Download Bonding and Molecular Structure - PART 1

Bonding and Molecular Structure - PART 1 - VSEPR
Objectives:
1. Understand and become proficient at using VSEPR to predict the
geometries of simple molecules and ions.
2. Become proficient at predicting bond angles and polarity of simple
molecules.
The basis of this model is that valence electrons arrange themselves around a central atom
in such a way as to minimize repulsions.
Valence electrons are considered to be localized into regions called electron domains.
(Some textbooks use the term electron groups rather than electron domains.)
An electron domain around an atom is:
a single bond,
a double bond,
a triple bond,
a lone pair, or
a lone electron (recall that free radicals contain unpaired electrons).
The best arrangement of a given number of electron domains is the one that minimizes the
electrostatic repulsions between them.
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ELECTRON Geometries Predicted by VSEPR
Valence Shell Electron Pair Repulsion predicts the following molecular
geometries around a central atom.
These are the FIVE BASE ELECTRON GEOMETRIES.
These 5 geometries minimize the
repulsions between the electron
domains on each central atom.
These Last two 2 geometries requiring an Expanded Octet
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Example Electron Domain Geometry - Tetrahedral
From the Lewis Structure we can count
electron domains around the central atom.
The number of electron domains
determines the basic arrangement of the
electron domains around the central atom.
In this case four electron domains (4 single
bonds) gives the tetrahedral geometry
with bond angles of 109.5°
To determine the geometry around a
central atom you must first be able to draw
the LEWIS STRUCTURE!
Tetrahedral
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Details of the 5 Base Electron Domain Geometries from
VSEPR
Bond angles are the angles made by the lines
joining the nuclei of the atoms in a molecule.
Each of the five basic geometries has specific
bond angles associated with it that you must
memorize (see Table 9.1). These “ideal” bond
angles may be distorted by certain conditions as
we shall see later.
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The MOLECULAR GEOMETRY describes the spatial arrangement of ATOMS
around a central atom. This is a subset of the ELECTRON GEOMETRY
The arrangement of electron domains about a central atom is called the electron-domain
geometry (or electron-group geometry) as previously discussed.
The molecular geometry is the arrangement of only the atoms in a molecule or
polyatomic ion.
Describes arrangement of all
Electron Domains
Describes arrangement
of Bonds
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Details of Molecular Geometries Derived from Linear and
Trigonal Planar Electron Domain Geometries
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Details of Molecular Geometries Derived from a
Tetrahedral Electron Domain Geometry
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Details of Molecular Geometries Derived from a Trigonal
Bipyramidal Electron Domain Geometry
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Details of Molecular Geometries Derived from an
Octahedral Electron Domain Geometry
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VSEPR Bond Angle Detail #1:
Lone Pairs and Slight Changes to Bond Angles
Lone pairs on a central atom will cause the bonding groups to move closer
together, decreasing the bond angle. Lone pairs occupy more space and
are more repulsive than bonding pairs.
Less
repulsive
More
repulsive
Decrease in bond angle as lone pairs are added
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VSEPR Bond Angle Detail #2:
Double Bonds Change Bond Angles
A double bond on a central atom cause adjacent single bonding groups to move closer
together, decreasing the bond angle between them. Double bonds occupy more space and
are more repulsive than single bonds.
Conclusion for Repulsive Energies:
Lone Pair > Double Bond > Single Bond > Single e–
Give It Some Thought
One of the resonance structures of the nitrate
ion is shown here. The bond angles in this ion
are exactly 120°. Considering the CCl2O
example shown above, explain why all the
bond angles are the same for the nitrate ion.
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More VSEPR Details:
5 Electron Groups and Axial vs. Equatorial Positions
When we form the trigonal bipyramidal electron domain
geometry we have inequivalent bonding positions, axial
and equatorial.
Lone pairs prefer the equatorial positions since they
minimize the strong 90° repulsions for the lone pairs.
1 lone pair
Seesaw Molecular
Geometry
Equatorial
lone pairs
2 lone pairs
3 lone pairs
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Geometries of Larger Molecules
The VSEPR model can be extended to consider every central
atom in a more complex, larger molecule.
Consider Acetic Acid:
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Molecular Geometries of Complex Molecules - DNA
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Dipole Moments and Polar Molecules
Many molecules are polar. They have a dipole moment and will align
themselves in an applied electric field.
