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1 Hybridization and Molecular Orbital (MO) Theory Chapter 10 Historical Models •Valence bond theory (VB) - a molecule arises from interaction of complete atoms, bound together through localized overlap of valence-shell atomic orbitals which retain their original character. •Valence shell electron pair repulsion theory (VSEPR) – predicts molecular shapes based on valence electrons, lewis dot structures and electron repulsions. •Molecular orbital theory (MO) – a molecule is formed by the overlap of atomic orbitals to form molecular orbitals, electrons are then distributed into MOs. A molecule is a collection of nuclei with the orbitals delocalized over the entire molecule. •G.N.Lewis and I. Langmuir (~1920) laid out foundations •Ionic species were formed by electron transfer •Covalent molecules arise from electron sharing Two Theories of Bonding Two Theories of Bonding VALENCE BOND VALENCE BOND THEORY THEORY Linus Linus Pauling Pauling valence electrons are valence electrons are localized between atoms (or localized between atoms (or are lone pairs). are lone pairs). half half-filled atomic filled atomic orbitals orbitals overlap to form bonds. overlap to form bonds.

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Page 1: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

1

Hybridization and Molecular Orbital (MO)

Theory

Chapter 10

Historical Models

•Valence bond theory (VB) - a molecule arises from interaction of complete atoms, bound together through localized overlap of valence-shell atomic orbitals which retain their original character.

•Valence shell electron pair repulsion theory (VSEPR) – predicts molecular shapes based on valence electrons, lewis dot structures and electron repulsions.

•Molecular orbital theory (MO) – a molecule is formed by the overlap of atomic orbitals to form molecular orbitals, electrons are then distributed into MOs. A molecule is a collection of nuclei with the orbitals delocalized over the entire molecule.

•G.N.Lewis and I. Langmuir (~1920) laid out foundations•Ionic species were formed by electron transfer•Covalent molecules arise from electron sharing

Two Theories of BondingTwo Theories of Bonding

•• VALENCE BOND VALENCE BOND THEORYTHEORY —— LinusLinus PaulingPauling

•• valence electrons are valence electrons are localized between atoms (or localized between atoms (or are lone pairs).are lone pairs).

•• halfhalf--filled atomic filled atomic orbitalsorbitalsoverlap to form bonds.overlap to form bonds.

Page 2: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Valence Bond (VB) Theory

• Covalent bonds are formed by the overlapoverlap of atomic orbitals.

• Atomic orbitals on the central atom can mix and exchange their character with other atoms in a molecule.– Process is called hybridizationhybridization.

� Hybrid Orbitals have the same shapes as predicted by VSEPR.

Valence Bond (VB) Theory

sp3d2Octahedral6

sp3dTrigonal

bipyramidal

5

sp3Tetrahedral4

sp2Trigonal

planar

3

spLinear2

HybridizationElectronic

Geometry

Regions of High

Electron Density

Molecular Shapes and BondingMolecular Shapes and Bonding

• In the next sections we will use the following terminology:A = central atom

B = bonding pairs around central atom

U = lone pairs around central atom

• For example:AB3U designates that there are 3 bonding pairs and 1

lone pair around the central atom.

Page 3: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Sigma Bond Formation by Sigma Bond Formation by Orbital OverlapOrbital Overlap

Two s Two s orbitalsorbitals

overlapoverlap

Sigma Bond FormationSigma Bond FormationSigma Bond Formation

Two s Two s

orbitalsorbitals

overlapoverlap

Two p Two p

orbitalsorbitals

overlapoverlap

Linear Electronic Geometry:AB2

Species (No Lone Pairs of Electrons on A)

• Some examples of molecules with this geometry are: BeCl

2, BeBr

2, BeI

2, HgCl

2, CdCl

2

• All of these examples are linear, nonpolarmolecules.

• Important exceptions occur when the two substituents are not the same!BeClBr or BeIBr will be linear and polar!

Page 4: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Linear Electronic Geometry:AB2

Species (No Lone Pairs of Electrons on A)

Trigonal Planar Electronic Geometry: AB3 Species (No

Lone Pairs of Electrons on A)• Some examples of molecules with this geometry

are: BF3, BCl3

• All of these examples are trigonal planar, nonpolarmolecules.

• Important exceptions occur when the three substituents are not the same!BF2Cl or BCI2Br will be trigonal planar and polar!

