
molecular geometry ≠ electron geometry
Chapter 10
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Chapter 10
c.
Try H2O.
d.
Try ozone, O3.
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e.
Try SF4.
Put unshared pairs on equatorial position, not axial, to minimize repulsion.
Unshared pairs are “fatter” than bonded regions.
Chapter 10
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D. Summary of Electronic and Molecular Geometries.
Chapter 10
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E.
Dipole Moment and Molecular Geometry.
1.
Dipole moment is a quantitative measure of the degree of charge separation in a
molecule.
a.
Measured in Debye units 1 D = 3.34x10-30 Coul-m
b.
What is dipole moment of diatomic molecule represented as a +e and -e charge
separated by 1 Angstrom?
e = 1.6 x 10-19 Coulombs
2.
Chapter 10
Polar bonds may or may not give rise to an overall dipole moment in a molecule,
depending on its geometry.
Page 10
3.
Example: Water vs Carbon dioxide.
4.
Example: Chloroform vs carbon tetrachloride.
5.
Polar molecules tend to align in an electric field. And tend to attract other polar
molecules. Tend to dissolve in polar fluids, so important in solubility.
6.
Now that we can predict molecular geometry, we can predict polarity in a molecule.
Chapter 10
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F.
Descriptions of Multiple Bonding using VB Theory.
1.
In VB theory a pair of electrons forms one bond.
2.
Have seen molecules with more than a single bond connecting some of the atoms:
example: ethene
3.
Chapter 10
How do we describe its bonding using VB theory?
a.
uses sp2 hybridization on each Carbon atom, leaving an uninvolved p orbital
perpendicular to the plane of the sp2 trigonal planar framework:
b.
sp2 framework is made of σ (sigma) bonds:
around the axis between the two bonded atoms.
c.
The un-used p orbitals may also overlap to form a π (pi) bond = one that is not
cylindrically symmetric around the bond axis.
bonds cylindrically symmetric
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