1 Molecular Geometry and VSEPR Theory Chapter 4 Pages Mrs. Weston Advanced Chemistry

2 Molecular Shapes The shape of a molecule plays a large part in determining its properties and reactivities. We can predict shapes by examining the Lewis structure for orientation of electron pairs. Electron pairs arrange themselves to minimize electrical repulsion.

3 VSEPR Theory  In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimized this repulsion.  Electron pairs arrange themselves as far as possible from each other.  We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.

4 Why is VSEPR Theory Important? Gives a specific shape due to the number of bonded and non-bonded electron pairs in a molecule Tells us the actual 3-D structure of a molecule Again in bonding, electron pairs want to be as far away from each other as possible.

5 The VSEPR Model Predicting Molecular Geometries

6 How does VSEPR THEORY work? We can use VSEPR theory using 4 steps. 1.Draw the Lewis Structure for the molecule. Example: SiF 4 F F-Si-F F

7 How does VSEPR THEORY work? We can use VSEPR theory using 4 steps 1.Draw the Lewis Structure for the molecule. 2.Tally the number of bonding pairs and lone (non-bonding) pairs on the center atom. F F-Si-F F Bonding pairs: 4 Lone pairs on central atom: 0

8 How does VSEPR THEORY work? We can use VSEPR theory using 4 steps 1.Draw the Lewis Structure for the molecule 2.Tally the number of bonding pairs and lone pairs on the center atom. 3.Arrange the rest of the atoms so that they are as far away from each other as possible. Si F F F F

9 How does VSEPR THEORY work? We can use VSEPR theory using 4 steps 1.Draw the Lewis Structure for the molecule 2.Tally the number of bonding pairs and lone pairs on the center atom. (Double and triple bonds only count as one pair.) 3.Arrange the rest of the atoms so that they are as far away from each other as possible 4.Give the type of geometry the molecule has: Tetrahedral

10 The VSEPR Model Difference between geometry and shape Electron Pair Geometry: We determine the geometry only looking at electrons. All the atoms that obey the octet rule and have single bonds have the same tetrahedral-like geometry. Shape or Molecular Structure: We name the shape by the positions of atoms. We ignore lone pairs in the shape.

11 VSEPR and Resulting Shapes

12 The VSEPR Model Predicting Shape Shape

13 The VSEPR Model Predicting Shape Shape

14 The VSEPR Model The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles By experiment, the H-X-H bond angle decreases on moving from C to N to O: Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. Therefore, the bond angle decreases as the number of lone pairs increase.

15 The VSEPR Model The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles Similarly, electrons in multiple bonds repel more than electrons in single bonds.

16 The VSEPR Model Molecules with More than One Central Atom In acetic acid, CH 3 COOH, there are three central atoms. We assign the geometry about each central atom separately.

17 Polarity of Molecules A molecule is POLAR if its centers of positive and negative charges do not coincide. The molecule behaves as a dipole (Having two ends of opposite charge)

18 Polarity of Molecules Dipole: def: two charges, equal in magnitude and opposite in sign, are separated by a distance Polarity of Polyatomic Molecules Each bond can be polar. The orientation of these polar bonds determines whether the molecule is polar overall. It is possible for a molecule with polar bonds to be either polar or non-polar.

19 Polarity of Molecules Dipole Moments of Polyatomic Molecules Example: in CO 2, each C-O dipole is canceled because the molecule is linear. In H 2 O, the H-O dipoles do not cancel because the molecule is bent.

20 Polarity of Molecules Dipole Moments of Polyatomic Molecules

21 Hybrid Orbitals s and p orbitals are used in bonding. It is easy to tell which ones are used by looking at our molecule. (2s and 2p) For example, CH 4. Looking again at the Lewis structure, we see that there are 4 bonds. The s orbital and three p orbitals can combine and form 4 sp 3 bonds.

22 Hybrid Orbitals Regions of electron density-EACH BOND AND LONE PAIR OF ELECTRONS ON THE CENTRAL ATOM IS KNOWN AS A REGION OF ELECTRON DENSITY. 4 regions of electron density-sp 3 hybridized 3 regions of electron density-sp 2 hybridized 2 regions of electron density-sp hybridized

23 Hybrid Orbitals

24 Hybrid Orbitals Summary To assign hybridization: 1.Draw a Lewis structure. 2.Assign the electron pair geometry using VSEPR theory. 3.Use the electron pair geometry to determine the hybridization. 4.Name the shape by the positions of the atoms.