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Chapter 10 Chemical Bonding II Chemistry II. Chemical Bonding II Molecular Shapes.

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Presentation on theme: "Chapter 10 Chemical Bonding II Chemistry II. Chemical Bonding II Molecular Shapes."— Presentation transcript:

1 Chapter 10 Chemical Bonding II Chemistry II

2 Chemical Bonding II Molecular Shapes

3 From the model of EDTA we can infer that the 3-Dimensional structure of a molecule determines its __________ the structure displays many molecular properties: Skeletal structure Bonding Shape Bond theory allows you to predict the shapes of molecules

4 Chemical Bonding II VSEPR Theory e - groups (lone pairs and bonds) are most stable when they are as far apart as possible – v________ s____ e_______ p_____ r_________ theory Maximum separation 3-D representation allows us to predict the shapes and bond angles in the molecule

5 Chemical Bonding II VSEPR Theory e.g. draw the 2 possible Lewis dot structures for NO 2 - and discuss the behavior of the associated e- groups there are _____ e - groups on N ____ lone pair ____ single bond ____ double bond (counted as 1 group )

6 Chemical Bonding II 2 e - Groups: Linear Geometry 5 basic shapes of molecules: linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral

7 Chemical Bonding II 2 e - Groups: Linear Geometry Draw both 2-dimensional and 3-dimensional pictures of the molecules in the following slides

8 Chemical Bonding II 2 e - Groups: Linear Geometry occupy positions opposite, around the central atom linear geometry - bond angle is ________ e.g. CO 2

9 Chemical Bonding II 3 e - Groups: Trigonal Geometry occupy triangular positions trigonal planar geometry - bond angle is __________ e.g. BF 3

10 Chemical Bonding II 3 e - Groups: Trigonal Geometry 3 e – groups around central atom – why not 120° ? e.g. Formaldehyde, CH 2 O

11 Chemical Bonding II 4 e - Groups: Tetrahedral Geometry occupy tetrahedron positions around the central atom tetrahedral geometry - bond angle is ________ e.g. CH 4

12 Chemical Bonding II 5 e - Groups: Trigonal Bipyramidal Geometry occupy positions in the shape of a two tetrahedra that are base-to-base trigonal bipyramidal geometry e.g. PCl 5

13 Chemical Bonding II 6 e - Groups: Octahedral Geometry occupy positions in the shape of two square-base pyramids that are base-to-base octahedral geometry e.g. SF 6

14 Chemical Bonding II The Effect of Lone Pairs lone pair groups “occupy more space” on the central atom 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

15 Chemical Bonding II 3 e - Groups with Lone Pairs: Derivative of Trigonal Geometry when there are 3 e- groups around central atom, and 1 of them is a lone pair trigonal planar - bent shape - bond angle < 120° e.g. SO 2

16 Chemical Bonding II 4 e - Groups with Lone Pairs : Derivatives of Tetrahedral Geometry when there are 4 e - groups around the central atom, and 1 is a lone pair trigonal pyramidal shape – bond angle is 107 ° e.g. NH 3

17 Which species has the smaller bond angle, Perchlorate (ClO 4 - ) or Chlorate (ClO 3 - )?

18 Chemical Bonding II 4 e - Groups with Lone Pairs: Derivatives of Tetrahedral Geometry when there are 4 e - groups around the central atom, and 2 are lone pairs tetrahedral-bent shape – bond angle is 104.5 ° e.g. H 2 O

19 Chemical Bonding II Tetrahedral-Bent Shape

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21 Chemical Bonding II 5 e - Groups with Lone Pairs Derivatives of Trigonal Bipyramidal Geometry when there are 5 e - groups around the central atom, and some are lone pairs, they will occupy the equatorial positions because there is more room when there are 5 e - groups around the central atom, and 1 is a lone pair, the result is called see-saw shape aka distorted tetrahedron when there are 5 e - groups around the central atom, and 2 are lone pairs, the result is called T-shaped when there are 5 e - groups around the central atom, and 3 are lone pairs, the result is called a linear shape the bond angles between equatorial positions is < 120° the bond angles between axial and equatorial positions is < 90° linear = 180° axial-to-axial

22 Chemical Bonding II Replacing Atoms with Lone Pairs in the Trigonal Bipyramid System

23 Chemical Bonding II See-Saw Shape

24 Chemical Bonding II T-Shape

25 Tro, Chemistry: A Molecular Approach25 Chemical Bonding II Linear Shape

26 when there are 6 e - groups around the central atom, and 1 is a lone pair, the result is called a square pyramid shape the bond angles between axial and equatorial positions is < 90° Chemical Bonding II 6 e - Groups with Lone Pairs: Derivatives of Octahedral Geometry

27 when there are 6 e - groups around the central atom, and 2 are lone pairs, the result is called a square planar shape the bond angles between equatorial positions is 90° 6 e - Groups with Lone Pairs Derivatives of Octahedral Geometry

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29 Chemical Bonding II 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