Using VB TheoryUsing VB TheoryBonding in BFBonding in BF33

planar triangleplanar triangle

angle = 120angle = 120oo

F

F F

Boron configuration

↑↑↑↑↑↑↑↑↓↓↓↓↑↑↑↑↓↓↓↓

2p2s1s•• ••

••••

••

•• ••

••••

B

Page 5: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Bonding in BF3Bonding in BFBonding in BF33

•• How to account for 3 bonds 120How to account for 3 bonds 120oo apart using a apart using a spherical s orbital and p spherical s orbital and p orbitalsorbitals that are 90that are 90oo apart?apart?

•• Pauling said to modify VB approach with Pauling said to modify VB approach with ORBITAL ORBITAL HYBRIDIZATIONHYBRIDIZATION

•• —— mix available mix available orbitalsorbitals to form a new set of to form a new set of orbitalsorbitals—— HYBRID ORBITALSHYBRID ORBITALS —— that will give the that will give the maximum overlap in the correct geometry. maximum overlap in the correct geometry.

Bonding in BFBonding in BF33

rearrange electronshydridize orbs.

unused porbital

three sp2

hybrid orbitals

2p2s

•• The three hybrid The three hybrid orbitalsorbitals are made are made

from 1 s orbital and 2 p from 1 s orbital and 2 p orbitalsorbitals →→→→→→→→ 3 sp3 sp22

hybrids.hybrids.

Bonding in BF3Bonding in BFBonding in BF33

•• Now we have 3, halfNow we have 3, half--filled HYBRID filled HYBRID orbitalsorbitals

that can be used to form Bthat can be used to form B--F sigma bonds.F sigma bonds.

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Trigonal Planar Electronic Geometry: AB3 Species (No Lone

Pairs of Electrons on A)

BFBF33, Planar , Planar TrigonalTrigonal

Tetrahedral Electronic Geometry: AB

4Species (No Lone Pairs of

Electrons on A)

• Some examples of molecules with this geometry are: CH

4, CF

4, CCl

4, SiH

4, SiF

4

• All of these examples are tetrahedral, nonpolarmolecules.

• Important exceptions occur when the four substituents are not the same!CF3Cl or CH2CI2 will be tetrahedral and polar!

Page 7: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of

Electrons on A)

Bonding in CHBonding in CH44How do we account for 4 How do we account for 4

CC——H sigma bonds 109H sigma bonds 109oo

apart? apart?

Need to use 4 atomic Need to use 4 atomic orbitalsorbitals

—— s, s, ppxx, , ppyy, and , and ppzz —— to to

form 4 new hybrid form 4 new hybrid orbitalsorbitals

pointing in the correct pointing in the correct

direction.direction.

109o109o

4 C atom orbitals

hybridize to form

four equivalent sp3

hybrid atomic

orbitals.

4 C atom 4 C atom orbitalsorbitals

hybridize to form hybridize to form

four equivalent spfour equivalent sp33

hybrid atomic hybrid atomic

orbitalsorbitals..

Bonding in a Tetrahedron Bonding in a Tetrahedron ——Formation of Hybrid Atomic Formation of Hybrid Atomic

OrbitalsOrbitals

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Tetrahedral Electronic Geometry: AB

4Species (No Lone Pairs of

Electrons on A)

Bonding in CHBonding in CH44

Figure 10.6Figure 10.6

Tetrahedral Electronic Geometry: AB4

Species (No Lone Pairs of Electrons on A)

Page 9: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of

Electrons on A)• Some examples of molecules with this geometry

are: NH3, NF3, PH3, PCl3, AsH3

• These molecules are our first examples of central atoms with lone pairs of electrons.Thus, the electronic and molecular geometries are

different.

All three substituents are the same but molecule is polarpolar.

• NH3 and NF3 are trigonal pyramidal, polar molecules.

Steps in predicting the hybrid orbitals used by an atom in bonding:

1. Draw the Lewis structure

2. Determine the electron pair geometry using the VSEPR model

3. Specify the hybrid orbitals needed to accommodate the electron pairs in the

geometric arrangement

NH3

1. Lewis structure

2. VSEPR indicates tetrahedral geometry

with one non-bonding pair of electrons

(structure itself will be trigonal pyramidal)

3. Tetrahedral arrangement indicates four

equivalent electron orbitals

Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of

Electrons on A)

• Some examples of molecules with this geometry are: H2O, OF2, H2S

• These molecules are our first examples of central atoms with two lone pairs of electrons.Thus, the electronic and molecular geometries are different.

Both substituents are the same but molecule is polarpolar.

• Molecules are angular, bent, or V-shaped and polar.

