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    Molecular Orbital

    Theory

    p][nJo[to au[to[mo

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    Covalent Bonding Theory

    Valence Bond Theory

    useful for deriving shapes/polarity

    simple but inaccurate/deficient

    Molecular Orbital Theory

    more complex/more accurate all valence electrons are delocalized

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    Molecular Orbital Theory

    The goal of molecular orbital theory is todescribe molecules in a similar way to howwe describe atoms, that is, in terms of

    orbitals, orbital diagrams, and electronconfigurations

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    Orbitals

    Atomic Orbital

    Hybrid Orbital

    Molecular Orbital

    space to find out the electron with thehighest possibility.

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    Atomic orbital

    Pure orbital of an atom

    Immixing orbital with other atomic orbital

    Characteristic for one atom

    1s1 characteristic of hydrogenatomic orbital

    1s2 characteristic of helium atomicorbital

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    Hybrid orbital

    Mixing of atomics orbital to form a newset orbital

    The properties and the energy of hybridorbital are intermediate of non-hybridorbitals

    sp 50% of s orbital & 50% of p

    orbital sp2 33.7% of s orbital &

    66.3% of p orbital

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    Molecular orbital

    Result of the overlapping of orbitals andthe mixing of different atomic orbitals in

    the molecule

    Electrons belong to the molecule as a

    whole

    , *, , *

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    Orbitals

    Atomic orbitals

    1s2, 1s2 2s2 2p1

    Hybrid orbitals

    sp, sp2, sp3, dsp2

    Molecular orbitals

    (1s)2, (*1s)

    2

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    Molecular Orbital

    Bonding orbital

    Antibonding orbital

    Nonbonding orbital

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    Bonding orbital

    The electron density localized betweenthe two nuclei

    Has higher stability and lower energythan the origin atomic orbital

    b = A + B (b = bonding)

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    Antibonding orbital

    The electron density = zero

    Has nodal plane

    Has lower stability and higher energythan the origin atomic orbital

    ab = A - B (ab = antibonding)

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    Nonbonding orbital

    The electron localized to each atomicorbital

    Electrons in nonbonding lobe repel tothose in bonding lobe

    Influent the geometry shape of themolecule

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    Orbitals Interaction

    Interactions of the atomic orbitals

    In-phase interaction

    Out-of-phase interaction

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    In-phase interaction

    Intensity of the negative charge >>>

    Negative charge between 2 nuclei >>>

    plus-minus attraction between theelectron charge and the nuclei >>>

    Attraction>>>

    Potential energy

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    Out-of-phase interaction

    intensity of the negative charge

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    1. Head-on overlap (end-on overlap) of

    atomic orbital

    produces (sigma) orbitals.

    2. Side-on overlap (parallel overlap) of

    atomic orbital produces (pi) orbitals.

    2 types of overlap of the atomic orbitals

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    Head-on-overlap of two 1s orbitals

    Two 1s atomic orbitals that overlapproduce two molecular orbitals designatedas:

    1. 1s bonding molecular orbital

    2. *1s antibonding molecular orbital

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    Overlap of Wave functions of two s orbitals

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    ConstructiveOverlapof two 1s orbitals

    Destructive

    Overlap of two1s orbitals

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    Bonding orbital

    Lower energyStableFavorable for electronsElectrons exist betweennuclei

    1s (bonding)

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    Anti-bonding orbital

    Higher energy

    UnstableUnfavorable for electronsElectrons exist outside

    *1s (antibonding)

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    Energetically, the molecular orbitals split.

    1. The 1s is lower in energy.2. The 1s* is higher in energy. Molecular

    orbitaldiagram for

    the union oftwo s orbitals

    *1s(antibonding)

    1s (bonding)

    1s1s

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    Head-on-overlap of two 2px orbitals

    Head-on (end-on) overlap of two patomic orbitals on different atoms, such as2px with 2px, produces:

    1. 2px bonding molecular orbital

    2. *2px antibonding molecular orbital

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    Side on overlap of two 2p or 2p

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    Side-on-overlap of two 2py or 2pz

    orbitals

    Side-on (parallel)overlap of two p atomicorbitals on different atoms (such as 2pywith 2py or 2pz with 2pz) produces:

    1. 2py or 2pz (both are bondingorbitals)

    2. *2py or *2pz (both are antibondingorbitals)

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    Graphically theseorbitals look like this:

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    How many molecular orbitals are formed

    when

    two 1s orbitals interact?

    two 2p orbitals interact?

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    and orbitals

    Overlap in

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    * and * orbitals

    Less overlap leads to less shift of electroncharge from between the nuclei.