30 Practice – Predict the Molecular Geometry and Bond Angles in SiF 5 -

31 Practice – Predict the Molecular Geometry and Bond Angles in SiF 5 ─ Si = 4e ─ F 5 = 5(7e ─ ) = 35e ─ (─) = 1e ─ total = 40e ─ 5 Electron Groups on Si 5 Bonding Groups 0 Lone Pairs Shape = Trigonal Bipyramid - Bond Angles F eq -Si-F eq = 120° F eq -Si-F ax = 90° Si Least Electronegative Si Is Central Atom

32 Practice – Predict the Molecular Geometry and Bond Angles in ClO 2 F (Chloryl Fluoride)

33 Practice – Predict the Molecular Geometry and Bond Angles in ClO 2 F Cl = 7e ─ O 2 = 2(6e ─ ) = 12e ─ F = 7e ─ Total = 26e ─ 4 Electron Groups on Cl 3 Bonding Groups 1 Lone Pair Shape = Trigonal Pyramidal Bond Angles O-Cl-O < 109.5° O-Cl-F < 109.5° Cl Least Electronegative Cl Is Central Atom

34 Chemical Bonding II 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

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36 SF 6 S F F F FF F

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38 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 e.g. acetic acid shape around left C is tetrahedral shape around center C is trigonal planar shape around right O is tetrahedral-bent

39 Describing the Geometry of Methanol

40 Describing the Geometry of Glycine

41 Tro, Chemistry: A Molecular Approach41 Practice – Predict the Molecular Geometries in H 3 BO 3

42 42 Practice – Predict the Molecular Geometries in H 3 BO 3 B = 3e ─ O 3 = 3(6e ─ ) = 18e ─ H 3 = 3(1e ─ ) = 3e ─ Total = 24e ─ 3 Electron Groups on B B has 3 Bonding Groups 0 Lone Pairs Shape on B = Trigonal Planar B Least Electronegative B Is Central Atom oxyacid, so H attached to O 4 Electron Groups on O O has 2 Bonding Groups 2 Lone Pairs Shape on O = Bent

43 Tro, Chemistry: A Molecular Approach43 Practice – Predict the Molecular Geometries in C 2 H 4

44 44 Practice – Predict the Molecular Geometries in C 2 H 4 C = 2(4e ─ ) = 8e ─ H = 4(1e ─ ) = 4e ─ Total = 12e ─ 3 Electron Groups on C Shape on each C = Trigonal Planar 0 Lone Pairs

45 Practice – Predict the Molecular Geometries in CH 3 OCH 3

46 Practice – Predict the Molecular Geometries in Dimethyl Ether (CH 3 OCH 3 ) C = 2(4e ─ ) = 8e ─ H = 6(1e ─ ) = 6e ─ O = 6(1e ─ ) = 6e ─ Total = 20e ─ 4 Electron Groups on C Shape on each C = Tetrahedral 2 Lone Pairs on O Shape on O = Bent

47 Reminder about Eletronegativity! Electronegativity, is a chemical property that describes the tendency of an atom to e - towards itself

48 Polarity of Molecules in order for a molecule to be polar it must 1)have polar bonds  electronegativity difference  dipole moments (charge x distance) 2)have an unsymmetrical shape  vector addition polarity affects the intermolecular forces of attraction therefore boiling points and solubilities  like dissolves like nonbonding pairs strongly affect molecular polarity

49 Molecule Polarity The H-Cl bond is polar Bonding e - are pulled toward the Cl end of the molecule Net result is a polar molecule.

50 Vector Addition

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52 Molecule Polarity The O-C bond is polar The bonding e - are pulled equally toward both O’s Symmetrical molecule Net result is a nonpolar molecule

53 Molecule Polarity The H-O bond is polar Both sets of bonding e - are pulled toward the O Net result is a polar molecule

54 Molecule Polarity

55 The H-N bond is polar All the sets of bonding electrons are pulled toward the N Not symmetrical Net result is a polar molecule

56 Molecule Polarity The C-H bond is polar Four equal dipoles cancel each other out due to symmetry Net result is a non-polar molecule

57 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

58 Molecular Polarity Affects Solubility in Water Oil and water do not mix! Mutual attraction causes polar molecules to clump together

59 Water shrinks on melting (ice floats on water) Unusually high melting point Unusually high boiling point Unusually high surface tension Unusually high viscosity Unusually high heat of vaporization Unusually high specific heat capacity And more… Unique Properties

60 Molecular Polarity Affects Solubility in Water some molecules have both polar and nonpolar parts e.g. soap

61 Practice - Decide Whether the Following Are Polar EN O = 3.5 N = 3.0 Cl = 3.0 S = 2.5

62 Practice - Decide Whether the Following Are Polar polar nonpolar 1) polar bonds, N-O 2) asymmetrical shape 1) polar bonds, all S-O 2) symmetrical shape Trigonal Bent Trigonal Planar Cl N O 3.0 3.5 O O O S 2.5


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