Chapter 10 Chemical Bonding II 2008, Prentice Hall Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA

Tro, Chemistry: A Molecular Approach2 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 Approach3 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 Approach4 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 Approach5 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 Approach6

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 there are 3 electron groups on N 1 lone pair 1 single bond 1 double bond

Tro, Chemistry: A Molecular Approach8 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 Approach9 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 Approach10 Linear Geometry

Tro, Chemistry: A Molecular Approach11 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 Approach12 Trigonal Geometry

Tro, Chemistry: A Molecular Approach13 Not Quite Perfect Geometry Because the bonds are not identical, the observed angles are slightly different from ideal.

Tro, Chemistry: A Molecular Approach14

Tro, Chemistry: A Molecular Approach15 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 Approach16 Tetrahedral Geometry

Tro, Chemistry: A Molecular Approach17 Methane

Tro, Chemistry: A Molecular Approach18 Trigonal Bipyramidal Geometry when there are 5 electron groups around the central atom, they will occupy positions in the shape of a two tetrahedra that are base-to-base with the central atom in the center of the shared bases this results in the molecule 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 Approach19 Trigonal Bipyramid

Tro, Chemistry: A Molecular Approach20 Trigonal Bipyramidal Geometry

Tro, Chemistry: A Molecular Approach21

Tro, Chemistry: A Molecular Approach22 Octahedral Geometry when there are 6 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 molecule taking an octahedral geometry it is called octahedral because the geometric figure has 8 sides all positions are equivalent the bond angle is 90°

Tro, Chemistry: A Molecular Approach23 Octahedral Geometry

Tro, Chemistry: A Molecular Approach24 Octahedral Geometry

Tro, Chemistry: A Molecular Approach25

Tro, Chemistry: A Molecular Approach26 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 Approach27 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 Approach28 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

Tro, Chemistry: A Molecular Approach29 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 Approach30 Derivatives of Tetrahedral Geometry when there are 4 electron groups around the central atom, and 1 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 4 electron groups around the central atom, and 2 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 < 109.5°

Tro, Chemistry: A Molecular Approach31 Pyramidal Shape

Tro, Chemistry: A Molecular Approach32 Bond Angle Distortion from Lone Pairs

Tro, Chemistry: A Molecular Approach33 Tetrahedral-Bent Shape

Tro, Chemistry: A Molecular Approach34 Pyramidal Shape

Tro, Chemistry: A Molecular Approach35 Bond Angle Distortion from Lone Pairs

Tro, Chemistry: A Molecular Approach36 Tetrahedral-Bent Shape

Tro, Chemistry: A Molecular Approach37 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 the bond angles between equatorial positions is < 120° the bond angles between axial and equatorial positions is < 90° linear = 180° axial-to-axial

Tro, Chemistry: A Molecular Approach38 Replacing Atoms with Lone Pairs in the Trigonal Bipyramid System

Tro, Chemistry: A Molecular Approach39 See-Saw Shape

Tro, Chemistry: A Molecular Approach40 T-Shape

Tro, Chemistry: A Molecular Approach41 T-Shaped

Tro, Chemistry: A Molecular Approach42 Linear Shape

Tro, Chemistry: A Molecular Approach43 Derivatives of the Octahedral 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 90°

Tro, Chemistry: A Molecular Approach44 Square Pyramidal Shape

Tro, Chemistry: A Molecular Approach45 Square Planar Shape

Tro, Chemistry: A Molecular Approach46

Tro, Chemistry: A Molecular Approach47 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 Approach48 Practice – Predict the Molecular Geometry and Bond Angles in SiF 5 -1

Tro, Chemistry: A Molecular Approach49 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

Tro, Chemistry: A Molecular Approach50 Practice – Predict the Molecular Geometry and Bond Angles in ClO 2 F

Tro, Chemistry: A Molecular Approach51 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

Tro, Chemistry: A Molecular Approach52 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 Approach53

Tro, Chemistry: A Molecular Approach54 SF 6 S F F F FF F

Tro, Chemistry: A Molecular Approach55 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 Approach56 Describing the Geometry of Methanol

Tro, Chemistry: A Molecular Approach57 Describing the Geometry of Glycine

Tro, Chemistry: A Molecular Approach58 Practice – Predict the Molecular Geometries in H 3 BO 3

59 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 = Tetrahedral Bent

Tro, Chemistry: A Molecular Approach60 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 Approach61 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.

Tro, Chemistry: A Molecular Approach62 Vector Addition

Tro, Chemistry: A Molecular Approach63

Tro, Chemistry: A Molecular Approach64 Molecule 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.

