1. 2008, Prentice Hall Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA

2. Structure Determines Properties! properties of molecular substances depend on the structure of the molecule the structure includes many factors, including: the skeletal arrangement of the atoms the kind of bonding between the atoms ionic, polar covalent, or covalent the shape of the molecule bonding theory should allow you to predict the shapes of molecules Tro, Chemistry: A Molecular Approach2

3. Molecular Geometry Molecules are 3-dimensional objects We often describe the shape of a molecule with terms that relate to geometric figures These geometric figures have characteristic “corners” that indicate the positions of the surrounding atoms around a central atom in the center of the geometric figure The geometric figures also have characteristic angles that we call bond angles Tro, Chemistry: A Molecular Approach3

4. Using Lewis Theory to Predict Molecular Shapes Lewis theory predicts there are regions of electrons in an atom based on placing shared pairs of valence electrons between bonding nuclei and unshared valence electrons located on single nuclei this idea can then be extended to predict the shapes of molecules by realizing these regions are all negatively charged and should repel Tro, Chemistry: A Molecular Approach4

5. VSEPR Theory electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory since electrons are negatively charged, they should be most stable when they are separated as much as possible the resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule Tro, Chemistry: A Molecular Approach5

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7. Electron Groups the Lewis structure predicts the arrangement of valence electrons around the central atom(s) each lone pair of electrons constitutes one electron group on a central atom each bond constitutes one electron group on a central atom regardless of whether it is single, double, or triple Tro, Chemistry: A Molecular Approach7 there are 3 electron groups on N 1 lone pair 1 single bond 1 double bond

8. Molecular Geometries there are 5 basic arrangements of electron groups around a central atom based on a maximum of 6 bonding electron groups though there may be more than 6 on very large atoms, it is very rare each of these 5 basic arrangements results in 5 different basic molecular shapes in order for the molecular shape and bond angles to be a “perfect” geometric figure, all the electron groups must be bonds and all the bonds must be equivalent for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular geometry will be the same Tro, Chemistry: A Molecular Approach8

9. Linear Geometry when there are 2 electron groups around the central atom, they will occupy positions opposite each other around the central atom this results in the molecule taking a linear geometry the bond angle is 180° Tro, Chemistry: A Molecular Approach9

10. Trigonal Geometry when there are 3 electron groups around the central atom, they will occupy positions in the shape of a triangle around the central atom this results in the molecule taking a trigonal planar geometry the bond angle is 120° Tro, Chemistry: A Molecular Approach10

11. Not Quite Perfect Geometry Tro, Chemistry: A Molecular Approach11 Because the bonds are not identical, the observed angles are slightly different from ideal.

12. Tetrahedral Geometry when there are 4 electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom this results in the molecule taking a tetrahedral geometry the bond angle is 109.5° Tro, Chemistry: A Molecular Approach12

13. Trigonal Bipyramidal Geometry Tro, Chemistry: A Molecular Approach13

14. Octahedral Geometry Tro, Chemistry: A Molecular Approach14

15. 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 effects the bond angles, making them smaller than expected Tro, Chemistry: A Molecular Approach15

16. Effect of Lone Pairs Tro, Chemistry: A Molecular Approach16 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.

17. Derivative of Trigonal Geometry when there are 3 electron groups around the central atom, and 1 of them is a lone pair, the resulting shape of the molecule is called a trigonal planar - bent shape the bond angle is < 120° Tro, Chemistry: A Molecular Approach17

18. Pyramidal Shape Tro, Chemistry: A Molecular Approach18

19. Bond Angle Distortion from Lone Pairs Tro, Chemistry: A Molecular Approach19

20. Tetrahedral-Bent Shape Tro, Chemistry: A Molecular Approach20

21. Bond Angle Distortion from Lone Pairs Tro, Chemistry: A Molecular Approach21

22. Tetrahedral-Bent Shape Tro, Chemistry: A Molecular Approach22

23. Replacing Atoms with Lone Pairs in the Trigonal Bipyramid System Tro, Chemistry: A Molecular Approach23

24. T-Shape Tro, Chemistry: A Molecular Approach24

25. Linear Shape Tro, Chemistry: A Molecular Approach25

26. Tro, Chemistry: A Molecular Approach26

27. 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 1 group 4) Use Table 10.1 to Determine the Shape and Bond Angles Tro, Chemistry: A Molecular Approach27

28. 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 Approach28

29. 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 Tro, Chemistry: A Molecular Approach29 shape around left C is tetrahedral shape around center C is trigonal planar shape around right O is tetrahedral-bent

30. Describing the Geometry of Methanol Tro, Chemistry: A Molecular Approach30

31. Describing the Geometry of Glycine Tro, Chemistry: A Molecular Approach31

32. Polarity of Molecules in order for a molecule to be polar it must 1) have polar bonds electronegativity difference - theory bond dipole moments - measured 2) have an unsymmetrical shape vector addition polarity affects the intermolecular forces of attraction therefore boiling points and solubilities like dissolves like nonbonding pairs affect molecular polarity, strong pull in its direction Tro, Chemistry: A Molecular Approach32

33. Molecule Polarity Tro, Chemistry: A Molecular Approach33 The H-Cl bond is polar. The bonding electrons are pulled toward the Cl end of the molecule. The net result is a polar molecule.

34. Tro, Chemistry: A Molecular Approach34

35. Molecule Polarity Tro, Chemistry: A Molecular Approach35 The O-C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule.

36. Molecule Polarity Tro, Chemistry: A Molecular Approach36 The H-O bond is polar. The both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.

37. Molecule Polarity Tro, Chemistry: A Molecular Approach37 The H-N bond is polar. All the sets of bonding electrons are pulled toward the N end of the molecule. The net result is a polar molecule.

38. Molecular Polarity Affects Solubility in Water polar molecules are attracted to other polar molecules since water is a polar molecule, other polar molecules dissolve well in water and ionic compounds as well some molecules have both polar and nonpolar parts Tro, Chemistry: A Molecular Approach38

39. Problems with Lewis Theory Lewis theory gives good first approximations of the bond angles in molecules, but usually cannot be used to get the actual angle Lewis theory cannot write one correct structure for many molecules where resonance is important Lewis theory often does not predict the correct magnetic behavior of molecules e.g., O 2 is paramagnetic, though the Lewis structure predicts it is diamagnetic Tro, Chemistry: A Molecular Approach39