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Lecture 21 Part 1

Lecture 21 Part 1. VSEPR (cont.) Table 10.1 (in detail). Tro: Chemistry: A Molecular Approach, 2/e. Linear Electron Geometry. When there are two electron groups around the central atom, they will occupy positions on opposite sides of the central atom

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Lecture 21 Part 1

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  1. Lecture 21 Part 1 VSEPR (cont.) Table 10.1 (in detail)

  2. Tro: Chemistry: A Molecular Approach, 2/e

  3. Linear Electron Geometry • When there are two electron groups around the central atom, they will occupy positions on opposite sides of the central atom • This results in the electron groups taking a linear geometry • The bond angle is 180° Tro: Chemistry: A Molecular Approach, 2/e

  4. Linear Geometry Tro: Chemistry: A Molecular Approach, 2/e

  5. Trigonal Planar Electron Geometry • When there are three electron groups around the central atom, they will occupy positions in the shape of a triangle around the central atom • This results in the electron groups taking a trigonal planar geometry • The bond angle is 120° Tro: Chemistry: A Molecular Approach, 2/e

  6. Trigonal Geometry Tro: Chemistry: A Molecular Approach, 2/e

  7. Tetrahedral Electron Geometry • When there are four electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom • This results in the electron groups taking a tetrahedral geometry • The bond angle is 109.5° Tro: Chemistry: A Molecular Approach, 2/e

  8. Tetrahedral Geometry Tro: Chemistry: A Molecular Approach, 2/e

  9. Trigonal Bipyramidal Electron Geometry • When there are five electron groups around the central atom, they will occupy positions in the shape of two tetrahedra that are base-to-base with the central atom in the center of the shared bases • This results in the electron groups taking a trigonal bipyramidal geometry • The positions above and below the central atom are called the axial positions • The positions in the same base plane as the central atom are called the equatorial positions • The bond angle between equatorial positions is 120° • The bond angle between axial and equatorial positions is 90° Tro: Chemistry: A Molecular Approach, 2/e

  10. Trigonal Bipyramid Tro: Chemistry: A Molecular Approach, 2/e

  11. Trigonal Bipyramidal Geometry Tro: Chemistry: A Molecular Approach, 2/e

  12. Tro: Chemistry: A Molecular Approach, 2/e

  13. Octahedral Electron Geometry • When there are six electron groups around the central atom, they will occupy positions in the shape of two square-base pyramids that are base-to-base with the central atom in the center of the shared bases • This results in the electron groups taking an octahedral geometry • it is called octahedral because the geometric figure has eight sides • All positions are equivalent • The bond angle is 90° Tro: Chemistry: A Molecular Approach, 2/e

  14. Octahedral Geometry Tro: Chemistry: A Molecular Approach, 2/e

  15. Octahedral Geometry Tro: Chemistry: A Molecular Approach, 2/e

  16. Tro: Chemistry: A Molecular Approach, 2/e

  17. Molecular Geometry • The actual geometry of the molecule may be different from the electron geometry • When the electron groups are attached to atoms of different size, or when the bonding to one atom is different than the bonding to another, this will affect the molecular geometry around the central atom • Lone pairs also affect the molecular geometry • they occupy space on the central atom, but are not “seen” as points on the molecular geometry Tro: Chemistry: A Molecular Approach, 2/e

  18. Not Quite Perfect Geometry Because the bonds and atom sizes are not identical in formaldehyde, the observed angles are slightly different from ideal Tro: Chemistry: A Molecular Approach, 2/e

  19. The Effect of Lone Pairs • Lone pair groups “occupy more space” on the central atom • because their electron density is exclusively on the central atom rather than shared like bonding electron groups • Relative sizes of repulsive force interactions is Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair • This affects the bond angles, making the bonding pair – bonding pair angles smaller than expected Tro: Chemistry: A Molecular Approach, 2/e

  20. Effect of Lone Pairs The bonding electrons are shared by two atoms, so some of the negative charge is removed from the central atom The nonbonding electrons are localized on the central atom, so area of negative charge takes more space Tro: Chemistry: A Molecular Approach, 2/e

  21. Bond Angle Distortion from Lone Pairs Tro: Chemistry: A Molecular Approach, 2/e

  22. Bond Angle Distortion from Lone Pairs Tro: Chemistry: A Molecular Approach, 2/e

  23. Bent Molecular Geometry:Derivative of Trigonal Planar Electron Geometry • When there are three electron groups around the central atom, and one of them is a lone pair, the resulting shape of the molecule is called a trigonal planar — bent shape • The bond angle is less than 120° • because the lone pair takes up more space Tro: Chemistry: A Molecular Approach, 2/e

  24. Pyramidal & Bent Molecular Geometries:Derivatives of Tetrahedral Electron Geometry • When there are four electron groups around the central atom, and one is a lone pair, the result is called a pyramidal shape • because it is a triangular-base pyramid with the central atom at the apex • When there are four electron groups around the central atom, and two are lone pairs, the result is called a tetrahedral—bent shape • it is planar • it looks similar to the trigonal planar—bent shape, except the angles are smaller • For both shapes, the bond angle is less than 109.5° Tro: Chemistry: A Molecular Approach, 2/e