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Orbital HybridizationFigure 10.5

Orbital HybridizationOrbital HybridizationFigure 10.5Figure 10.5

BONDSBONDS SHAPESHAPE HYBRID REMAINHYBRID REMAIN

22 linearlinear spsp 2 2 pp’’ss

33 trigonaltrigonal spsp22 1 p1 pplanarplanar

44 tetrahedral sptetrahedral sp33 nonenone

Compounds Containing Double Bonds

Valence Bond Theory (Hybridization)

C atom has four electrons.

Three electrons from each C atom are in sp2

hybrids.

One electron in each C atom remains in an unhybridized p orbital

2s 2p three sp2 hybrids 2p

C ↑↓ ↑ ↑ ⇒ ↑ ↑ ↑ ↑

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Compounds Containing Double Bonds

• The single 2p orbital is perpendicular to the trigonal planar sp2 lobes.The fourth electron is in the p orbital.

Side view of sp2 hybrid

with p orbital included.

Compounds Containing Double Bonds

• An sp2 hybridized C atom has this shape.Remember there will be one electron in each of the three

lobes.

Top view of

an sp2 hybrid

Compounds Containing Double Bonds

• The portion of the double bond formed from the head-on overlap of the sp2 hybrids is designated as a σ bond.

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Sometimes it is not necessary for all the valence electron orbitals to hybridize. For

example, ethylene has the following structure:

The bonds between C and H

are all sigma bonds between

sp2 hybridized C atoms and

the s-orbitals of Hydrogen.

The double bond between the

two C atoms consists of a

sigma bond (where the

electron pair is located

between the atoms) and a pi

bond (where the electron pair

occupies the space above

and below the sigma bond.

σσ and and ππ Bonding inBonding in CHCH22OO

Compounds Containing Triple Bonds

• Ethyne or acetylene, C2H2, is the simplest triple bond containing organic compound.

• Compound must have a triple bond to obey octet rule.

Page 13: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Compounds Containing Triple Bonds

Lewis Dot Formula

C C HHCH HC··

······

·· orCH HC·

·······

··

VSEPR Theory suggests regions of high

electron density are 180o apart.

Compounds Containing Triple Bonds

Valence Bond Theory (Hybridization)

Carbon has 4 electrons.

Two of the electrons are in sp hybrids.

Two electrons remain in unhybridized p orbitals.

2s 2p two sp hybrids 2p

C [He] ↑↓ ↑ ↑ ⇒ ↑ ↑ ↑ ↑

σσ and and ππ Bonding inBonding in CC22HH22

Figure 10.12Figure 10.12

Page 14: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Compounds Containing Triple Bonds

A σ bond results from the head-on overlap of two sp hybrid orbitals.

Compounds Containing Triple Bonds

• The unhybridized p orbitals form two π bonds.� Note that a triple bond consists of one σ and

two π bonds.

Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2,

and AB2U3� Some examples of molecules with this geometry

are: PF5, AsF5, PCl5, etc.

• These molecules are examples of central atoms with five bonding pairs of electrons.The electronic and molecular geometries are the same.

• Molecules are trigonal bipyramidal and nonpolarwhen all five substituents are the same.If the five substituents are not the same polarpolar molecules

can result, AsF4Cl is an example.

Page 15: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2,

and AB2U3Valence Bond Theory (Hybridization)

4s 4p 4d

As [Ar] 3d10 ↑↓ ↑ ↑ ↑ ___ ___ ___ ___ ___

⇓five sp3 d hybrids 4d

↑ ↑ ↑ ↑ ↑ ___ ___ ___ ___ ___

Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and

AB2U3

• If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new shapes.

1. One lone pair - Seesaw shape

2. Two lone pairs - T-shape

3. Three lone pairs – linear

• The lone pairs occupy equatorial positions because they are 120o

from two bonding pairs and 90o from the other two bonding pairs.

– Results in decreased repulsions compared to lone pair in axial position.

Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2,

and AB2U3

• AB4U molecules have:1. trigonal bipyramid electronic geometry

2. seesaw shaped molecular geometry

3. and are polar

• One example of an AB4U molecule is

SF4

• Hybridization of S atom is sp3d.

Page 16: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2,

and AB2U3

Molecular Geometry

H

C

HH

H

Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2,

and AB2U3• AB3U2 molecules have:

1. trigonal bipyramid electronic geometry

2. T-shaped molecular geometry

3. and are polar

• One example of an AB3U2 molecule is

IF3

• Hybridization of I atom is sp3d.

Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2,

and AB2U3Molecular Geometry

H

C

HH

H

Page 17: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2,

and AB2U3• AB2U3 molecules have:

1.trigonal bipyramid electronic geometry

2.linear molecular geometry

3.and are nonpolar

• One example of an AB3U2 molecule isXeF2

• Hybridization of Xe atom is sp3d.

Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2,

and AB2U3Molecular Geometry

H

C

HH

H

Octahedral Electronic Geometry: AB6, AB5U, and AB4U2

• AB5U molecules have:1.octahedral electronic geometry

2.Square pyramidal molecular geometry

3.and are polar.

• One example of an AB4U molecule is

IF5

• Hybridization of I atom is sp3d2.

Page 18: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Octahedral Electronic Geometry: AB6, AB5U, and AB4U2

Molecular Geometry

H

C

HH

H

Octahedral Electronic Geometry: AB6, AB5U, and AB4U2

• AB4U2 molecules have:1.octahedral electronic geometry

2.square planar molecular geometry

3.and are nonpolar.

• One example of an AB4U2 molecule is

XeF4

• Hybridization of Xe atom is sp3d2.

Octahedral Electronic Geometry: AB6, AB5U, and AB4U2

Molecular Geometry Polarity

H

C

HH

H

Page 19: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Summary of Electronic & Molecular Geometries

•• MOLECULAR MOLECULAR ORBITAL THEORYORBITAL THEORY ——Robert Mullikan (1896Robert Mullikan (1896--1986)1986)

•• valence electrons are valence electrons are delocalizeddelocalized

•• valence electrons are in valence electrons are in orbitalsorbitals (called molecular (called molecular orbitalsorbitals) spread over ) spread over entire molecule.entire molecule.

Two Theories of BondingTwo Theories of Bonding

Review of Atomic Orbitals - s, p and d

Page 20: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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The Need for MO

�VSEPR and VB theory are good to explain the molecular shape.

�BUT they did not explain the magnetic or spectral properties of molecules.

�Molecular orbital theory is needed.

Homonuclear Diatomic Molecules: Molecular Orbital (MO) Theory

�MOs are derived from a linear combination (addition and subtraction) of atomic orbitals represented as wavefunctions of nearby atoms to form molecular orbitals.

•There are two possible combinations

•Adding two atomic orbitals forms a bonding MO.

•Subtracting two atomic orbitals forms an antibondingMO.

•Basic Tenant –•The number of atomic orbitals contributed equals the number of molecular orbitals generated.

Electron Wave Functions – Wave-Particle DualityLinear Combination of Wavefunctions – Ψ

Ψ(1) + Ψ (2)

Ψ(1) + Ψ (2)

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If we look at H2, we see that each hydrogen atom has a 1s atomic orbital

that is half-filled. Remembering that orbitals are mathematical functions,

they can combine by adding or subtracting to give two new combinations

which we call molecular orbitals.

Homonuclear Diatomic MoleculesMolecular Orbital TheoryIn Phase / Out of Phase Overlap

σσσσ

HHHHaaaaHHHHbbbb

σσσσ****

Ψ(1) + Ψ (2)

Ψ(1) − Ψ (2)

The energy of the H2 molecule with the two electrons in the bonding

orbital is lower by 435 kJ/mole than the combined energy of the two

separate H-atoms.

On the other hand, the energy of the H2 molecule with two electrons in

the antibonding orbital is higher than two separate H-atoms. To show the

relative energies we use diagrams like this:

Page 22: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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Homonuclear Diatomic Molecules: Molecular Orbital Theory

σ label implies rotation of MO about internuclearaxis (z axis) generates no phase change

*label implies a nodal plane between the nuclei which is orthogonal to the z axis

π label implies rotation of orbital about internuclear axis generates a phase change

In the H2 molecule, the bonding and anti-bonding orbitals are

called sigma orbitals ( σ )

Sigma Orbital: A bonding molecular orbital with cylindrical symmetry about

an internuclear axis.

When atomic orbitals are combined to give molecular orbitals, the

number of molecular orbitals formed equals the number of atomic orbitals

used.

A molecular orbital (like an atomic orbital) can contain no more than two

electrons (Pauli Exclusion Principle), and are filled starting with the

lowest energy orbital first.

In general, the energy difference between a bonding and anti-bonding

orbital pair becomes larger as the overlap of the atomic orbitals increase.

Example: H2 molecule

Each hydrogen atom contributes one electron. These go in the bonding

molecular orbital because we fill the lowest energy orbital first.

Electrons fill MOs by standard rules - aufbau, pauli, etc.

Page 23: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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σ*1s

σ1s

Bond Order / Electron Configurationfor H2 Molecule

φΗ1 φΗ1

Ψbσ1s

Ψaσ∗1s -Bond Order (B.O.)B.O. = 1/2 (Nb - Na)Nb = bonding electronsNa = antibonding electrons

-Molecular electron configurations - analogous to atomic configurations

- H2 = σ21s

Example: He2 molecule

Not observed because there is no energy benefit to bonding these two atoms

together.