Polar molecule
No alignment
Alignment
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Dipole Moments for Polyatomic Molecules
For a molecule that consists of more than two atoms (a polyatomic molecule), the
dipole moment depends upon both the individual bond polarities and the molecular
geometry. We can determine polarity of polyatomic molecules as follows:
• View bond dipoles and dipole moments as vector quantities; that is they have both a
magnitude and a direction.
• The overall dipole moment of a polyatomic molecule is the vector sum of the bond
dipoles. Both the magnitudes and the directions of the bond dipoles must be considered.
(Molecular Geometry analysis is necessary!)
• It is possible to have a nonpolar molecule that contains polar bonds if the polar bond
dipoles are arranged in such a way as to “cancel” each other.
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Dipole Moment Depends on Bond Polarity and Electron Geometry
To have a dipole moment a molecule must have:
1. Polar bonds and/or lone pairs.
2. A molecular geometry where the polar bonds/lone pairs do not
offset (cancel) each other.
Polar bonds cancel
Polar bonds do not cancel
Give It Some Thought
The molecule OCS has a Lewis structure analogous to that of CO2 and is a linear molecule.
Will it necessarily have a zero dipole moment like CO2?
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Polarity of Some Molecules
Give It Some Thought
Why is ozone (O3) a polar molecule?
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Polarity of Molecules
Dipole
molecules:
Text moments of some
Dipole
Moments of Some Molecules
Remember: The units of the dipole moment are debyes, D: 1D = 3.34x10–30 C•m
Give It Some Thought
Based on differences in electronegativites, the N–F bond is slightly more polar than the N–
H bond. Why then is NF3 a much less polar molecule compared to NH3?
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Microwave Ovens - How do they work?
The microwaves are oscillating
electric fields that cause the water
molecules to flip back and forth as
the wave passes through them.
Remember, water has a relatively
large large dipole moment to interact
with the electric field.
Each water molecule might flip a
billion times a second!
The friction from the flipping water
molecules heats your food.
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Predicting Electron Domain Geometries, Molecular Geometries, Bond
Angles and Dipole Moments
We can generalize the steps we follow in using the VSEPR model to predict the electron
domain geometries, molecular geometries, bond angles and dipole moments. Use the
VSEPR worksheet to guide you as you learn this process.
1.
Draw the Lewis structure of the molecule or ion, and count the total number of electron domains around the
central atom. Each nonbonding electron pair, each single bond, each double bond, and each triple bond counts
as an electron domain.
2.
Determine the electron-domain geometry by arranging the electron domains about the central atom so that the
repulsions among them are minimized, as shown in Table 9.1.
3.
Use the arrangement of the bonded atoms to determine the molecular geometry as shown in Tables 9.2 and 9.3.
4.
Look at the arrangement and types of electron domains. Predict if any bond angles will vary from their “ideal
values”.
5.
Determine if the molecule has a net dipole moment. Use the flow diagram on the VSEPR worksheet. Note: Since
IONS have a nonzero charge, dipole moments do not apply.
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Problem 1: Acrolein
1. Determine the molecular geometry around each central atom. Draw the molecule,
showing the correct geometry.
2. Indicate the “ideal” bond angles.
3. Does this molecule have a net dipole moment? If it does, indicate the direction of the
dipole using an arrow.
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Problem 2: Acetonitrile
1. Determine the molecular geometry around each central atom. Draw the molecule,
showing the correct geometry.
2. State the indicated “ideal” bond angles.
3. Does this molecule have a net dipole moment? If it does, indicate the direction of the
dipole using an arrow.
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Problems
Text question 9.95: An AB5 molecule adopts the geometry shown to the right.
(a) What is the name of this geometry?
(b) Do you think there are any nonbonding electron pairs on atom A? Why or why
not?
(c) Suppose the atoms B are halogen atoms. Can you determine uniquely to
which group in the periodic table atom A belongs?
Text question 9.24: An AB3 molecule is described as having a trigonal-bipyramidal electron-domain
geometry. How many nonbonding domains are on atom A? Explain
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Problems
Text question 9.34: The three species NH2−, NH3, and NH4+, have H–N–H bond angles of 105°, 107°,
and 109°, respectively. Explain this variation in bond angles.
Additional Question: Dichloroethylene (C2H2Cl2) has three forms (isomers), each of which is a different
substance. A pure sample of one of these substances is found experimentally to have a dipole
moment of zero. Can we identify the sample?
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