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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.
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