Chapter 5 Molecular Structure and Orbitals

Chapter 5 Table of Contents 5.1 Molecular Structure: The VSEPR Model 5.2 Hybridization and the Localized Electron Model 5.3 The Molecular Orbital Model 5.4 Bonding in Homonuclear Diatomic Molecules 5.5 Bonding in Heteronuclear Diatomic Molecules 5.6Combining the Localized Electron and Molecular Orbital Models

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 3 VSEPR Model VSEPR: Valence Shell Electron-Pair Repulsion. The structure around a given atom is determined principally by minimizing electron pair repulsions.

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 4 Steps to Apply the VSEPR Model 1.Draw the Lewis structure for the molecule. 2.Count the electron pairs and arrange them in the way that minimizes repulsion (put the pairs as far apart as possible. 3.Determine the positions of the atoms from the way electron pairs are shared (how electrons are shared between the central atom and surrounding atoms). 4.Determine the name of the molecular structure from positions of the atoms.

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 5 Concept Check Determine the shape for each of the following molecules, and include bond angles: HCN PH 3 SF 4 HCN – linear, 180 o PH 3 – trigonal pyramid, o (107 o ) SF 4 – see saw, 90 o, 120 o

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 6 Concept Check Determine the shape for each of the following molecules, and include bond angles: O3O3 KrF 4 O 3 – bent, 120 o KrF 4 – square planar, 90 o, 180 o

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 7 Concept Check True or false: A molecule that has polar bonds will always be polar. -If true, explain why. -If false, provide a counter-example.

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 8 Let’s Think About It Draw the Lewis structure for CO 2. Does CO 2 contain polar bonds? Is the molecule polar or nonpolar overall? Why?

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 9 Concept Check True or false: Lone pairs make a molecule polar. -If true, explain why. -If false, provide a counter-example.

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 10 Let’s Think About It Draw the Lewis structure for XeF 4. Does XeF 4 contain lone pairs? Is the molecule polar or nonpolar overall? Why?

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 11 Arrangements of Electron Pairs Around an Atom Yielding Minimum Repulsion

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 12 Arrangements of Electron Pairs Around an Atom Yielding Minimum Repulsion

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 13 Structures of Molecules That Have Four Electron Pairs Around the Central Atom

Section 5.1 Molecular Structure: The VSEPR Model Return to TOC Copyright © Cengage Learning. All rights reserved 14 Structures of Molecules with Five Electron Pairs Around the Central Atom

Section 5.2 Hybridization and the Localized Electron Model Return to TOC VSEPR Model : a) method for predicting the geometry of molecules. b) treats all covalent bonds the same. ex. H-H, F-F Localized Electron Model (LE)  Valence Bond Theory (VB) 1930  covalent bond : overlapping of atomic orbitals. : orbitals share a common region in space.

Section 5.2 Hybridization and the Localized Electron Model Return to TOC why the bond energies, lengths in H 2, F 2, HF are different? H 2 : 1S orbitals F 2 : 2P orbitals HF : 1S + 2P orbitals Hybridization : the mixing of different atomic orbitals to form a new set of orbitals that are equivalent. hybrid orbitals

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 17 Exercise Draw the Lewis structure for methane, CH 4.  What is the shape of a methane molecule? tetrahedral  What are the bond angles? o s2p2s2p2

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 18 Exercise Draw the Lewis structure for C 2 H 4 (ethylene).  What is the shape of an ethylene molecule? trigonal planar  What are the approximate bond angles around the carbon atoms? 120 o

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 19 An Energy-Level Diagram Showing the Formation of Four sp 3 Orbitals

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 20 The Formation of sp 3 Hybrid Orbitals

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 21 Tetrahedral Set of Four sp 3 Orbitals

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 22 An Orbital Energy-Level Diagram for sp 2 Hybridization

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 23 The Hybridization of the s, p x, and p y Atomic Orbitals

Section 5.2 Hybridization and the Localized Electron Model Return to TOC

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 25 The Orbital Energy-Level Diagram for the Formation of sp Hybrid Orbitals on Carbon

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 26 When One s Orbital and One p Orbital are Hybridized, a Set of Two sp Orbitals Oriented at 180 Degrees Results

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 27 The Orbitals for CO 2

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 28 Set of dsp 3 Hybrid Orbitals on a Phosphorus Atom

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Copyright © Cengage Learning. All rights reserved 29 How is the Xenon Atom in XeF 4 Hybridized?

