11/14/2014. Chemical Bonding. Richard Philips Feynman, Nobel Laureate in Physics ( )

Transcription

1 Chemical Bonding Lewis Theory Valence Bond VSEPR Molecular rbital Theory 1 "...he [his father] knew the difference between knowing the name of something and knowing something" Richard Philips eynman, Nobel Laureate in Physics ( ) 2 BNDING MDELS 1

2 4 ormation of dihydrogen, H 2 2

3 nonpolar covalent bond polar covalent bond 8 9 3

4

5 The element with the larger electronegativity will carry the partial negative charge 13 The degree of polarity, or ionic character, varies continuously with the electronegativity difference 14 igure 9.19 Percent ionic character of electronegativity difference (EN). 15 5

6 BEGIN 11/18 16 Bond Type and Electronegativity ΔEN = nonpolar covalent ΔEN = polar covalent ΔEN = ionic 17 H EN 2.1 EN 4.0 H H 18 6

7 Bond Polarity EN Cl = = 0 Pure Covalent EN Cl = 3.0 EN H = = 0.9 Polar Covalent EN Cl = 3.0 EN Na = = 2.1 Ionic Bond Dipole Moments the dipole moment is a quantitative way of describing the polarity of a bond a dipole is a material with positively and negatively charged ends measured dipole moment,, is a measure of bond polarity it is directly proportional to the size of the partial charges and directly proportional to the distance between them = (q)(r) measured in Debyes, D the percent ionic character is the percentage of a bond s measured dipole moment to what it would be if full ions 20 Dipole Moments 7

8 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 22 Molecule Polarity 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. 23 Vector Addition 24 8

9 25 Molecule Polarity The C bond is polar. The bonding electrons are pulled equally toward both ends of the molecule. The net result is a nonpolar molecule. 26 Molecule Polarity The H bond is polar. The both sets of bonding electrons are pulled toward the end of the molecule. The net result is a polar molecule. 27 9

10 Water a VERY Polar Molecule stream of water attracted to a charged glass rod stream of hexane not attracted to a charged glass rod Molecule Polarity 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. 29 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 30 10

11 A Soap Molecule Sodium Stearate 31 Practice Decide Whether the ollowing Are Polar N Cl S EN = 3.5 N = 3.0 Cl = 3.0 S = Practice Decide Whether the ollowing Are Polar N Cl Trigonal S Bent N 3.5 Trigonal Planar Cl S ) polar bonds, N 2) asymmetrical shape 1) polar bonds, all S polar 2) symmetrical shape nonpolar 33 11

12 VIDE 34 Lewis Structures of Molecules shows pattern of valence electron distribution in the molecule useful for understanding the bonding in many compounds allows us to predict shapes of molecules allows us to predict properties of molecules and how they will interact together 35 Lewis Structures use common bonding patterns C = 4 bonds & 0 lone pairs, N = 3 bonds & 1 lone pair, = 2 bonds & 2 lone pairs, H and halogen = 1 bond, Be = 2 bonds & 0 lone pairs, B = 3 bonds & 0 lone pairs often Lewis structures with line bonds have the lone pairs left off their presence is assumed from common bonding patterns structures which result in bonding patterns different from common have formal charges B C N 36 12

13 Practice Lewis Structures C 2 16 e H 3 P 4 H P H :::C::: 32 e H Se 2 S e 26 e Se S N 1 2 P 2 H 4 H H 18 e N 14 e H P P H 37 ormal Charge during bonding, atoms may wind up with more or less electrons in order to fulfill octets this results in atoms having a formal charge C = valence e nonbonding e ½ bonding e left C = 6 4 ½ (4) = 0 S C = 6 2 ½ (6) = +1 S right C = 6 6 ½ (2) = 1 sum of all the formal charges in a molecule = 0 in an ion, total equals the charge 38 Common Bonding Patterns B C N C + N B - C - N

14 Resonance when there is more than one Lewis structure for a molecule that differ only in the position of the electrons, they are called resonance structures the actual molecule is a combination of the resonance forms a resonance hybrid it does not resonate between the two forms, though we often draw it that way look for multiple bonds or lone pairs S S Dinitrogen monoxide 42 14

15 43 H : : S H : : Structure I H : : S H : : Structure II Structure I obeys the octet rule, but is not consistent with experiment Structure II violates the octet rule, but is consistent with experiment

16 Structure Determines Properties! the structure includes many factors, including: the skeletal arrangement of the atoms the kind of bonding between the atoms the shape of the molecule bonding theory should allow you to predict the shapes of molecules 46 Molecular Geometry 3 dimensional objects shape of a molecule as 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 47 Using Lewis Theory to Predict Molecular Shapes shared pairs of valence electrons between bonding nuclei unshared valence electrons located on single nuclei predict the shapes of molecules by realizing these regions are all negatively charged and should repel one another 48 16

