Chapter - Winthrop Chemistrychem.winthrop.edu/faculty/mcintosh/link_to_webpages/courses/che… ·...
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Chapter
Gilbert Kirss Foster
Bonding TheoriesExplaining Molecular Geometry
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Chapter Outline
• 5.1 Molecular Shape• 5.2 Valence-Shell Electron-Pair Repulsion
Theory (VSEPR)• 5.3 Polar Bonds and Polar Molecules• 5.4 Valence Bond Theory and Hybrid Orbitals• 5.5 Molecules with Multiple Central Atoms• 5.6 Chirality and Molecular Recognition• 5.7 Molecular Orbital Theory
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• Chemical/physical properties related to molecular shape.• Lewis Structures
• Show atoms and bonds, but not spatial orientations (3-D)
• Molecular Models • Show orientations and bond angles; help
us understand physical and chemical properties
Molecular Shape
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Lewis Structures vs. Models
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• Bond Angle:• Angle (in degrees) defined by
lines joining the centers of two atoms to the center of a third atom to which they are covalently bonded
• Not always predictable from Lewis structures
Molecular Shape
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Chapter Outline
• 5.1 Molecular Shape• 5.2 Valence-Shell Electron-Pair Repulsion Theory
(VSEPR)• 5.3 Polar Bonds and Polar Molecules• 5.4 Valence Bond Theory and Hybrid Orbitals• 5.5 Molecules with Multiple Central Atoms• 5.6 Chirality and Molecular Recognition• 5.7 Molecular Orbital Theory
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• VSEPR Theory• A model predicting that the arrangement of
valence electron pairs around a central atom minimizes repulsion to produce the lowest energy orientation
• Electron(ic) Geometry• 3-dimensional arrangement of bonding e- pairs
and lone pairs electrons about a central atom• Molecular Geometry
• 3-dimensional arrangement of atoms in a molecule
Valence Shell Electron Pair Repulsion Theory (VSEPR)
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• To determine electronic geometry:• Draw Lewis structure (see Chapter 8)• From Lewis structure, determine steric
number (SN):
• Determine optimal spatial arrangement of electron pairs (bonding + nonbonding) to minimize repulsion
VSEPR: Electronic Geometry
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• Molecular Geometry = Electronic Geometry
• Determine Steric Number (SN):• SN = 2 (two atoms bonded to central atom)
• geometry = linear• SN = 3 (three atoms bonded to central atom)
• geometry = trigonal planar• SN = 4 tetrahedral• SN = 5 trigonal bipyramidal• SN = 6 octahedral
Molecular Geometry: Central Atom with No Lone Pairs
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Central Atom with No Lone Pairs
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Central Atom with No Lone Pairs
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• Examples: CO2 BF3 CCl4 PF5 SF6
Geometric Forms
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• Determine the molecular geometry of: a) H2CO (C is central atom)
b) CH4
Practice: Molecular Geometry (No Lone Pairs)
• Collect and Organize:• Analyze:• Solve:• Think about It:
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Central Atoms with Lone Pairs
• Molecular Geometry Electronic Geometry• Replace bonding pair(s) with lone pair(s)
• Example: SO2 (SN = 3)• Three electron pairs (2 bonding + 1 lone pair)
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• Bond angles less than predicted.
• Electron group repulsion!• Lone pair – Lone pair = Greatest repulsion
• Lone pair – Bonding pair
• Bonding pair – Bonding pair = Least repulsion
• Mulitple bonds > single bonds
Central Atoms with Lone Pairs
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Note: Bond angles decrease as # of lone pairs increases.
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Molecular Geometry: SN = 4
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Two lone pairs = greater repulsion, decreased bond angle.
Molecular Geometry: SN = 4
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With SN = 5, we see there are two distinctly different positions.
Molecular Geometry: SN = 5
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9 -19
Note: Lone pairs occupy equatorial positions.
Molecular Geometry: SN = 5
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Note: – bond angles = 90
(geometries with more than 2 lone pairs possible)
Molecular Geometry: SN = 6
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What are the molecular geometries of the ions SCN- and NO2
- ?
