Chapter 9 Molecular Geometry. Lewis Theory-VSEPR Valence Bond Theory Molecular Orbital Theory

Transcription

1 Chapter 9 Molecular Geometry Lewis Theory-VSEPR Valence Bond Theory Molecular Orbital Theory

2 Sulfanilamide Lewis Structures and the Real 3D-Shape of Molecules

3 Lewis Theory of Molecular Shape and Polarity

4 Structure Determines Properties! Properties of molecular substances depend on the structure of the molecule. The structure includes many factors, such as: Skeletal arrangement of the atoms Kind of bonding between the atoms Shape of the molecule

5 Molecular Geometry We can describe the shape of a molecule with terms that relate to geometric figures These geometric figures have characteristic corners (indicating the positions of atoms) The geometric figures also have characteristic angles that we call bond angles.

6 Lewis Theory Predicts Electron Groups Electron groups - regions of electrons around an atom Regions result from placing shared pairs of valence electrons between bonding nuclei Regions result from placing unshared valence electrons on a single nuclei

7 Lewis Theory of Molecular Shapes Electron groups repel each other. Predicting the shapes of molecules 1) The arrangement of the electron groups will be determined by trying to minimize repulsions between them. 2) The arrangement of atoms ( molecular shape ) surrounding a central atom will be determined by where the bonding electron groups are. 3) 1 and 2 are not necessarily the same

8 VSEPR Theory Valence shell electron pair repulsion (VSEPR) - Electron groups around the central atom will be most stable when they are as far apart as possible.

9 Electron Groups A Lewis structure predicts the number of valence electron pairs around a central atom(s). Each lone pair of electrons or odd electron constitutes one electron group on a central atom. Each bond constitutes one electron group, regardless of whether it is single, double, or triple O N O O N O There are three electron groups There are three electron groups around N steric number = 3 around N one lone pair one odd electron one single bond one single bond one double bond one double bond

10 Electron Group Geometry There are five basic arrangements of electron groups around a central atom. For molecules that exhibit resonance, it doesn t matter which resonance form you use the electron group geometry will be the same. linear triangular tetrahedral trigonal bipyramidal octahedral

11 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. Cl Be Cl O C O

12 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 in a trigonal planar geometry. The bond angle is 120 F Be F F

13 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 F F C F F

14 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 in a trigonal bipyramidal geometry.

15 Trigonal Bipyramidal Electron 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.

16 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 baseto-base with the central atom in the center of the shared bases This results in the electron groups taking an octahedral geometry. All positions around the central atom are equivalent. The bond angle is 90

17 Molecular Geometry 1) The actual geometry ( molecular geometry ) of a molecule may be different from the electron geometry. 2) 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. 3) Lone pairs occupy space on the central atom, but are not seen as points on the molecular geometry.

18 Not Quite Perfect Geometry O H C H Because the bonds and atom sizes are not identical in formaldehyde, the observed angles are slightly different from ideal.

19 The 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.

20 The Effect of Lone Pairs Lone pair groups occupy more space on the central atom than bonding electrons. Relative sizes of repulsive force interactions: 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.

21 Bond Angle Distortion from Lone Pairs

22 Molecular geometries derived from tetrahedral electron geometry.

23 VSEPR Theory

24 Bond Angle Distortion from Lone Pairs Tetrahedral molecular shape Pyramidal molecular shape Bent molecular shape

25 Bent or Angular Molecular Geometry: a 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 angular or bent shape. The bond angle is less than 120. O S O SO2

26 Bent Molecular Geometry ClO2 - O Cl O º

27 Trigonal Bipyramidal Electron Geometry

28 Molecular geometries derived from trigonal bipyramidal electron geometry.

29 See Saw Moleclular Geometry a Derivatives of Trigonal Bipyramidal Electron Geometry When there are five electron groups around the central atom, and some are lone pairs, the lone pairs will occupy the equatorial positions because there is more room. F F S F F SF4

30 See Saw Moleclular Geometry a Derivatives of Trigonal Bipyramidal Electron Geometry When there are five electron groups around the central atom, and one is a lone pair, the result is called the seesaw shape. F F S F F SF4

31 T-Shaped Molecular Geometry a Derivative of Trigonal Bipyramidal Electron Geometry When there are five electron groups around the central atom, and two are lone pairs, the result is called the T-shaped. BrF 3

32 Linear Molecular Geometry a Derivatives of Trigonal Bipyramidal Electron Geometry When there are five electron groups around the central atom, and three are lone pairs, the result is a linear shape. XeF 2

33 Molecular geometries derived from octahedral electron geometry.

34 Square Pyramidal Molecular Geometry a Derivatives of Octahedral Electron Geometry BrF 5 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

35 Square Planar Molecular Geometry a Derivatives of Octahedral Electron Geometry XeF 4 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.