Section 5.2 Hybridization and the Localized Electron Model Return to TOC Effective pairs

Section 5.3 The Molecular Orbital Model Return to TOC Copyright © Cengage Learning. All rights reserved 31 Regards a molecule as a collection of nuclei and electrons, where the electrons are assumed to occupy orbitals much as they do in atoms, but having the orbitals extend over the entire molecule. The electrons are assumed to be delocalized rather than always located between a given pair of atoms.

Section 5.3 The Molecular Orbital Model Return to TOC ex.Lewis structure paramagnetic !! O = O : : : :

Section 5.3 The Molecular Orbital Model Return to TOC ex. H 2 (1)MO 1 = 1S A + 1S B = σ bonding MO 2 = 1S A – 1S B = σ * antibonding (2)Energy : σ < σ * (3)Electrons are assigned to MO according to the Pauli principle & Hund’s rule (4)Bond Order number of bonding es – number of antibonding e 2 Larger bond order indicates greater bond strength

Section 5.3 The Molecular Orbital Model Return to TOC Copyright © Cengage Learning. All rights reserved 34 Combination of Hydrogen 1s Atomic Orbitals to form MOs

Section 5.3 The Molecular Orbital Model Return to TOC Copyright © Cengage Learning. All rights reserved 35 MO Energy-Level Diagram for the H 2 Molecule

Section 5.3 The Molecular Orbital Model Return to TOC Copyright © Cengage Learning. All rights reserved 36 Example: H 2

Section 5.3 The Molecular Orbital Model Return to TOC Copyright © Cengage Learning. All rights reserved 37 Example: H 2 –

Section 5.3 The Molecular Orbital Model Return to TOC p. 220, Fig H 2 - energy-level diagram

Section 5.3 The Molecular Orbital Model Return to TOC He2 energy-level diagram

Section 5.3 The Molecular Orbital Model Return to TOC He 2 + energy-level diagram

Section 5.3 The Molecular Orbital Model Return to TOC Li 2 energy-level diagram

Section 5.4 Bonding in Homonuclear Diatomic Molecules Return to TOC 2p orbitals

Section 5.4 Bonding in Homonuclear Diatomic Molecules Return to TOC

Section 5.4 Bonding in Homonuclear Diatomic Molecules Return to TOC

Section 5.4 Bonding in Homonuclear Diatomic Molecules Return to TOC

Section 5.4 Bonding in Homonuclear Diatomic Molecules Return to TOC Copyright © Cengage Learning. All rights reserved 46 Apparatus Used to Measure the Paramagnetism of a Sample Paramagnetism – substance is attracted into the inducing magnetic field.  Unpaired electrons (O 2 ) Diamagnetism – substance is repelled from the inducing magnetic field.  Paired electrons (N 2 )

Section 5.4 Bonding in Homonuclear Diatomic Molecules Return to TOC

Section 5.4 Bonding in Homonuclear Diatomic Molecules Return to TOC Copyright © Cengage Learning. All rights reserved 48 Molecular Orbital Summary of Second Row Diatomic Molecules

Section 5.5 Bonding in Heteronuclear Diatomic Molecules Return to TOC NO energy-level diagram

Section 5.5 Bonding in Heteronuclear Diatomic Molecules Return to TOC NO + /CN - diagram

Section 5.5 Bonding in Heteronuclear Diatomic Molecules Return to TOC Copyright © Cengage Learning. All rights reserved 51 Orbital Energy-Level Diagram for the HF Molecule

Section 5.6 Combining the Localized Electron and Molecular Orbital Models Return to TOC Copyright © Cengage Learning. All rights reserved 52 Resonance in Benzene

Section 5.6 Combining the Localized Electron and Molecular Orbital Models Return to TOC Copyright © Cengage Learning. All rights reserved 53 The Sigma System for Benzene

Section 5.6 Combining the Localized Electron and Molecular Orbital Models Return to TOC Copyright © Cengage Learning. All rights reserved 54 The Pi System for Benzene