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. Molecular Geometries and Bonding Theory 1 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 Molecular Geometries and Bonding Theory 2
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 Molecular Geometries and Bonding Theory 3 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. Molecular Geometries and Bonding Theory 4
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. All Electron Domains counted Only Bonds counted, lone pairs ignored Molecular Geometries and Bonding Theory 5 Details of Molecular Geometries Derived from Linear and Trigonal Planar Electron Domain Geometries Molecular Geometries and Bonding Theory 6
Details of Molecular Geometries Derived from a Tetrahedral Electron Domain Geometry Molecular Geometries and Bonding Theory 7 Details of Molecular Geometries Derived from a Trigonal Bipyramidal Electron Domain Geometry Molecular Geometries and Bonding Theory 8
Details of Molecular Geometries Derived from an Octahedral Electron Domain Geometry Molecular Geometries and Bonding Theory 9 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 Molecular Geometries and Bonding Theory 10
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, NO 3, is The bond angles in this ion are exactly 120. Is this observation consistent with the above discussion of the effect of multiple bonds on bond angles Molecular Geometries and Bonding Theory 11 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 Geometry Equatorial lone pairs 2 lone pairs Molecular Geometries and Bonding Theory 12 3 lone pairs
Geometries of Larger Molecules The VSEPR model can be extended to consider every central atom in a more complex, larger molecule. Consider Acetic Acid: Molecular Geometries and Bonding Theory 13 Molecular Geometries of Complex Molecules - DNA Molecular Geometries and Bonding Theory 14
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 Molecular Geometries and Bonding Theory 15 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. Bond dipoles and dipole moments are 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. Molecular Geometries and Bonding Theory 16
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 cancel. Polar bonds cancel Polar bonds do not cancel Molecular Geometries and Bonding Theory 17 Polarity of Some Molecules Give It Some Thought The molecule OCS has a Lewis structure analogous to that of CO 2 and is a linear molecule. Will it necessarily have a zero dipole moment like CO 2? Molecular Geometries and Bonding Theory 18
Polarity of Molecules Dipole Moments of Some Molecules Molecular Geometries and Bonding Theory 19 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. Molecular Geometries and Bonding Theory 20
Problem 1: Acrolein 1. Give the molecular geometry around each central atom. 2. Identify all bonds as polar or nonpolar. 3. Does this molecule have a net dipole moment? Molecular Geometries and Bonding Theory 21 Problem 2: Acetonitrile 1. Give the molecular geometry around each central atom. 2. State the indicated bond angles. 3. Identify all bonds as polar or nonpolar. 4. Does this molecule have a net dipole moment? Molecular Geometries and Bonding Theory 22
Problems Text question 9.3: An AB 5 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 9.18: The AB 3 molecule is described as having a trigonal-bipyramidal electron-domain geometry. How many nonbonding domains are on atom A? Explain Molecular Geometries and Bonding Theory 23 Problems Text question 9.28: The three species NH 2, NH 3, and NH 4+, have H N H bond angles of 105, 107, and 109, respectively. Explain this variation in bond angles. Additional Question: Dichloroethylene (C 2 H 2 Cl 2 ) 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 which of the three isomers was used? Molecular Geometries and Bonding Theory 23 24