Tro, Chemistry: A Molecular Approach65 Molecule Polarity 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.

Tro, Chemistry: A Molecular Approach66 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.

Tro, Chemistry: A Molecular Approach67 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 Approach68 A Soap Molecule Sodium Stearate

Tro, Chemistry: A Molecular Approach69 Practice - Decide Whether the Following Are Polar EN O = 3.5 N = 3.0 Cl = 3.0 S = 2.5

Tro, Chemistry: A Molecular Approach70 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 O O O S 2.5

Tro, Chemistry: A Molecular Approach71 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 Approach72 Valence Bond Theory Linus Pauling and others applied the principles of quantum mechanics to molecules they reasoned that bonds between atoms would arise when the orbitals on those atoms interacted to make a bond the kind of interaction depends on whether the orbitals align along the axis between the nuclei, or outside the axis

Tro, Chemistry: A Molecular Approach73 Orbital Interaction as two atoms approached, the partially filled or empty valence atomic orbitals on the atoms would interact to form molecular orbitals the molecular orbitals would be more stable than the separate atomic orbitals because they would contain paired electrons shared by both atoms the interaction energy between atomic orbitals is negative when the interacting atomic orbitals contain a total of 2 electrons

Tro, Chemistry: A Molecular Approach74 Orbital Diagram for the Formation of H 2 S + ↑ H 1s1s ↑↓ H-S bond ↑ H 1s1s S ↑↑↑↓ 3s3s3p3p H-S bond Predicts Bond Angle = 90° Actual Bond Angle = 92°

Tro, Chemistry: A Molecular Approach75 Valence Bond Theory - Hybridization one of the issues that arose was that the number of partially filled or empty atomic orbital did not predict the number of bonds or orientation of bonds C = 2s 2 2p x 1 2p y 1 2p z 0 would predict 2 or 3 bonds that are 90° apart, rather than 4 bonds that are 109.5° apart to adjust for these inconsistencies, it was postulated that the valence atomic orbitals could hybridize before bonding took place one hybridization of C is to mix all the 2s and 2p orbitals to get 4 orbitals that point at the corners of a tetrahedron

Tro, Chemistry: A Molecular Approach76 Unhybridized C Orbitals Predict the Wrong Bonding & Geometry

Tro, Chemistry: A Molecular Approach77 Valence Bond Theory Main Concepts 1. the valence electrons in an atom reside in the quantum mechanical atomic orbitals or hybrid orbitals 2. a chemical bond results when these atomic orbitals overlap and there is a total of 2 electrons in the new molecular orbital a)the electrons must be spin paired 3. the shape of the molecule is determined by the geometry of the overlapping orbitals

Tro, Chemistry: A Molecular Approach78 Hybridization some atoms hybridize their orbitals to maximize bonding hybridizing is mixing different types of orbitals to make a new set of degenerate orbitals sp, sp 2, sp 3, sp 3 d, sp 3 d 2 more bonds = more full orbitals = more stability better explain observed shapes of molecules same type of atom can have different hybridization depending on the compound C = sp, sp 2, sp 3

Tro, Chemistry: A Molecular Approach79 Hybrid Orbitals H cannot hybridize!! the number of standard atomic orbitals combined = the number of hybrid orbitals formed the number and type of standard atomic orbitals combined determines the shape of the hybrid orbitals the particular kind of hybridization that occurs is the one that yields the lowest overall energy for the molecule in other words, you have to know the structure of the molecule beforehand in order to predict the hybridization

Tro, Chemistry: A Molecular Approach80 Orbital Diagrams with Hybridization place electrons into hybrid and unhybridized valence orbitals as if all the orbitals have equal energy when bonding,  bonds form between hybrid orbitals and  bonds form between unhybridized orbitals that are parallel

Tro, Chemistry: A Molecular Approach81 Carbon Hybridizations Unhybridized 2s2s 2p2p  sp hybridized 2sp  sp 2 hybridized 2p2p sp 3 hybridized    2p2p 2sp 2  2sp 3 

Tro, Chemistry: A Molecular Approach82 sp 3 Hybridization atom with 4 areas of electrons tetrahedral geometry 109.5° angles between hybrid orbitals atom uses hybrid orbitals for all bonds and lone pairs

Tro, Chemistry: A Molecular Approach83 sp 3 Hybridization of C

Tro, Chemistry: A Molecular Approach84

Tro, Chemistry: A Molecular Approach85 sp 3 Hybridized Atoms Orbital Diagrams Unhybridized atom 2s2s 2p2p  sp 3 hybridized atom 2sp 3  C 2s2s 2p2p  2sp 3   N   

