Molecular Geometry and Bonding Theories Chapter 9
Molecular Shapes CCl 4 Lewis structures give atomic connectivity; The shape of a molecule is determined by its bond angles
VSEPR Model Valence Shell Electron Pair Repulsion (VSEPR) theory 1)Lewis structure 2)Electron-electron repulsion Assume: a) The valence electrons repel each other b) the molecule adopts whichever 3D geometry minimized this repulsion.
VSEPR model Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear B B Cl Be Cl
VSEPR Model Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear AB 3 3 0 trigonal planar trigonal planar
VSEPR Model Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear AB 3 3 0 trigonal planar trigonal planar AB 4 4 0 tetrahedral tetrahedral
VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear AB 3 3 0 trigonal planar trigonal planar AB 4 4 0 tetrahedral tetrahedral AB 5 5 0 trigonal bipyramidal trigonal bipyramidal
VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AB 2 2 0 linear linear AB 3 3 0 trigonal planar trigonal planar AB 4 4 0 tetrahedral tetrahedral AB 5 5 0 trigonal bipyramidal trigonal bipyramidal AB 6 6 0 octahedral octahedral 10.1
The VSEPR Model The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles: By experiment, the H-X-H bond angle decreases on moving from C to N to O: H H C H109.5 H O H H N H 107 O O H H 104.5 O Electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. 1) The bond angle decreases as the number of lone pairs increase.
The VSEPR Model The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles: 2) Electrons in multiple bonds repel more than electrons in single bonds. Cl 111.4 o C O Cl 124.3 o
The VSEPR Model Predicting Molecular Geometries Molecular shapes for molecules with 2, 3, 4, 5 and 6 electron pairs about the central atom
The VSEPR Model Predicting Molecular Geometries Molecular shapes for molecules with 2, 3, 4, 5 and 6 electron pairs about the central atom
The VSEPR Model Predicting Molecular Geometries Molecular shapes for molecules with 2, 3, 4, 5 and 6 electron pairs about the central atom
The VSEPR Model Molecules with Expanded Valence Shells To minimize e e repulsion, lone pairs are always placed in equatorial positions
The VSEPR Model Predicting Molecular Geometries Molecular shapes for molecules with 2, 3, 4, 5 and 6 electron pairs about the central atom
The VSEPR Model Predicting Molecular Geometries To determine the electron pair geometry: Draw the Lewis structure Count the total number of electron pairs around the central atom Arrange the electron pairs in one of the above geometries to minimize e -e repulsion Multiple bonds count as one bonding pair
Predicting Molecular Geometry Molecules with more than one central atoms H O H C C O H H
Predicting Molecular Geometry Molecules with More than One Central Atom
Dipole Moments and Polar Molecules electron poor region electron rich region Dipole Moments µ= Q r H F δ+ δ Q: the charge r : the distance between charges 1 D = 3.36 x 10-30 C m
Polarity of Molecules Polar molecules interact with electric fields:
Polarity of Molecules Dipole Moments of Polyatomic Molecules 1) Diatomic molecules: polar bonds always result in an overall dipole moment.
Polarity of Molecules Dipole Moments of Polyatomic Molecules: O C O Non-polar molecule O S O polar molecule Non-polar molecule H O H polar molecule
Covalent bond and orbital overlap Valence bond theory bonds are formed by sharing of e - from overlapping atomic orbitals. Change in electron density as two hydrogen atoms approach each other.
Hybridization Hybridization mixing of two or more atomic orbitals to form a new set of hybrid orbitals. 1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals. 2. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process.
sp Hybrid Orbitals
sp Hybrid Orbitals sp Hybrid Orbitals
sp 2 Hybrid Orbitals Ground state excited state sp 2 hybrids
Formation of sp 2 Hybrid Orbitals
sp 2 Hybrid Orbitals
sp 3 Hybrid Orbitals
Formation of sp 3 Hybrid Orbitals
sp 3 Hybrid Orbitals
Hybridized O atom in H 2 O
Hybridized N atom in NH3
sp 3 Hybrid Orbitals
Hybrid Orbitals
Hybrid Orbitals
Hybrid Orbitals
Summary To assign hybridization: Hybrid Orbitals Draw a Lewis structure; Assign the electron pair geometry using VSEPR theory; From the electron pair geometry, determine the hybridization; Name the geometry by the positions of the atoms.
sp 2 Hybridization of a Carbon atom
Sigma bond (σ) electron density between the 2 atoms Pi bond (π) electron density above and below plane of nuclei of the bonding atoms
sp Hybridization of a C atom in Acetylene
Bonding in Acetylene
Second-Row Diatomic Molecules Molecular Orbitals from 2p Atomic Orbitals: end-on sideways
Multiple Bonds σ-bonds: electron density lies on the axis between the nuclei. end-on All Single bonds σ bonds
Multiple Bonds π-bonds: electron density lies above and below the plane of the nuclei. sideways
Delocalized π Bonding Multiple Bonds
Multiple Bonds Delocalized π Bonding Localized delocalized between over the C atoms entire ring (i.e. the π electrons are shared by all 6 C atoms)
Molecular Orbitals Some aspects of bonding are not explained by (1) Lewis structures, (2) VSEPR theory (3) hybridization. Why does O interact with magnetic field? 2 Why are some molecules colored?
Molecular Orbitals Molecular Orbital (MO) Theory: Electrons in atoms atomic orbitals(ao) Electrons in molecules molecular orbitals(mo) Molecular orbitals: each contain a maximum of two electrons; have definite energies; MO are associated with an entire molecule.
Molecular Orbitals The Hydrogen Molecule : Two AOs two MOs
Molecular Orbitals in Hydrogen (H 2 ) A bonding molecular orbital has lower energy and greater stability than the atomic orbitals from which it was formed. An antibonding molecular orbital has higher energy and lower stability than the atomic orbitals from which it was formed.
Molecular Orbitals in Helium (He 2 )
Bond Order Bond Ord er = 1 (bonding electrons - antibonding electrons) 2 Bond order of H 1 2 = (2 0) = 1 2 H has a single bond 2 Bond order of He 1 2 = (2 2) = 0 2 He is not a stable molecule 2
bond order = 1 2 Number of electrons in bonding MOs Number of electrons in antibonding MOs ( - ) bond order ½ 1 ½ 0
Molecular Orbitals in Li2
P Orbitals and the Corresponding Molecular Orbitals
Second-Row Diatomic Molecules Electron Configurations for O 2 through Ne 2:
Second-Row Diatomic Molecules Electron Configurations for B 2 through Ne 2 Small 2s-2p 2p interaction larger 2s-2p 2p interaction
Second-Row Diatomic Molecules Electron Configurations for B 2 through Ne 2
Second-Row Diatomic Molecules Electron Configurations and Molecular Properties Two types of magnetic behavior: paramagnetism (unpaired electrons in molecule): strong attraction between magnetic field and molecule; (P340) diamagnetism (no unpaired electrons in molecule): weak repulsion between magnetic field and molecule.
Homework 9.12, 9.20, 9.26, 9.34, 9.38, 9.48, 9.52, 9.76