Shapes of Molecules. Lewis structures are useful but don t allow prediction of the shape of a molecule.

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1 Shapes of Molecules Lewis structures are useful but don t allow prediction of the shape of a molecule. H O H H O H Can use a simple theory based on electron repulsion to predict structure (for non-transition elements). Shapes of Molecules Molecular shape - the way bonded atoms arrange themselves in three dimensions. Molecular shape affects molecular properties, including reactivity. Valence Shell Electron Pair Repulsion (VSEPR) theory - molecules assume a shape that allows them to minimize the repulsions between electron pairs in the valence shell of the central atom. Electron pairs include both lone pair electrons and bonding pair electrons. Electron group geometry distribution of e - pairs. Molecular geometry distribution of nuclei. 1

2 2 Electron Pairs 2 Pairs: e.g. BeH 2 the molecule is LINEAR to maximize the distance between bond pairs. H Be H bond angle is 180 # Electron Groups Electron Group Geometry Lone Pairs Molecular Geometry Bond Angles Example 2 linear 0 linear 180 BeCl 2 3 Electron Pairs 3 Pairs: e.g. BH 3 the electron pairs arrangement is trigonal planar the H B H bond angle is 120 # Electron Groups 3 Electron Group Geometry Trigonal Planear Lone Pairs 0 Molecular Geometry Trigonal Planear Ideal Bond Angles Example 120 BF 3 1 Bent 120 O 3 2

3 4 Electron Pairs 4 Pairs: e.g. CH 4 the electron pairs arrangement is tetrahedral the H C H bond angle is # Electron Groups Electron Group Geometry Lone Pairs Molecular Geometry Ideal Bond Angles Example 4 Tetrahedral 0 Tetrahedral CH 4 1 Trigonal Pyramidal NH 3 2 Bent H 2 O Structures of Molecules that Have Four Electron Pairs Around the Central Atom 3

4 The Molecular Structure of Methane The Molecular Structure of Ammonia is a Trigonal Pyramid 4

5 The Structure and Charge Distribution of the Ammonia Molecule The Tetrahedral Arrangement of Oxygen In a Water Molecule 5

6 The Charge Distribution in the Water Molecule Lone Pairs vs. Bond Pairs In a tetrahedron, the expected bond angles are H C H bond angle = H N H bond angle = 107 H O H bond angle = The slight differences are explained by the lone pairs occupying more space than bond pairs around the central atoms 6

7 Bonding Pair vs Lone Pair Electrons a) In a bonding pair of electrons the electrons are shared by two nuclei b) In a lone pair, both electrons must be close to a single nucleus The Bond Angles In the CH 4, NH 3, and H 2 O Molecules 7

8 Multiple Bonds for the purposes of VSEPR multiple bonds behave like single bonds Consider CO 2 O C O so the central carbon would have 2 bonding pairs of electrons around it carbon dioxide is linear. The Carbon Dioxide Molecule 8

9 5 Electron Pairs 5 Pairs: e.g. PCl 5 the electron pairs arrangement is trigonal bipyramidal the Cl P Cl bond angles are 90 and120 # Electron Groups 5 Electron Group Geometry Trigonal bipyramidal Lone Pairs 0 Molecular Geometry Trigonal bipyramidal Ideal Bond Angles Example 90 and 120 PCl 5 1 Seesaw 90 and 120 SF 4 2 T-shaped 90 IF 3 3 Linear 180 XeF 2 Location of Lone Pairs: the I 3- Ion Three distinct choices for the structure: The order of repulsive forces is: LP-LP > LP-BP > BP-BP LP = lone-pair; BP = bond pair Therefore the correct structure is c (linear) 9

10 Structures of Molecules with Five Electron Pairs Around the Central Atom 6 Pairs: e.g. SF 6 the electron pairs arrangement is trigonal bipyramidal the F S F bond angles are 90 6 Electron Pairs # Electron Groups Electron Group Geometry Lone Pairs Molecular Geometry Ideal Bond Angles Example 6 Octahedral 0 Octahedral 90 SF Square pyramidal Sqaure Planar 90 IF 5 90 XeF 4 10

