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1 CH301 Chapter 14b Molecular Orbitals 2 Molecular Orbital Theory z The diatomic molecule of O 2 has the following Lewis structure: •• •• O=O •• •• z The molecular geometric shape indicates that the molecule has a double bond and each oxygen atom has two unshared pairs of electrons around the central atom. 3 Molecular Orbital Theory z Since there are no half-filled orbitals (unpaired electrons): z It would be expected that the molecule, when placed in a magnetic field, would be diamagnetic. z Diamagnetic substances are those substances in which all the electrons are paired and are weakly repelled by magnetic fields. sp 3 sp 3 sp 3 sp 3 4 Molecular Orbital Theory However, experimental procedures show that the molecule is paramagnetic. z Paramagnetic substances are those substances that contain unpaired electrons and are weakly attracted into magnetic fields. sp 3 sp 3 sp 3 sp 3 5 Figure 14.42: When liquid oxygen is poured into the space between the poles of a strong magnet, it remains there until it boils away. Source: Donald Clegg 6 Molecular Orbital Theory Therefore, the valence bond theory is not an acceptable explanation for the bonding that occurs in the oxygen molecule. z Molecular orbitals is a bonding theory that explains the phenomenon of the paramagnetic behavior of molecular oxygen by rationalization that O 2 has two unpaired electrons .
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7 Molecular Orbital Theory The MO (molecular orbitals) theory suggests that the combination of atomic orbitals on different atoms forms molecular orbitals so that electrons in MO’s belong to the molecule as a whole. Molecular Orbital Theory z The electrons that occupy orbitals called molecular orbitals : z are valence electrons that are not localized on the atoms or between pairs of atoms. z are valence electrons that are delocalized over the entire molecule. z The valence electrons of both bonding atoms are included when filling molecular orbitals. 9 Molecular Orbital Theory Bonding orbitals (Linear combination of atomic orbitals) LCAO – MO z The following assumptions lie at the core of this model. Molecular Orbital Theory z Molecular orbitals are formed from the overlap of atomic orbitals. z Only atomic orbitals of about the same energy interact to a significant degree. z When two atomic orbitals overlap, they interact in two extreme ways to form two molecular orbitals, a bonding molecular orbital and an antibonding molecular orbital. Molecular Orbital Theory z In the LCAO model, the two atomic orbitals are treated as waves that are centered on different nuclei. z If the waves between nuclei have a constructive interference with each other z The total amplitude of the wavefunction is increased where the orbitals overlap 12 Molecular Orbital Theory ± Waves that describe atomic orbitals have both positive and negative phases or amplitudes.
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This note was uploaded on 02/24/2010 for the course CH 301 taught by Professor Fakhreddine/lyon during the Spring '07 term at University of Texas at Austin.

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