  25. Methane Tro: Chemistry: A Molecular Approach, 2/e

  26. Pyramidal Shape Tro: Chemistry: A Molecular Approach, 2/e

  27. Pyramidal Shape Tro: Chemistry: A Molecular Approach, 2/e

  28. Tetrahedral–Bent Shape Tro: Chemistry: A Molecular Approach, 2/e

  29. Tetrahedral–Bent Shape Tro: Chemistry: A Molecular Approach, 2/e

  30. Derivatives of theTrigonal Bipyramidal Electron Geometry • When there are five electron groups around the central atom, and some are lone pairs, they will occupy the equatorial positions because there is more room • When there are five electron groups around the central atom, and one is a lone pair, the result is called the seesaw shape • aka distorted tetrahedron • When there are five electron groups around the central atom, and two are lone pairs, the result is called the T-shaped • When there are five electron groups around the central atom, and three are lone pairs, the result is a linear shape • The bond angles between equatorial positions are less than 120° • The bond angles between axial and equatorial positions are less than 90° • linear = 180° axial–to–axial Tro: Chemistry: A Molecular Approach, 2/e

  31. Replacing Atoms with Lone Pairsin the Trigonal Bipyramid System Tro: Chemistry: A Molecular Approach, 2/e

  32. Seesaw Shape Tro: Chemistry: A Molecular Approach, 2/e

  33. T–Shape Tro: Chemistry: A Molecular Approach, 2/e

  34. T–Shape Tro: Chemistry: A Molecular Approach, 2/e

  35. Linear Shape Tro: Chemistry: A Molecular Approach, 2/e

  36. Derivatives of theOctahedral Geometry • When there are six electron groups around the central atom, and some are lone pairs, each even number lone pair will take a position opposite the previous lone pair • When there are six electron groups around the central atom, and one is a lone pair, the result is called a square pyramid shape • the bond angles between axial and equatorial positions is less than 90° • When there are six electron groups around the central atom, and two are lone pairs, the result is called a square planar shape • the bond angles between equatorial positions is 90° Tro: Chemistry: A Molecular Approach, 2/e

  37. Square Pyramidal Shape Tro: Chemistry: A Molecular Approach, 2/e

  38. Square Planar Shape Tro: Chemistry: A Molecular Approach, 2/e

  39. Lecture 21 Part 2 Predicting the Shapes Around Central Atoms

  40. Predicting the Shapes Around Central Atoms 1. Draw the Lewis structure 2. Determine the number of electron groups around the central atom 3. Classify each electron group as bonding or lone pair, and count each type • remember, multiple bonds count as one group 4. Use Table 10.1 to determine the shape and bond angles Tro: Chemistry: A Molecular Approach, 2/e

  41. Example 10.2: Predict the geometry and bond angles of PCl3 1. Draw the Lewis structure a) 26 valence electrons 2. Determine the Number of electron groups around central atom a) four electron groups around P Tro: Chemistry: A Molecular Approach, 2/e

  42. Example 10.2: Predict the geometry and bond angles of PCl3 3. Classify the electron groups a) three bonding groups b) one lone pair 4. Use Table 10.1 to determine the shape and bond angles a) four electron groups around P = tetrahedral electron geometry b) three bonding + one lone pair = trigonal pyramidal molecular geometry c) trigonal pyramidal = bond angles less than 109.5° Tro: Chemistry: A Molecular Approach, 2/e

  43. Practice – Predict the molecular geometry and bond angles in SiF5─ Si least electronegative 5 electron groups on Si Si is central atom 5 bonding groups 0 lone pairs Si = 4e─ F5 = 5(7e─) = 35e─ (─) = 1e─ total = 40e─ Shape = trigonal bipyramid Bond angles Feq–Si–Feq = 120° Feq–Si–Fax = 90° Tro: Chemistry: A Molecular Approach, 2/e

  44. Practice – Predict the molecular geometry and bond angles in ClO2F Cl least electronegative 4 electron groups on Cl Cl is central atom 3 bonding groups 1 lone pair Cl = 7e─ O2 = 2(6e─) = 12e─ F = 7e─ Total = 26e─ Shape = trigonal pyramidal Bond angles O–Cl–O < 109.5° O–Cl–F < 109.5° Tro: Chemistry: A Molecular Approach, 2/e

  45. Representing 3-Dimensional Shapes on a 2-Dimensional Surface • One of the problems with drawing molecules is trying to show their dimensionality • By convention, the central atom is put in the plane of the paper • Put as many other atoms as possible in the same plane and indicate with a straight line • For atoms in front of the plane, use a solid wedge • For atoms behind the plane, use a hashed wedge Tro: Chemistry: A Molecular Approach, 2/e

  46. Tro: Chemistry: A Molecular Approach, 2/e

  47. F F F S F F F SF6 Tro: Chemistry: A Molecular Approach, 2/e

  48. Multiple Central Atoms • Many molecules have larger structures with many interior atoms • We can think of them as having multiple central atoms • When this occurs, we describe the shape around each central atom in sequence shape around left C is tetrahedral shape around center C is trigonal planar shape around right O is tetrahedral-bent Tro: Chemistry: A Molecular Approach, 2/e

  49. Describing the Geometryof Methanol Tro: Chemistry: A Molecular Approach, 2/e

  50. Describing the Geometryof Glycine Tro: Chemistry: A Molecular Approach, 2/e

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