11 Possible Electron Pair Arrangements for XeF 4 The order of repulsive forces is: LP-LP > LP-BP > BP-BP LP = lone-pair; BP = bond pair The lone pairs will locate as far apart as possible Therefore correct structure is b (square planar) Predicting a VSEPR Structure 1. Draw Lewis structure. 2. Put electron pairs as far apart as possible. Double and triple bonds count as one bonding pair. 3. Determine positions of atoms from the way electron pairs are shared. 4. Determine the name of molecular structure from positions of the atoms. 11

12 Molecular Shape Example Determine the shape for each of the following molecules, and include bond angles: HCN PH 3 SF 4 O 3 KrF 4 Shapes of Molecules Molecular geometry for larger molecules is possible. Geometry is assigned for each central atom. Hybridization on each central atom assists in determination of overall geometry. 12

13 The Molecular Structure of Methanol a) The arrangement of electron pairs around the carbon atom b) The arrangement of bonding and lone pairs around the oxygen atom c) The molecular structure Molecular Orbital Theory Atomic orbitals are isolated on atoms. Molecular orbitals span two or more atoms. LCAO Linear combination of atomic orbitals. Ψ 1 = c c 12 2 Ψ 2 = c c c 2 c c c

14 Combining Atomic Orbitals Electron Waves in Two Different Atoms Interact as the Atoms Come Together 14

15 Overlap as a Function of Atomic Distance (Left column) Two hydrogen atoms approaching with 1s orbitals in phase result in an enhanced amplitude in the internuclear space (Right column) Out-ofphase orbitals in the two approaching hydrogen atoms cancel each out in the internuclear space, resulting in diminished amplitude (a node) between the atoms Molecular Orbitals of Hydrogen 15

16 Basic Ideas Concerning MOs Number of MOs = Number of AOs. Bonding and antibonding MOs formed from AOs. e - fill the lowest energy MO first. Pauli exclusion principle is followed. Hund s rule is followed Bond Order Stable species have more electrons in bonding orbitals than antibonding. Bond Order = No. e- in bonding MOs - No. e - in antibonding MOs 2 16

17 Molecular Orbitals of the Second Period First period use only 1s orbitals. Second period have 2s and 2p orbitals available. p orbital overlap: End-on overlap is best sigma bond (σ). Side-on overlap is good pi bond (π). The Relative Sizes of the Lithium 1s and 2s Atomic Orbitals 17

18 The Molecular Orbital Energy-Level Diagram of the Li 2 Molecule Molecular Orbitals of the Second Period 18

19 Expected MO Diagram of C 2 The s Orbital and the pz Orbital The s bonding orbital and the pz orbital are located in a similar region of space and have the same symmetry with respect to the molecule 19

20 Modified MO Diagram of C 2 The Responses of Different Materials to a Magnetic Field Reveal Pairing of Electrons in the Bonds 20

21 Apparatus Used to Measure the Paramagnetism of a Sample Molecular Orbital Summary of Second Row Diatomics 21

22 Orbital Energy-Level Diagram for the HF Molecule The Electron Probability Distribution in the Bonding Molecular Orbital of the HF Molecule 22

23 MO Diagrams of Heteronuclear Diatomics The molecular orbital energy-level diagram for NO Illustrates the effect of differing electronegativities on bonding The Molecular Orbitals of NO The portion of the orbital in the vicinity of the oxygen atom is more compact and more concentrated than that near to the nitrogen, reflecting the greater electronegativity of oxygen 23

24 Delocalized Electrons The Sigma System for Benzene 24

25 The Pi System for Benzene Benzene 25

26 Ozone The Molecular Orbital System of the NO 3- Ion (a) The p Orbitals used to form the bonding system in the NO 3-. (b) A representation of the delocalization of the electrons in the molecular orbital system of the NO 3 - ion 26