• Collect and Organize:• Analyze:• Solve:• Think about It:
Practice: Molecular Geometry
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Chapter Outline
• 5.1 Molecular Shape• 5.2 Valence-Shell Electron-Pair Repulsion Theory
(VSEPR)• 5.3 Polar Bonds and Polar Molecules• 5.4 Valence Bond Theory and Hybrid Orbitals• 5.5 Molecules with Multiple Central Atoms• 5.6 Chirality and Molecular Recognition• 5.7 Molecular Orbital Theory
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• Requirements for Polar Molecule:• 1. Polar bonds (i.e., covalent bonds between atoms with ΔEN).
• 2. Nonuniform distribution of polar bonds.
Polar Bonds and Polar Molecules
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•Exactly symmetric molecules are not polar
•Molecules with unshared electron pairs will almost always be polar
Polar Bonds and Polar Molecules
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• Bond Dipole:• Separation of electrical charge created
when atoms with different EN form a covalent bond
• Polar Molecule:• Sum of bond dipole vectors > zero
Polar Molecules
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• Dipole Moment (): A quantitative expression of the polarity of a molecule.Units = debyes (D); 1 D = 3.34 × 10-30 coul·m )
Measuring Polarity
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Chapter Outline
• 5.1 Molecular Shape• 5.2 Valence-Shell Electron-Pair Repulsion Theory
(VSEPR)• 5.3 Polar Bonds and Polar Molecules• 5.4 Valence Bond Theory and Hybrid Orbitals• 5.5 Molecules with Multiple Central Atoms• 5.6 Chirality and Molecular Recognition• 5.7 Molecular Orbital Theory
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• Valence Bond Theory (Linus Pauling)• Quantum mechanics-based model• Covalent bond = overlap of orbitals
• Sigma () Bond: • Covalent bond having highest electron
density between the two atoms along the bond axis.
Overlap of 1s orbitals
Atomic Orbitals and Bonds
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• Hybridization: Mixing of atomic orbitals to generate new sets of equivalent orbitals that form covalent bonds with other atoms
Hybridization: sp3 Orbitals
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• Tetrahedral orientation of sp3 hybridized orbitals = tetrahedral molecular geometry
Overlap of 1s with sp3 orbitals
Tetrahedral Sigma Bonds
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Note: lone pairs (non-bonding)
Other sp3 Examples
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Unhybridized p orbitals – form double bonds.
Trigonal Planar: sp2 Hybridization
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• Pi () Bond:• Electron density is concentrated above / below
the bonding axis.
sp2 Hybridization
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form triple bond (one and two π bonds)
Linear: sp Hybridization
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Formed by mixing one s, one d, and three p orbitals. Example: PF5 – five sigma bonds.
Trigonal Bipyramidal: sp3d Hybridization
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Formed by mixing one s, two d, and three p orbitals. Example: SF6 – six sigma bonds.
Octahedral: sp3d2 Hybridization
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What are the hybridizations of the central atoms of the ions: SCN- and NO2
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• Collect and Organize:• Analyze:• Solve:• Think about It:
Practice: Hybrid Orbitals
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Chapter Outline
• 5.1 Molecular Shape• 5.2 Valence-Shell Electron-Pair Repulsion Theory
(VSEPR)• 5.3 Polar Bonds and Polar Molecules• 5.4 Valence Bond Theory and Hybrid Orbitals• 5.5 Molecules with Multiple Central Atoms• 5.6 Chirality and Molecular Recognition• 5.7 Molecular Orbital Theory
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• Molecular Recognition:• Process by which molecules interact with
other molecules in living tissues to produce a biological effect
• Example: Ethylene (ripening agent)
All atoms in same plane.
