Chapter 9 Covalent Bonding: Orbitals

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Chapter 9Covalent Bonding Orbitals

• Hybridization
• The mixing of atomic orbitals to form special
  orbitals for bonding.
• The atoms are responding as needed to give the
  minimum energy for the molecule.

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The Valence Orbitals on a Free Carbon Atom 2s,
2px, 2py, and 2pz
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The Formation of sp3 Hybrid Orbitals
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An Energy-Level Diagram Showing the Formation of
Four sp3 Orbitals
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Tetrahedral Set of Four sp3 Orbitals
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The Nitrogen Atom in Ammonia is sp3 Hybridized
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sp3 Hybridization

• The experimentally known structure of CH4
  molecule can be explained if we assume that the
  carbon atom adopts a special set of atomic
  orbitals. These new orbital are obtained by
  combining the 2s and the three 2p orbitals of the
  carbon atom to produce four identically shaped
  orbital that are oriented toward the corners of a
  tetrahedron and are used to bond to the hydrogen
  atoms.
• Whenever a set of equivalent tetrahedral atomic
  orbitals is required by an atom, this model
  assumes that the atom adopts a set of sp3
  orbitals the atom becomes sp3 hybridized.

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The Hybridization of the s, px, and py Atomic
Orbitals
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An Orbital Energy-Level Diagram for sp2
Hybridization
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• A sigma (?) bond centers along the internuclear
  axis.
• A pi (?) bond occupies the space above and below
  the internuclear axis.

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An sp2 Hybridized C Atom
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The s Bonds in Ethylene
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Sigma and Pi Bonding
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The Orbitals for C2H4
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When One s Orbital and One p Orbital are
Hybridized, a Set of Two sp Orbitals Oriented at
180 Degrees Results
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The Hybrid Orbitals in the CO2 Molecule
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The Orbital Energy-Level Diagram for the
Formation of sp Hybrid Orbitals on Carbon
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The Orbitals of an sp Hybridized Carbon Atom
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The Orbital Arrangement for an sp2 Hybridized
Oxygen Atom
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The Orbitals for CO2
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The Orbitals for N2
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A Set of dsp3 Hybrid Orbitals on a Phosphorus Atom
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An Octahedral Set of d2sp3 Orbitals on a Sulfur
Atom
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The Relationship of the Number of Effective
Pairs, Their Spatial Arrangement, and the Hybrid
Orbital Set Required
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The Localized Electron Model

• Three Steps
• Draw the Lewis structure(s)
• Determine the arrangement of electron pairs
  (VSEPR model).
• Specify the necessary hybrid orbitals.

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Molecular Orbitals (MO)

• Analagous to atomic orbitals for atoms, MOs are
  the quantum mechanical solutions to the
  organization of valence electrons in molecules.
• Molecular orbitals have many of the same
  characteristics as atomic orbitals, such as they
  can hold two electrons with opposite spins and
  the square of the molecular orbital wave function
  indicates electron probability.

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The Combination of Hydrogen 1s Atomic Orbitals
to Form Molecular Orbitals
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The Molecular Orbitals for H2
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Types of MOs

• bonding lower in energy than the atomic
  orbitals from which it is composed. Electrons in
  this type of orbital will favor the molecule.
• antibonding higher in energy than the atomic
  orbitals from which it is composed. Electrons in
  this type of orbital will favor the separated
  atoms.

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Bonding and Antibonding Molecular Orbitals (MOs)
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The Molecular Orbital Energy-Level Diagram for
the H2 Molecule
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The Molecular Orbital Energy-Level Diagram for
the H2- Ion
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Bond Order (BO)

• Difference between the number of bonding
  electrons and number of antibonding electrons
  divided by two.
• Bonds order is an indication of bond strength.
  Large bond order means greater bond strength.

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The Molecular Orbital Energy-Level Diagram for
the He2 Molecule
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Bonding in Homonuclear Diatomic Molecules

• In order to participate in MOs, atomic orbitals
  must overlap in space. (Therefore, only valence
  orbitals of atoms contribute significantly to
  MOs.)

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The Relative Sizes of the Lithium 1s and 2s
Atomic Orbitals
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The Molecular Orbital Energy-Level Diagram for
the Li2 Molecule
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The Molecular Orbitals from p Atomic Orbitals
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The Expected Molecular Orbital Energy-Level
Diagram Resulting from the Combination of the 2p
Orbitals on Two Boron Atoms
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The Expected Molecular Orbital Energy-Level
Diagram for the B2 Molecule
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Paramagnetism

• unpaired electrons
• attracted to induced magnetic field
• much stronger than diamagnetism

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Diamagnetism

• paired electrons
• repelled from induced magnetic field
• much weaker than paramagnetism

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Diagram of the Kind of Apparatus Used to Measure
the Paramagnetism of a Sample
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The Correct Molecular Orbital Energy-Level
Diagram for the B2 Molecule
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Molecular Orbital Summary of Second Row Diatomics
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Outcomes of MO Model

• As bond order increases, bond energy increases
  and bond length decreases.
• Bond order is not absolutely associated with
• a particular bond energy.
• N2 has a triple bond, and a correspondingly
• high bond energy.
• O2 is paramagnetic. This is predicted by the MO
  model, not by the LE model, which predicts
  diamagnetism.

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Combining LE and MO Models

• ? bonds can be described as being localized.
• ? bonding must be treated as being delocalized.

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The Resonance Structures for O3 and NO3-
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A Benzene Ring
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The Sigma System for Benzene
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The Pi System for Benzene
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The NO3- Ion