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    Molecular Orbital Theory

    Combination of atomic orbitals on different atoms(may be several) forms molecular orbitals (MO)

    Electrons in MO belong to the molecule as a wholeElectron cloud distribution Bond energies Magnetic properties

    (Valence bond theorycombination of atomicorbitals for same atom)

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    Molecular Orbit Theory

    ++

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    Molecular Orbit Theory

    Same bonding orbital as more detailedbonding orbital of first MO slide

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    No nodes

    1 node

    Start with 2 atomic orbitals;end with 2 molecular orbitals

    The greater the atomic orbitaloverlapthe stronger the bond

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    E n e r g y

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    1*

    s

    1 s

    2 s

    2 x p

    2*

    x p

    2*

    s

    2*

    y p 2*

    z p

    2 y p

    2 z p

    For H 2 through N 2

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    2H+

    Ground state Bond order?

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    - -Number bonding e Number antibonding e

    2

    Bond order =

    Single: 1

    Double: 2Triple: 3

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    2H+

    Ground state Bond order?

    Paramagneticunpaired spins

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    2H+

    Excited state

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    2H+

    More highlyexcited state

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    2H

    Bond order: ?

    Diamagneticno unpaired spins

    1

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    2He+

    Bond order: ?

    Paramagnetic

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    2He

    Bond order: ?

    If existed, diamagnetic

    0Never observed.

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    2Li

    Bond order: 1

    Diamagnetic

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    2Be

    Bond order: 0

    Diamagnetic

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    2B

    Bond order: 1

    Paramagnetic

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    2C

    Bond order: 2

    Diamagnetic

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    2N

    Bond order: 3

    Diamagnetic

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    2O

    Bond order: ?

    Paramagnetic

    2

    .. ..:O::O:

    Aside:VSEPR predictsdiamagnetic

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    Liquid Oxygen

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    E n e r g y

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    N2

    O2

    Bond order O 2: 2

    Bond order O 2-: 1.5

    Bond order O 2+: 2.5

    MOLECULAR ELECTRON CONFIGURATIONS

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    HF

    DIATOMIC HETERONUCLEAR MOLECULE

    212

    =

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    Sometimes the valence bond and molecular orbital

    models are combined.

    Sigma bonds are treated using valence bond theory.

    Pi bonds are treated using molecular orbital theory.

    Benzene

    C6H6

    Hybridization for carbonsingle bonds ( C C andC H)? sp 2

    H

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    2

    bonds in benzene

    from C sp orbitals and

    H 1s orbitals

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    Ground state molecular orbital

    How many electrons in this orbital? 2

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    Number valenceelectrons in C 6H6?

    6x4 +6 = 30

    Number electronsin bonds?

    12x2 = 24

    6

    Numberelectrons in bonds?

    Bond order of e - is 3. Bond order of e - per CC bond is .

    *

    E N R I C H M E N T

    (Number of nodesfrom top view)

    BENZENE PI BOND ORDER

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    Ozone, O 3

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    ENRICHMENT

    NEED TO KNOW

    MOLECULAR ORBITALS FOR OZONE

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    2-3CO

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    CO

    OPTIONAL EXAMPLE: DIATOMIC HETERONUCLEAR MOLECULE

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    What about Be 1s ?What about Be 1s it is too small to overlap appreciably withH1s . It remains localized on Be and is a nb orbital.

    OPTIONAL EXAMPLE: MOLECULAR ORBITALS FOR TRIATOMIC BeH 2

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    What about Be 2px and Be 2py?What about Be 2py and Be 2pz there orientation is such thatthere is little overlap with H 1s . They remain localized onBe and are nb orbitals.

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    E n e r g y

    nb 1s(no overlap)

    Molecular orbital diagram of BeH 2. Bond order formolecule is 2. Bond order each bond is 2/2 = 1.