σ*1s

σ1s

Bond Order / Electron Configurationfor He2 Molecule

φΗ1 φΗ1

Ψbσ1s

Ψaσ∗1s -Bond Order (B.O.)B.O. = 1/2 (Nb - Na)Nb = bonding electronsNa = antibonding electrons

-Molecular electron configurations - analogous to atomic configurations

- H2 = σ21s σ∗21s

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MO of He2

+

σ*1s

σ1s

Energy

He2+ bond order = ??

MO Diagram for He2+ and H2

-

MO of H2

-

H2- bond order = ??

AO of He

AO of He+

AO of

H-

AO of H

σ*1s

σ1s

Summary Data for First Row Homo - Diatomics

----022He2

2301.08½12He2+

4580.74102H2

2691.06½01H2+

Bond Energy (kJ/mol)

Bond length (Å)

Bond Order

Antibond. e-

Bonding e-Molecule

σ*1s

σ1s

Orbital Interaction for Li2 MoleculeLi atom - 1s22s1

σ2s

σ*2s

1s

2s Bond order for Li2?

Molecular electronconfiguration?

Be2?

Li2+?

Page 25: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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σ*1s

σ1s

Orbital Interaction for Li2 MoleculeLi atom - 1s22s1

σ2s

σ*2s

1s

2s Bond order for Li2 = ½(4-2) = 1σ21s σ∗21sσ22s

Be2 = ½(4-4) = 0σ21s σ∗21sσ22s σ∗22s

Li2+ = ½(3-2) = ½

σ21s σ∗21sσ12s

MO of He2

+

σ*1s

σ1s

AO of He+

1s

Energy

He2+ bond order = 1/2

AO of He

1s

MO Diagram for He2+ and H2

-

MO of H2

-

σ*1s

σ1s

AO of H

1s

H2- bond order = 1/2

AO of H-

1s

We can also form bonding orbitals using other atomic orbitals. For

example, we can combine two p orbitals to form a sigma bond:

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Using p orbitals a second type of orbital called a π orbital can also be formed. These exist above and below the internuclear axis. We see πbonds used for the second bond of a double bond or the second and

third of a triple bond. π bonds limit rotation of the atoms in space.

Relative MO Energy Levels for Period 2

Homonuclear Diatomic Molecules

MO energy levels for

O2, F2, and Ne2

MO energy levels for

B2, C2, and N2

No 2s-2p repulsion Effect of 2s-2p

repulsion

Homonuclear Diatomic Molecules Molecular Orbital Theory - p Orbital Set

Page 27: Hybridization and Molecular Orbital (MO) Theorychemphys.armstrong.edu/nivens/GeneralChemistry/Chapter10kotz.pdfHybridization and Molecular Orbital (MO) Theory Chapter 10 Historical

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O2 molecule is an example

with sigma and pi bonds

forming between atoms. MO

theory predicts that oxygen

will be paramagnetic.

Molecular Oxygen (O2)

Using the following MO Diagram

σ21s σ∗21sσ22s σ∗22sπ42p π∗22p

BO = ½(8-4)= 2

Orbital Energies for Second Row Homodiatomics

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VBT describes O2 as a double bond (O=O), however experimentindicates the molecule is paramagnetic.

MOT describes the bonding and accounts for the paramagnetism.

Paramagnetic= > 1 unpaired electronDiamagnetic = 0 unpaired electrons

Experimental Data for Homodinuclear Diatomics Li to F

D11591.41F2

P24981.21O2

D39451.10N2

D26071.24C2

P12971.59B2

--0----Be2

D11102.67Li2

Magnetic Info

Bond Order

Bond Diss. Enthalpy (kJ/mol)

Bond Length (Å)

Diatomic

Energy

MO of

HF

AO of H

1s

σ

2px 2py

σ∗

AO of F

2p

Two non-bonding orbitals

are the lone pairs on F

seen in The Lewis structure

for HF

Note the H1Sis less stable

than the F2P

The MO Diagram for HF

Note: 2s non-bondingorbital (F) not shown

Energy

The MO Diagram

for NO

2s

AO’s of

N

2p

σσσσ*2s

σσσσ2s

2sAO’s of

O

2p

ππππ2pxy

σσσσ2pz

ππππ*2pxy

σσσσ*2pz

PARAMAGNETIC

1 unpaired e-

Note AO’s of the more

electronegative O are

More stable than those

of N

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Heteronuclear Diatomic Molecules - CO

Homonuclear Diatomic Molecules Review of Bonding Types

sigma - σ

pi - π delta - δ