Tro, Chemistry: A Molecular Approach86 Methane Formation with sp 3 C

Tro, Chemistry: A Molecular Approach87 Ammonia Formation with sp 3 N

Tro, Chemistry: A Molecular Approach88 CH 3 NH 2 Orbital Diagram  sp 3 C  sp 3 N   1s H    

Tro, Chemistry: A Molecular Approach89 Practice - Draw the Orbital Diagram for the sp 3 Hybridization of Each Atom Unhybridized atom 3s3s 3p3p  Cl 2s2s 2p2p  O 

Tro, Chemistry: A Molecular Approach90 Practice - Draw the Orbital Diagram for the sp 3 Hybridization of Each Atom Unhybridized atom 3s3s 3p3p  Cl 2s2s 2p2p  O  sp 3 hybridized atom 3sp 3  2sp 3    

Tro, Chemistry: A Molecular Approach91 Types of Bonds a sigma (  ) bond results when the bonding atomic orbitals point along the axis connecting the two bonding nuclei either standard atomic orbitals or hybrids  s-to-s, p-to-p, hybrid-to-hybrid, s-to-hybrid, etc. a pi (  ) bond results when the bonding atomic orbitals are parallel to each other and perpendicular to the axis connecting the two bonding nuclei between unhybridized parallel p orbitals the interaction between parallel orbitals is not as strong as between orbitals that point at each other; therefore  bonds are stronger than  bonds

Tro, Chemistry: A Molecular Approach92

Tro, Chemistry: A Molecular Approach93

Tro, Chemistry: A Molecular Approach94 Bond Rotation because orbitals that form the  bond point along the internuclear axis, rotation around that bond does not require breaking the interaction between the orbitals but the orbitals that form the  bond interact above and below the internuclear axis, so rotation around the axis requires the breaking of the interaction between the orbitals

Tro, Chemistry: A Molecular Approach95

Tro, Chemistry: A Molecular Approach96

Tro, Chemistry: A Molecular Approach97 sp 2 atom with 3 areas of electrons trigonal planar system  C = trigonal planar  N = trigonal bent  O = “linear” 120° bond angles flat atom uses hybrid orbitals for  bonds and lone pairs, uses nonhybridized p orbital for  bond

Tro, Chemistry: A Molecular Approach98

Tro, Chemistry: A Molecular Approach99 + sp 2   Hybrid orbitals overlap to form  bond Unhybridized p orbitals overlap to form  bond

Tro, Chemistry: A Molecular Approach100

Tro, Chemistry: A Molecular Approach101 sp 2 Hybridized Atoms Orbital Diagrams Unhybridized atom 2s2s 2p2p  sp 2 hybridized atom 2sp 2  2p2p C 3  1  2s2s 2p2p  2sp 2  2p2p    N 2  1 

Tro, Chemistry: A Molecular Approach102 CH 2 NH Orbital Diagram  sp 2 C   sp 2 N   1s H    p Cp N 

Tro, Chemistry: A Molecular Approach103 Practice - Draw the Orbital Diagram for the sp 2 Hybridization of Each Atom. How many  and  bonds would you expect each to form? Unhybridized atom 2s2s 2p2p  B 2s2s 2p2p  O 

Tro, Chemistry: A Molecular Approach104 Practice - Draw the Orbital Diagram for the sp 2 Hybridization of Each Atom. How many  and  bonds would you expect each to form? Unhybridized atom 2s2s 2p2p  B 3  0  2s2s 2p2p  O 1  1   sp 2 hybridized atom 2sp 2  2p2p  2p2p   

Tro, Chemistry: A Molecular Approach105 sp atom with 2 areas of electrons linear shape 180° bond angle atom uses hybrid orbitals for  bonds or lone pairs, uses nonhybridized p orbitals for  bonds   

Tro, Chemistry: A Molecular Approach106

Tro, Chemistry: A Molecular Approach107

Tro, Chemistry: A Molecular Approach108 sp Hybridized Atoms Orbital Diagrams Unhybridized atom 2s2s 2p2p  sp hybridized atom 2sp  2p2p C22C22 2s2s 2p2p  2sp  2p2p    N12N12

Tro, Chemistry: A Molecular Approach109 HCN Orbital Diagram  sp C   sp N    1s H   p Cp N 

Tro, Chemistry: A Molecular Approach110 sp 3 d atom with 5 areas of electrons around it trigonal bipyramid shape See-Saw, T-Shape, Linear 120° & 90° bond angles use empty d orbitals from valence shell d orbitals can be used to make  bonds