17 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 the resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule 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 N there are 3 electron groups on N 1 lone pair 1 single bond 1 double bond 51 17

18 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 52 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 Cl Be Cl C 53 Linear Geometry 54 18

19 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 B 55 Trigonal Geometry 56 Not Quite Perfect Geometry Because the bonds are not identical, the observed angles are slightly different from ideal

20 58 Tetrahedral Geometry 4 electron groups around the central atom will occupy positions in the shape of a tetrahedron around the central atom tetrahedral geometry C 59 Tetrahedral Geometry 60 20

21 Methane 61 Trigonal Bipyramidal Geometry 5 electron groups around the central atom will occupy positions in the shape of a two tetrahedra that are base tobase with the central atom in the center of the shared bases 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 Trigonal Bipyramid 63 21

22 Trigonal Bipyramidal Geometry Cl Cl Cl Cl P Cl ctahedral Geometry 6 electron groups around the central atom 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 octahedral thd geometry it is called octahedral because the geometric figure has 8 sides all positions are equivalent the bond angle is

23 ctahedral Geometry 67 ctahedral Geometry S

24 The Effect of Lone Pairs lone pair groups occupy more space on the central atom 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 70 Effect of Lone Pairs The nonbonding electrons electrons are shared localized by two on atoms, the central so some atom, of the so area negative of negative charge is charge removed takes from more the space. central atom. 71 Derivative Shapes the molecule s shape will be one of basic molecular geometries if all the electron groups are bonds and all the bonds are equivalent molecules with lone pairs or different kinds of surrounding atoms will have distorted bond angles and different bond lengths, but the shape will be a derivative of one of the basic shapes 72 24

25 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 S S S 73 Derivatives of Tetrahedral Geometry 4 electron groups around the central atom and 1 is a lone pair: pyramidal shape because it is a triangular base pyramid with the central atom at the apex the bond angle is < Pyramidal Shape 75 25

26 Bond Angle Distortion from Lone Pairs 76 Derivatives of Tetrahedral Geometry 4 electron groups around the central atom and 2 are lone pairs: tetrahedral bent shape it is planar it looks similar to the trigonal planar bent shape, except the angles are smaller the bond angle is < Tetrahedral Bent Shape 78 26

27 Pyramidal Shape As 79 Bond Angle Distortion from Lone Pairs 80 Tetrahedral Bent Shape Cl

28 Derivatives of the Trigonal Bipyramidal Geometry when there are 5 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 5 electron 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 electron groups around the central atom, and 2 are lone pairs, the result is called T shaped when there are 5 electron groups around the central atom, and 3 are lone pairs, the result is called a linear shape 82 Replacing Atoms with Lone Pairs in the Trigonal Bipyramid System 83 See Saw Shape S 84 28

29 T Shape 85 Cl T Shaped 86 Linear Shape 87 29

30 Derivatives of the ctahedral Geometry when there are 6 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 6 electron 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 when there are 6 electron 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 Square Pyramidal Shape Br 89 Square Planar Shape Xe 90 30

31 91 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 92 Practice Predict the Molecular Geometry and Bond Angles in Si

32 Practice Predict the Molecular Geometry and Bond Angles in Si Electron Groups on Si Si 5 Bonding Groups ( ) 0 Lone Pairs Si Least Electronegative Si Is Central Atom Si = 4e 5 = 5(7e ) = 35e ( ) = 1e total = 40e Shape = Trigonal Bipyramid Bond Angles eq Si eq = 120 eq Si ax = Practice Predict the Molecular Geometry and Bond Angles in Cl 2 95 Practice Predict the Molecular Geometry and Bond Angles in Cl 2 4 Electron Groups on Cl Cl 3 Bonding Groups ( ) 1 Lone Pair Cl Least Electronegative Cl Is Central Atom Cl = 7e 2 = 2(6e ) = 12e = 7e Total = 26e Shape = Trigonal Pyramidal Bond Angles Cl < Cl <

33 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 S S 6 S 99 33

34 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 is tetrahedral bent H H C C H H 100 Describing the Geometry of Methanol 101 Describing the Geometry of Glycine

35 Practice Predict the Molecular Geometries in H 3 B Practice Predict the Molecular Geometries in H 3 B 3 Boric Acid oxyacid, so H attached to B Least Electronegative B Is Central Atom B = 3e 3 = 3(6e ) = 18e H 3 = 3(1e ) = 3e Total = 24e H B H H 4 Electron Groups on 3 Electron Groups on B B has 3 Bonding Groups 0 Lone Pairs has 2 Bonding Groups 2 Lone Pairs Shape on B = Trigonal Planar Shape on = Tetrahedral electron Geometry Bent molecular geometry 104 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., 2 is paramagnetic, though the Lewis structure predicts it is diamagnetic