covalent bonding orbitals orbitals just got stranger copyright©2000 by houghton mifflin company....

Covalent Bonding Orbitalsorbitals just got stranger

Copyright©2000 by Houghton Mifflin Company. All rights

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1

Localized Electron Model

Lewis structures, VSEPR, and now, what type of atomic orbitals are used to share electrons.


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Problem: all 4 bonds identical But C uses 2s, 2p orbitals. C 1s22s22p2


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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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Figure 9.2The Valence Orbitals on a Free Carbon Atom: 2s, 2px, 2py, and 2pz


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Figure 9.3 The Formation of sp3 Hybrid Orbitals from the s and p

orbitals

sp3 hybridization (tetrahedron)

We say that “carbon is sp3 hybridized” or

“carbon undergoes sp3 hybridization”

All 4 hybrid orbitals are identical


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Figure 9.4Cross Section of an sp3 Orbital


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Figure 9.5An Energy-Level Diagram Showing the Formation of Four sp3 Orbitals


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Figure 9.6 Tetrahedral Set of Four sp3 Orbitals showing CH4 arrangement


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What about trig. pyramidal?The Nitrogen Atom in Ammonia is sp3 Hybridized.

sp2 hybridization (trig planar)

ethylene


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Figure 9.8 The Hybridization of the s, px, and py Atomic Orbitals


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Figure 9.9 An Orbital Energy-Level Diagram for sp2

Hybridization


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Figure 9.10An sp2 Hybridized C Atom

notice that the extra P is perpen. To the 3 sp2 orbitals

What does the extra “p” do?

A sigma () bond centers along the internuclear axis. (in ethylene these are the bonds using the sp2 orbitals)

A pi () bond occupies the space above and below the internuclear axis. (in ethylene these are the bonds use the extra “p” orbitals)

Note: a double bond is always a and a

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16

CCH H

HH


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Figure 9.11The Bonds in Ethylene


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Figure 9.12 Sigma and Pi Bonding


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Figure 9.13The Orbitals for C2H4

sp hybridization (linear)


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Figure 9.14When One s Orbital and One p Orbital

are Hybridized, a Set of Two sp Orbitals Oriented at 180 Degrees Results


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Figure 9.15 The Hybrid Orbitals in the CO2 Molecule

Note O is sp2 hybridized, trig planar


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The Orbital Energy-Level Diagram for the Formation of sp Hybrid

Orbitals on Carbon(note: two 2p orbitals unchange)


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Figure 9.17The Orbitals of an sp Hybridized

Carbon Atom


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Figure 9.18The Orbital Arrangement for an sp2

Hybridized Oxygen Atom


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Figure 9.19The Orbitals for CO2


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Figure 9.20The

Orbitals for N2

Note: sp

hybridation for

each N atom


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dsp3 hybridization trig. bipyramidal

PCl5


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Set of dsp3 Hybrid Orbitals on a Phosphorus Atom


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Figure 9.22 (a) The Structure of the PCl5 Molecule (b) The

Orbitals Used to Form the Bonds in PCl5 (note each Cl is sp3 hybridized)

Finally d2sp3 hybridization

SF6


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Figure 9.23An Octahedral Set of d2sp3 Orbitals

on a Sulfur Atom


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The Localized Electron Model

Draw the Lewis structure(s)

Determine the arrangement of electron pairs (VSEPR model).

Specify the necessary hybrid orbitals.


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Figure 9.24The Relationship of the Number of Effective Pairs, Their Spatial Arrangement, and the Hybrid Orbital Set Required


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MO BETTA MODELSince resonance is cumbersome and tedious,

And odd – electron structures are not well accommodated by the simplified L.E. model,

And no direct information about bond energies are given,

And magnetic properties predicted by the L.E. model are not seen in the lab,

We supplant the L.E. model with the M.O. model.


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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.


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Types of MOs

bonding: lower in energy than the atomic orbitals from which it is composed.

antibonding: higher in energy than the atomic orbitals from which it is composed.


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Figure 9.25 The Combination of Hydrogen 1s Atomic Orbitals to Form

Molecular Orbitals


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Figure 9.27 Bonding and Antibonding Molecular Orbitals

(MOs)


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Figure 9.26The Molecular Orbitals for H2


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Figure 9.28The Molecular Orbital Energy-Level Diagram for the H2 Molecule


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Bond Order (BO)

Difference between the number of bonding electrons and number of antibonding electrons divided by two.

BO = # bonding electrons # antibonding electons

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Figure 9.29The Molecular Orbital Energy-Level

Diagram for the H2- Ion


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Diagram for the He2 Molecule


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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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Figure 9.31 The Relative Sizes of the Lithium 1s and 2s Atomic

Orbitals


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Diagram for the Li2 Molecule


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Figure 9.34 The Molecular Orbitals from p Atomic Orbitals


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Figure 9.35The Expected Molecular Orbital Energy-Level Diagram Resulting from the Combination of

the 2p Orbitals on Two Boron Atoms


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Figure 9.36The 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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Figure 9.37Diagram of the Kind of Apparatus Used to Measure the Paramagnetism of a Sample


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Figure 9.38The Correct Molecular Orbital Energy-Level Diagram for the B2 Molecule


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Figure 9.39Molecular Orbital Summary of Second Row

Diatomics


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Figure 9.41The Molecule Orbital Energy-Level Diagram for the NO Molecule


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Figure 9.42The Molecular Orbital Energy-Level Diagram for Both the NO+ and CN- Ions


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Figure 9.43A Partial Molecular Orbital Energy-Level Diagram for the HF Molecule


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Figure 9.44 The Electron Probability Distribution in the Bonding Molecular

Orbital of the HF Molecule


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Outcomes of the MO Betta Model

1. As bond order increases, bond energy increases and bond length decreases.

2. Bond order is not absolutely associated with a particular bond energy.

3. N2 has a triple bond, and a correspondingly high bond energy.

4. 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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Figure 9.45 The Resonance Structures for O3 and NO3

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Figure 9.46A Benzene Ring


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Figure 9.47 The Sigma System for Benzene


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Figure 9.48The Pi System for Benzene


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Figure 9.49 The NO3- Ion

covalent bonding orbitals orbitals just got stranger copyright©2000 by houghton mifflin company....

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orbital slide

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extra p orbitals

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