    Overlap in * >> *

    Stabilization of * >>> *

    Potential energy of *

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    How are the orbitals filledwith the electrons

    Note: you consider all the

    electrons of each atom, not justthe valence electrons!

    ?

    !!!

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    Molecular Orbital (MO)

    Orbital filled by electrons based on

    LCAO

    (Linear Combination of Atomic Orbitals)

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    Keep these in mind!!!

    1. Molecular orbitals are formed from theoverlap of atomic orbitals.

    2. Only atomic orbitals of about the sameenergy interact to a significant degree.

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    3. The number of MO = the number ofcombined AO

    4. When two atomic orbitals overlap, andtwo molecular orbitals are formed, abonding MO and an antibonding MO

    Keep these in mind!!!

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    5. The filling of the orbitals by electrons inmolecular orbital like to those in atomicorbitals

    Aufbau Principle

    Hund's Rule

    Pauli Exclusion Principle

    Keep these in mind!!!

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    The Uses of Molecular Orbital

    Predicting:

    The stability of the molecule

    The magnetism of the molecule

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    Stability of the Molecule

    Molecule stability determined by itsBOND ORDER (BO)

    2

    orbitalsgantibondinineorbitalsbondingine=BO

    --

    2 gantibondinebondinge=BO

    --

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    Bond order reflects the bond strength

    BO = 0, the molecule is unstable (thereare equal numbers of electrons inbonding and antibonding orbitals)

    BO > 0, suggests a stable molecule(there are more electrons in bonding

    than antibonding orbitals)

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    Bond Order >>>

    Bond Length >

    Molecule Stability >>>

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    Magnetism of the Molecule

    Paramagnetic

    compounds that are weakly attractive inmagnetic fields

    unpaired electrons

    Diamagnetic

    compounds that are not attractive inmagnetic fields

    all electrons are paired

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    Amplitude

    Quadrate of wave functions

    Electron density

    The highest possibility to find theelectron

    bonding b2 = A2 + B2 + 2AB

    antibonding ab2 = A2 + B2 - 2AB

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    Energy Level Diagram

    2 types of energy level diagram:

    Homonuclear diatomic molecules

    Heteronuclear diatomic molecules

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    Homonuclear Diatomic Molecules

    Electronegativityof A = B

    Energy of A = B

    E

    EA B

    b = A + B

    ab = A - B

    bonding

    antibondingE

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    Li2 N2 O2 Ne2

    2s

    *2s

    2s

    *2s

    2py 2pz

    2px

    *2px

    *2py *2pz

    2s2s

    2p 2p

    2px

    2py 2pz

    *2py *2pz

    *2px

    2p 2p

    2s 2s

    Energy Level Diagram

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    Energy Level Diagram of H2 Molecule

    1H = 1s1

    1H = 1s1

    Molecular Electronic Configuration = (1s)2

    No unpairedelectron

    DIAMAGNETIC

    2

    e-eBO

    gantibondin-

    bonding-

    STABLE1

    2

    02BO

    AOHAO H MO H2

    1s1s

    1s

    *1s

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    Energy Level Diagram of He2 Molecule

    2He = 1s2

    2He = 1s2

    Molecular Electronic Configuration = (1s)2

    (*1s)2

    No unpairedelectron

    DIAMAGNETIC

    2e-eBO

    gantibondin-

    bonding-

    UNSTABLE0

    2

    22BO

    1s1s

    1s

    *1s

    AOHeAO He MO He2

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    Problem 1:

    Molecule of N2 (atomic number 7)

    Sketch the energy level diagram

    Determine the magnetism

    Determine the stability of the molecule

    Write the molecular electronic

    configuration

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    Problem 2:Molecule of O2 (atomic number 8)

    Sketch the energy level diagram

    Determine the magnetism Determine the stability of the molecule

    Write the molecular electronic

    configuration

    H t l Di t i M l l

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    Heteronuclear Diatomic Molecules

    Electronegativityof A < B

    Energy of A > B

    The moreelectronegativethe atom, the

    lower theenergy.

    E

    EA

    B

    E

    b = A + B

    ab = A - B

    bonding

    antibonding

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    Problem 3

    Sketch the energy level diagram of COmolecule.

    What type energy level diagram must be

    adopted? Li2 N2 type?

    OR

    O2 Ne2 type?

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    Problem 4

    Sketch the energy level diagram of NO-ion.

    Determine the stability and magnetism ofNO- ion.

    Write down the molecular electronicconfiguration.

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    Problem 5

    Sketch the energy level diagram of OH-ion.

    Determine the stability and magnetism ofOH- ion.

    Write down the molecular electronicconfiguration.