36 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 4. Determine the shape and bond angles

37 Predict the geometry and bond angles of PCl3 1. Draw the Lewis structure 26 valence electrons 2. Determine the number of electron groups around central atom Cl Cl P Cl four electron groups around P 3. Classify the electron groups a) three bonding groups b) one lone pair

38 Predict the geometry and bond angles of PCl3 4. 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

39 Predict the molecular geometry and bond angles in SiF5 1. Draw the Lewis structure 40 valence electrons 2. Determine the number of electron groups around central atom five electron groups around Si F F Si F F F - 3. Classify the electron groups a) five bonding groups b) 0 lone pairs

40 Predict the molecular geometry and bond angles in SiF5 4. Determine the shape and bond angles a) five electron groups around Si = trigonal bipyramidal electron geometry b) five bonding + 0 lone pairs = trigonal bipyramidal molecular geometry c) trigonal bipyramidal = bond angles of than 120 (eq-eq) and 90º (ax-eq)

41 Predict the molecular geometry and bond angles in ClO2F 1. Draw the Lewis structure 26 valence electrons 2. Determine the number of electron groups around central atom 4 electron groups around Cl 3. Classify the electron groups a) three bonding groups b) one lone pair

42 Predict the molecular geometry and bond angles in ClO2F 4. Determine the shape and bond angles a) four electron groups around Cl = tetrahedral electron geometry b) 3 bonding + 1 lone pair = trigonal pyramidal molecular geometry c) trigonal pyramidal = bond angles of <109.5

43 Molecules with Multiple Central Atoms Methanol H O H N C C O H H H Glycine

44 Polarity of Molecules

45 Polarity of Molecules For a molecule to be polar, it must have polar bonds, and have an unsymmetrical shape Polarity affects the intermolecular forces of attraction and therefore affects boiling points and solubilities Nonbonding pairs affect molecular polarity.

46 When describing the polarity of a molecule, we must consider bond polarities as VECTOR QUANTITIES quantities with magnitude and direction.

47 Common Cases of Adding Dipole Moments to Determine Whether a Molecule is Polar

48 Molecular Polarity 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.

49 Molecular Polarity The H O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.

50 Predicting Polarity of Molecules 1. Draw the Lewis structure and determine the molecular geometry. 2. Determine whether the bonds in the molecule are polar. 3. Determine whether the polar bonds add together to give a net dipole moment.

51 Predict whether NH3 is a polar molecule 1. Draw the Lewis structure and determine the molecular geometry a) eight valence electrons b) three bonding + one lone pair = trigonal pyramidal molecular geometry

52 Predict whether NH3 is a polar molecule 2. Determine if the bonds are polar a) electronegativity difference b) if the bonds are not polar, we can stop here and declare the molecule will be nonpolar ENN = 3.0 ENH = = 0.9 therefore the bonds are polar covalent

53 Predict whether NH3 is a polar molecule 3) Determine whether the polar bonds add together to give a net dipole moment a) vector addition b) generally, asymmetric shapes result in uncompensated polarities and a net dipole moment 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.

54 Decide whether the following molecule is polar EN O = 3.5 N = 3.0 Cl = 3.0 S = 2.5 Trigonal Bent 1. polar bonds, N-O 2. asymmetrical shape polar

55 Decide whether the following molecule is polar Trigonal Planar EN O = 3.5 S = polar bonds, all S-O 2. symmetrical shape nonpolar

56 What about Tetrahedral Geometry?

57 Some molecules are inherently polar because of the atoms which they contain and the arrangement of these atoms in space. H2O NH3 CH2O HCl δ δ+ A crude representation of a polar molecule

58 Other molecules are considered nonpolar CH4 BH3 C2H2 CO2 Nonpolarized electron clouds

59 Molecular Formula Structural Formula Dot Diagram Molecular Shape Molecular Polarity Intermolecular Forces Melting Point, Boiling Point, Solubility