Molecular Recognition
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• Delocalization: Spreading of electrons in alternating single and double bonds over three or more atoms in a molecule
a)
b)
Delocalization of Electrons
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• Aromatic Compound: • A cyclic, planar compound with delocalized
electrons above and below the plane of the molecule• Example – Polycyclic Aromatic Hydrocarbons (PAH)
• Planar shape may allow intercalation in DNA
Aromatic Compounds
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Chapter Outline
• 5.1 Molecular Shape• 5.2 Valence-Shell Electron-Pair Repulsion Theory
(VSEPR)• 5.3 Polar Bonds and Polar Molecules• 5.4 Valence Bond Theory and Hybrid Orbitals• 5.5 Molecules with Multiple Central Atoms• 5.6 Chirality and Molecular Recognition• 5.7 Molecular Orbital Theory
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• Isomers: Compounds with same molecular formula but with atoms arranged differently in 3-dimensional space
Optical isomers
Isomerism
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• Chirality: Property of a molecule that is not superimposable on its mirror image
Optical Isomers = Chirality
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Chapter Outline
• 5.1 Molecular Shape• 5.2 Valence-Shell Electron-Pair Repulsion Theory
(VSEPR)• 5.3 Polar Bonds and Polar Molecules• 5.4 Valence Bond Theory and Hybrid Orbitals• 5.5 Molecules with Multiple Central Atoms• 5.6 Chirality and Molecular Recognition• 5.7 Molecular Orbital Theory
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• Based on mixing of atomic orbitals of similar shapes and energies to form molecular orbitals (MOs) that belong to the molecule as a whole.• The number of MOs formed is equal to the
number of atomic orbitals combined• MOs represent discrete energy states;
orbitals spread out over entire molecule
Molecular Orbital (MO) Theory
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• Bonding Orbitals:• Region of increased electron density between
nuclear centers that hold atoms together• Are lower in energy (more stable) than atomic
orbitals from which they are formed• Antibonding Orbitals:
• Regions of electron density that destabilize the molecule because they do not increase electron density between nuclear centers
• Less stable than atomic orbitals from which they are formed
Types of Molecular Orbitals
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• MO Diagram:• Energy level diagram for showing the relative
energies and electron occupancy of the MOs for a molecule
• Sigma () Bond:• Covalent bond with the highest electron
density along the bond axis• Pi () Bond:
• Formed by mixing of atomic orbitals not oriented along the bonding axis in a molecule
Molecular Orbital Diagram
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• The two 1s orbitals mix to yield two sigma MOs (1 bonding / 1 antibonding).
Molecular Orbital Diagram: H2
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Bond Order = 1/2 (# bonding e- – # antibonding e-)
Bond order in H2- = ½
(Stable)Bond order in He2 = 0
(Not Stable)
Bond Order and Stability
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1. The total # of MOs = the # of AOs orbitals mixed.
2. Orbitals with similar energy/shape mix more effectively than do those of different energy/shape.
3. Orbitals of different n (different sizes/energies) result in less effective mixing.
4. A MO can accommodate two electrons.5. Electrons fill MO diagrams according to
Hund’s rule.
MO Guidelines
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• Electron configuration for O2: (σ2s)2(σ2s
*)2(σ2p)2(π2p)4(π2p*)2
• Bond order = ½ (8 – 4) = 2
• O2 has two bonds
• O2 has two unpaired electrons in 2p
*
MO Scheme for O2
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• Electron configuration for N2: (σ2s)2(σ2s
*)2(σ2p)2(π2p)4
• Bond order = ½ (8 – 2) = 3
N2 has three bonds.
N2 has no unpaired electrons.
MO Scheme for N2
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• Paramagnetism:• Atoms or molecules having unpaired
electrons are attracted to magnetic fields• Example: O2
• Diamagnetism:• Atoms or molecules having all paired
electrons are repelled by magnetic fields
• Example: N2
Paramagnetism vs. Diamagnetism
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• Zeff alters MO diagram; atomic orbitals for O are lower in energy.
• Odd electron in π*2p , closer in energy to the 2p atomic orbitals of N atom.
• Bond order = ½ (8 – 3) = 2.5
MO for NO
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Blue-violetCrimson red
MO for N2, N2+: Emission
Spectra
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ChemTours: Chapter 5
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GILBERTKIRSS
FOSTER
This concludes the Lecture PowerPoint
presentation for Chapter 5
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