Tro, Chemistry: A Molecular Approach111

Tro, Chemistry: A Molecular Approach112 sp 3 d Hybridized Atoms Orbital Diagrams Unhybridized atom 3s3s 3p3p  sp 3 d hybridized atom 3sp 3 d  S 3s3s 3p3p   3sp 3 d   P    3d3d 3d3d   (non-hybridizing d orbitals not shown)

Tro, Chemistry: A Molecular Approach113 sp 3 d 2 atom with 6 areas of electrons around it octahedral shape Square Pyramid, Square Planar 90° bond angles use empty d orbitals from valence shell d orbitals can be used to make  bonds

Tro, Chemistry: A Molecular Approach114

Tro, Chemistry: A Molecular Approach115 sp 3 d 2 Hybridized Atoms Orbital Diagrams Unhybridized atomsp 3 d 2 hybridized atom S 3s3s 3p3p 3sp 3 d 2 ↑↓ 3d3d ↑↑↑↑↑↑↑↑ I 5s5s 5p5p 5d5d ↑ 5sp 3 d 2 ↑↓↑↑↑↑↑ (non-hybridizing d orbitals not shown)

Tro, Chemistry: A Molecular Approach116

Tro, Chemistry: A Molecular Approach117 Example - Predict the Hybridization of All the Atoms in H 3 BO 3 H = can’t hybridize B = 3 electron groups = sp 2 O = 4 electron groups = sp 3

Tro, Chemistry: A Molecular Approach118 Practice - Predict the Hybridization and Bonding Scheme of All the Atoms in NClO

Tro, Chemistry: A Molecular Approach119 Practice - Predict the Hybridization and Bonding Scheme of All the Atoms in NClO N = 3 electron groups = sp 2 O = 3 electron groups = sp 2 Cl = 4 electron groups = sp 3

Tro, Chemistry: A Molecular Approach120 Predicting Hybridization and Bonding Scheme 1) Start by drawing the Lewis Structure 2) Use VSEPR Theory to predict the electron group geometry around each central atom 3) Use Table 10.3 to select the hybridization scheme that matches the electron group geometry 4) Sketch the atomic and hybrid orbitals on the atoms in the molecule, showing overlap of the appropriate orbitals 5) Label the bonds as  or 

Tro, Chemistry: A Molecular Approach121 Ex 10.8 – Predict the hybridization and bonding scheme for CH 3 CHO Draw the Lewis Structure Predict the electron group geometry around inside atoms C1 = 4 electron areas  C1= tetrahedral C2 = 3 electron areas  C2 = trigonal planar

Tro, Chemistry: A Molecular Approach122 Ex 10.8 – Predict the hybridization and bonding scheme for CH 3 CHO Determine the hybridization of the interior atoms C1 = tetrahedral  C1 = sp 3 C2 = trigonal planar  C2 = sp 2 Sketch the molecule and orbitals

Tro, Chemistry: A Molecular Approach123 Ex 10.8 – Predict the hybridization and bonding scheme for CH 3 CHO Label the bonds

Tro, Chemistry: A Molecular Approach124 Problems with Valence Bond Theory VB theory predicts many properties better than Lewis Theory bonding schemes, bond strengths, bond lengths, bond rigidity however, there are still many properties of molecules it doesn’t predict perfectly magnetic behavior of O 2

Tro, Chemistry: A Molecular Approach125 Molecular Orbital Theory in MO theory, we apply Schrödinger’s wave equation to the molecule to calculate a set of molecular orbitals in practice, the equation solution is estimated we start with good guesses from our experience as to what the orbital should look like then test and tweak the estimate until the energy of the orbital is minimized in this treatment, the electrons belong to the whole molecule – so the orbitals belong to the whole molecule unlike VB Theory where the atomic orbitals still exist in the molecule

Tro, Chemistry: A Molecular Approach126 LCAO the simplest guess starts with the atomic orbitals of the atoms adding together to make molecular orbitals – this is called the Linear Combination of Atomic Orbitals method weighted sum because the orbitals are wave functions, the waves can combine either constructively or destructively

Tro, Chemistry: A Molecular Approach127 Molecular Orbitals when the wave functions combine constructively, the resulting molecular orbital has less energy than the original atomic orbitals – it is called a Bonding Molecular Orbital ,  most of the electron density between the nuclei when the wave functions combine destructively, the resulting molecular orbital has more energy than the original atomic orbitals – it is called a Antibonding Molecular Orbital  *,  * most of the electron density outside the nuclei nodes between nuclei

