Understanding Molecular Shape: VSEPR Theory Explained

1. Chapter 8 Molecular Structure, Valence Bond Theory, and Hybridization

2. ValenceShellElectronPairRepulsionTheory Trigonal planar Tetrahedral Trigonal bipyramidal Octahedral

3. VSEPR Theory Explains how molecules obtain their shapes. Coulomb’s Law allows us to predict that regions of High Electron Density (bonds or lone pairs) arrange themselves around a central atom as far away from each other as possible so as to minimize repulsive forces.

4. VSEPR Theory The combination of Lewis diagrams with the VSEPR Theory gives a powerful model for predicting structural properties of many covalently bonded molecules and polyatomic ions, including: Molecular Geometry Bond Angles Presence of a dipole moment (polarity).

5. From Chapter 8 Note Supplement • Molecular Shape: When explaining molecular shape include the following 3 steps in your answer: • What is the geometry of the molecule: based on the number of regions of high electron density. (See Table 8.1 pp. 215). • Discuss VSEPR theory: The regions of high electron density will get as far apart as possible from one another in order to minimize repulsive forces. (The underlined statement is an essential part of any explanation of molecular shape and is the essence of VSEPR theory). • What is the shape of the molecule: based on the number of bonds vs. unshared pairs. (See Table 8.2 pp. 216). • Explain placement of unshared pairs and/or bonds. (Easiest to use bond angles here).

6. Table 8.1 page 215 Trigonal Planar

7. Table 8.1 Page 215 The Number of Regions of High Electron Density around the Central Atom Determine Molecular Geometry.... 2 regions  linear (Bond angle = 180o) 3 regions  trigonal planar (Bond angle = 120o) 4 regions  tetrahedral (Bond angle = 109.5o) 5 regions  trigonal bipyramidal (Bond angles = 90o and 120o ) 6 regions  octahedral (Bond angle = 90o)

8. Trigonal Bipyramidal Axial atoms are above and below the plane of the triangle on opposite sides of the molecule. Equatorial atoms are 120° apart and are within the plane of the triangle.

9. Molecular Shape and VSEPR Theory Helps to Determine • Bond Angles • Molecular Polarity

10. Table 8.1 page 215 Trigonal Planar

11. Bond Angles • Bond angles are based on the geometry of the molecule • Slight adjustments may be necessary because of various space requirements for different types of regions of high electron density. • Lone pairs have the greatest space requirement followed by triple bonds then double bonds and finally single bonds which require the least amount of space around the central atom.

12. Linear • Two atom molecule. • 2/0 central atom count. • Two atoms attached to the central atom and no lone pairs on the central atom. • Bond angle of 180°

14. Linear Lewis Structure Molecular Geometry O = C = O

15. Linear Lewis Structure CO2 has two regions of high electron density resulting in a linear geometry. In order to minimize the repulsive forces the two oxygen atoms are bonded 180˚apart resulting in a linear shape. Molecular Geometry

16. 3/0 or 2/1 count on central atom. • The geometry of the molecule is trigonal planar. • The base angle for this geometry is 120°. • The shape can be either trigonal planar or angular.

17. Trigonal Planar

18. Trigonal Planar BF3 has three regions of high electron density resulting in a trigonal planar geometry. In order to minimize the repulsive forces the three fluorine atoms are bonded 120° apart resulting in a trigonal planar shape.

19. Explain why NO2- has a bond angle of 115.4°?

20. NO2- has three regions of high electron density which results in a trigonal planar geometry and a base angle of 120°. However the greater space requirement of the lone pair repels the bonds and results in a bond angle of 115.4°.

21. 4/0 or 3/1 or 2/2 count on central atom. • The geometry of the molecule is tetrahedral. • The base angle for this geometry is 109.5°. • The shapes can be either tetrahedral, trigonal pyramidal, or angular.

22. Why does water have a bond angle of 105°?

23. Why does water have a bond angle of 105°? • The four regions of high electron density surrounding the oxygen tend to arrange themselves as far from each other as possible in order to minimize repulsive forces. This results in a tetrahedral geometry in which the H-O-H bond angle would be 109.5°. However, the two lone pairs around the oxygen atom, have a greater space requirement, effectively pushing the two hydrogen atoms closer together. The result is a H—O—H angle of 105°.

24. 5/0 or 4/1 or 3/2 or 2/3 count on central atom. • The geometry of the molecule is trigonal bipyramidal. • The base angles for this geometry are 120° and 90°. However be careful with the angles on this geometry as they can vary depending upon the shape. • The shapes can be either trigonal bipyramidal, seesaw, T- shaped, or linear.

25. It will be helpful when explaining shapes based on a Trigonal Bipyramidal geometry to remember the difference between axial and equatorial. Axial atoms are above and below the plane of the triangle on opposite sides of the molecule. Equatorial atoms are 120° apart and are within the plane of the triangle.

26. a

27. a

28. a

29. I3- I3- I3- a

30. 6/0 or 5/1 or 4/2 count on central atom. • The geometry of the molecule is octahedral. • The base angles for this geometry 90°. • The shapes can be either octahedral, square pyramidal, or square planar.

31. a

32. a

33. Xenon Tetrafluoride Octahedral Geometry Square Planar Shape

34. Explain the shape of IBr3 • Draw Lewis Structure

35. Explain the shape of IBr3 • Draw Lewis Structure • State VSEPR Theory (MRF) and Geometry • Place the Lone Pairs • Place the Bonds • State the Shape

36. Explain the shape of Xenon Tetrafluoride (XeF4)

37. Molecular Models

38. Skeletal Formulas (Models)

39. Skeletal Formulas (Models) •  A skeletal formula shows the atoms that make up a molecule and serves as a shorthand representation of its bonding. • Benefit: They are the most common models of molecules because they can relatively quickly allow us to show the bonding in a molecule. Drawback: They are ineffective at showing the three dimensional structure of the molecule and the relative size of the atoms.

40. Ball-and-Stick Models

41. Ball-and-Stick Models • The ball-and-stick model is used to display both the three-dimensional position of the atoms and the bonds between them. • Benefit: Bond angles and distances between atoms are more clearly shown. • Drawback: In a ball-and-stick model, the pegs used to represent bonds and spheres used to represent atoms are not to scale. As a consequence, the model does not provide a clear insight about the space occupied by the model.

42. Space-Filling Models

43. Space-Filling Models

44. Space-Filling Models • A space-filling model is a three-dimensional molecular model where the atoms are represented by spheres whose radii are proportional to the radii of the atoms and whose center-to-center distances are proportional to the distances between the atomic nuclei. • Benefit: They are useful for visualizing the effective shape and relative dimensions of the molecule, in particular the region of space occupied by it. • Drawback: Space-filling models do not show the chemical bonds between the atoms, nor the structure of the molecule beyond the outside layer of atoms.

48. Molecular Polarity • A dipole is anything with a positive end and a negative end. Another word for dipole is polar. • A bond is a dipole (polar) if it connects different atoms. • A polar molecule (dipole) is a molecule where the polar bonds are asymmetrically (not symmetrically) arranged (the dipoles do not cancel). • A nonpolar molecule is a molecule with no polar bonds or a molecule where the polar bonds are symmetrically arranged (the dipoles cancel).

49. + - H Cl Dipole Moment • The dipole moment is the measurement of a molecules polarity. • Arrow points toward the more electronegative atom.

50. O net dipole moment H2O H H Determining Molecular Polarity • Polar Molecule • Dipoles (Polar Bonds) are asymmetrically arranged and don’t cancel in this bent (angular) molecule.