Molecular Geometry. Valence Shell Electron Pair. What Determines the Shape of a Molecule? Repulsion Theory (VSEPR) Localized Electron Model

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1 Molecular Geometry Learn Shapes you will Because the physical and chemical properties of compounds are tied to their structures, the importance of molecular geometry can not be overstated. Localized Electron Model According to the Localized Electron Model, the arrangement of atoms, bonds and nonbonding electron pairs can be predicted for most molecules. One important reason for drawing these Lewis structures is to be able to predict the three-dimensional geometry of molecules and molecular ions. By noting the number of bonding and nonbonding electron pairs according to the Localized Electron Model, we can easily predict the shape of the Valence Shell Electron Pair Repulsion Theory (VSEPR) Developed by Ronald Nyholm (left) and Ronald Gillespie (right) in 1957 The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them. What Determines the Shape of a Molecule? Simply put, electron pairs, whether they be bonding or nonbonding, repel each other. By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the Valence Shell Electron Pair Repulsion Theory contends that the structure around a given atom is determined principally by minimizing electron pair repulsions. Meaning, bonded and non bonding pairs of electrons around a central (AB n ) atom will be as far apart as possible. Or, have the greatest possible bond angle. The VSEPR model uses electron-pair geometry around a central atom to predict molecular geometries. Electron-pair geometry: The arrangement of electron pairs, or electron domains, around a central atom. Molecular geometry: Includes the effect of non-bonding electrons. 1

2 Electron Domains This molecule has four electron domains around the central atom. We can refer to the electron pairs as electron domains. In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. Electron-Pair Geometries or (Electron-Domain Geometries) These are the electron-domain geometries for two through six electron domains around a central atom. Electron-Domain Geometries All one must do is count the number of electron domains in the Lewis structure. The geometry will be that which corresponds to that number of electron domains. # e- pairs Electron-pair Example Bond geometry angle 2 Liner BeCl o 3 Triagonal Planar BF o 4 Tetrahedral CH o 5 Triagonal Bipyramidal PCl o, 90 o 6 Octahedral SF 6 90 o, 90 o Remember, this is only electron pair geometry To determine the electron-pair geometry: 1. Draw the Lewis structure(s) 2. Count & total the number of electron pairs around the central atom. (double and triple bonds are only counted as one pair) 3. Describe the geometry in terms of greatest possible bond angle between electron-pairs 1. Determine the electron-pair geometry (electron-domain geometry) for the central atom in each of the following: CO 2 O 3 NH 3 H 2 O * Any resonance structure can be used to predict geometry. 2

3 Molecular Geometries The electron-domain geometry is often not the shape of the molecule, however. The molecular geometry refers to the positions of the atoms in the molecules as they are affected by the nonbonding pairs. Electron-pair geometry is excellent for predicting the general arrangement; however, it is not the same as the actual molecular geometry. Compare: CH 4 and NH o o Molecular geometry describes not only the geometry of the electron pairs, but it also attempts to describe the influence of any unbonded electron pairs on the total Experimentation has shown that un-bonded electron pairs occupy more space than bonded pairs, because they are not localized between two atoms; Therefore, the angles between them and other electron pairs, bonded or not, is increased. Molecular Geometries Within each electron domain, then, there might be more than one molecular geometry. Linear Electron Domain Trigonal Planar Electron Domain In this domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is. There are two molecular geometries: Trigonal planar, if all the electron domains are bonding Bent, if one of the domains is a nonbonding pair. 3

4 Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds. Therefore, they also affect bond angles. Tetrahedral Electron Domain There are three molecular geometries: Tetrahedral, if all are bonding pairs Trigonal pyramidal if one is a nonbonding pair Bent if there are two nonbonding pairs Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry: Axial Equatorial Trigonal Bipyramidal Electron Domain Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry. There are four distinct molecular geometries in this domain: Trigonal bipyramidal Seesaw T-shaped Linear 4

5 Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: Octahedral Square pyramidal Square planar In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole. Larger Molecules Larger Molecules This approach makes sense, especially because larger molecules tend to react at a particular site in the Although the VSEPR model is not perfect, it works for a wide range of molecules. The bond angles predicted are very close to those determined experimentally. Using VSEPR to predict molecular geometry: 1. Draw Lewis structure (most probable) 2. Count & total electron pairs (remember multiple bonds are counted as a single pair) 3. Determine the basic electron-pair geometry 4. Predict Distortion caused by lone pairs of electrons. 2.Predict the molecular geometries for the following molecules: H 2 O PCl 5 XeF 4 I 3 - NO 3 - Bond Polarity In Chapter 8 we discussed bond dipoles. But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. 5

6 Here are some common bonds with their relative dipole moments. Note however, that not all molecules with dipole moments are polar. Molecular Polarity describes the charge distribution along the entire molecule as the vector sum of the individual dipole moments. Compare the following for molecular polarity: Carbon Dioxide CO 2 is NOT polar even though the CO bonds are polar. CO 2 is symmetrical. 95 Types of molecules with polar bonds but no resulting molecular polarity Type Cancellation of Polar Bonds Example Linear CO 2 Positive C atom is reason CO 2 + H 2 O gives H 2 CO Triagonal Planer Tetrahedr al SO 3 CCl 4 Molecular Polarity Molecules will be polar if a) bonds are polar AND b) the molecule is NOT symmetric 9 Molecular Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the All above are NOT polar 6

7 3. Describe the polarity associated with: a.ch 2 Cl 2 b.bclf 2 4. Complete the following for a molecule of PF 4-. a) Draw the best Lewis structure. b) Show all dipole moments and assign all partial charges. c) Predict the proper electron-pair and molecular geometry. d) Describe the overall polarity of the 7