Tro, Chemistry: A Molecular Approach128 Interaction of 1s Orbitals

Tro, Chemistry: A Molecular Approach129 Molecular Orbital Theory Electrons in bonding MOs are stabilizing Lower energy than the atomic orbitals Electrons in anti-bonding MOs are destabilizing Higher in energy than atomic orbitals Electron density located outside the internuclear axis Electrons in anti-bonding orbitals cancel stability gained by electrons in bonding orbitals

Tro, Chemistry: A Molecular Approach130 MO and Properties Bond Order = difference between number of electrons in bonding and antibonding orbitals only need to consider valence electrons may be a fraction higher bond order = stronger and shorter bonds if bond order = 0, then bond is unstable compared to individual atoms - no bond will form. A substance will be paramagnetic if its MO diagram has unpaired electrons if all electrons paired it is diamagnetic

Tro, Chemistry: A Molecular Approach131 1s1s1s1s   Hydrogen Atomic Orbital Hydrogen Atomic Orbital Dihydrogen, H 2 Molecular Orbitals Since more electrons are in bonding orbitals than are in antibonding orbitals, net bonding interaction

Tro, Chemistry: A Molecular Approach132 H2H2  * Antibonding MO LUMO  bonding MO HOMO

Tro, Chemistry: A Molecular Approach133 1s1s1s1s   Helium Atomic Orbital Helium Atomic Orbital Dihelium, He 2 Molecular Orbitals Since there are as many electrons in antibonding orbitals as in bonding orbitals, there is no net bonding interaction BO = ½(2-2) = 0

134 1s1s1s1s   Lithium Atomic Orbitals Lithium Atomic Orbitals Dilithium, Li 2 Molecular Orbitals Since more electrons are in bonding orbitals than are in antibonding orbitals, net bonding interaction 2s2s2s2s   Any fill energy level will generate filled bonding and antibonding MO’s; therefore only need to consider valence shell BO = ½(4-2) = 1

Tro, Chemistry: A Molecular Approach135  bonding MO HOMO  Antibonding MO LUMO Li 2

Tro, Chemistry: A Molecular Approach136 Interaction of p Orbitals

Tro, Chemistry: A Molecular Approach137 Interaction of p Orbitals

Tro, Chemistry: A Molecular Approach139 O2O2 dioxygen is paramagnetic paramagnetic material have unpaired electrons neither Lewis Theory nor Valence Bond Theory predict this result

Tro, Chemistry: A Molecular Approach140 O 2 as described by Lewis and VB theory

Tro, Chemistry: A Molecular Approach141 Oxygen Atomic Orbitals Oxygen Atomic Orbitals 2s2s2s2s 2p2p2p2p       Since more electrons are in bonding orbitals than are in antibonding orbitals, net bonding interaction Since there are unpaired electrons in the antibonding orbitals, O 2 is paramagnetic O 2 MO’s BO = ½( 8 be – 4 abe) BO = 2

Tro, Chemistry: A Molecular Approach142 O2O2  Antibonding MO HOMO  Bonding MO HOMO-1  Antibonding MO LUMO+1  Antibonding MO LUMO

Tro, Chemistry: A Molecular Approach143 Draw a molecular orbital diagram of N 2 and predict its bond order and magnetic properties

Tro, Chemistry: A Molecular Approach144 Nitrogen Atomic Orbitals Nitrogen Atomic Orbitals 2s2s2s2s 2p2p2p2p       Since there are no unpaired electrons, N 2 is diamagnetic N 2 MO’s BO = ½( 8 be – 2 abe) BO = 3

Tro, Chemistry: A Molecular Approach145 Heteronuclear Diatomic Molecules the more electronegative atom has lower energy orbitals when the combining atomic orbitals are identical and equal energy, the weight of each atomic orbital in the molecular orbital are equal when the combining atomic orbitals are different kinds and energies, the atomic orbital closest in energy to the molecular orbital contributes more to the molecular orbital lower energy atomic orbitals contribute more to the bonding MO higher energy atomic orbitals contribute more to the antibonding MO nonbonding MOs remain localized on the atom donating its atomic orbitals

Tro, Chemistry: A Molecular Approach146 NO Free-Radical  2s Bonding MO mainly O’s 2s atomic orbital

Tro, Chemistry: A Molecular Approach147 HF

Tro, Chemistry: A Molecular Approach148 Polyatomic Molecules when many atoms are combined together, the atomic orbitals of all the atoms are combined to make a set of molecular orbitals which are delocalized over the entire molecule gives results that better match real molecule properties than either Lewis or Valence Bond theories

Tro, Chemistry: A Molecular Approach149 Ozone, O 3 Delocalized  bonding orbital of O 3