1 chapter 9 orbitals and covalent bond. 2 molecular orbitals n the overlap of atomic orbitals from...

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Chapter 9Chapter 9

Orbitals and Covalent BondOrbitals and Covalent Bond

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Molecular OrbitalsMolecular Orbitals The overlap of atomic orbitals from The overlap of atomic orbitals from

separate atoms makes molecular separate atoms makes molecular orbitalsorbitals

Each molecular orbital has room for two Each molecular orbital has room for two electronselectrons

Two types of MOTwo types of MO

– Sigma ( Sigma ( ) between atoms ) between atoms

– Pi ( Pi ( ) above and below atoms ) above and below atoms

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Sigma bonding orbitals Sigma bonding orbitals From s orbitals on separate atomsFrom s orbitals on separate atoms

+ +

s orbital s orbital

+ ++ +

Sigma bondingmolecular orbital

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Sigma bonding orbitals Sigma bonding orbitals From p orbitals on separate atomsFrom p orbitals on separate atoms

p orbital p orbital

Sigma bondingmolecular orbital

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Pi bonding orbitalsPi bonding orbitals p orbitals on separate atomsp orbitals on separate atoms

Pi bondingmolecular orbital

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Sigma and pi bondsSigma and pi bonds All single bonds are sigma bondsAll single bonds are sigma bonds A double bond is one sigma and one pi A double bond is one sigma and one pi

bondbond A triple bond is one sigma and two pi A triple bond is one sigma and two pi

bonds.bonds.

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Atomic Orbitals Don’t WorkAtomic Orbitals Don’t Work to explain molecular geometry.to explain molecular geometry. In methane, CHIn methane, CH44 , the shape s , the shape s

tetrahedral.tetrahedral. The valence electrons of carbon should The valence electrons of carbon should

be two in s, and two in p.be two in s, and two in p. the p orbitals would have to be at right the p orbitals would have to be at right

angles.angles. The atomic orbitals change when The atomic orbitals change when

making a molecule making a molecule

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HybridizationHybridization We blend the s and p orbitals of the We blend the s and p orbitals of the

valence electrons and end up with the valence electrons and end up with the tetrahedral geometry.tetrahedral geometry.

We combine one s orbital and 3 p We combine one s orbital and 3 p orbitals.orbitals.

spsp33 hybridization has tetrahedral hybridization has tetrahedral geometry.geometry.

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In terms of energyIn terms of energyIn terms of energyIn terms of energyE

nerg

y

2p

2s

Hybridization sp3

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How we get to hybridizationHow we get to hybridization We know the geometry from experiment.We know the geometry from experiment. We know the orbitals of the atomWe know the orbitals of the atom hybridizing atomic orbitals can explain hybridizing atomic orbitals can explain

the geometry.the geometry. So if the geometry requires a tetrahedral So if the geometry requires a tetrahedral

shape, it is spshape, it is sp33 hybridized hybridized This includes bent and trigonal pyramidal This includes bent and trigonal pyramidal

molecules because one of the spmolecules because one of the sp33 lobes lobes holds the lone pair.holds the lone pair.

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spsp22 hybridization hybridization CC22HH44

Double bond acts as one pair.Double bond acts as one pair. trigonal planartrigonal planar Have to end up with three blended Have to end up with three blended

orbitals.orbitals. Use one s and two p orbitals to make Use one s and two p orbitals to make

spsp22 orbitals. orbitals. Leaves one p orbital perpendicular.Leaves one p orbital perpendicular.

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nerg

y

2p

2s

sp2Hybridization

2p

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Where is the P orbital?Where is the P orbital? PerpendicularPerpendicular The overlap of The overlap of

orbitals makes a orbitals makes a sigma bond (sigma bond ( bond)bond)

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Two types of BondsTwo types of Bonds Sigma bonds from overlap of orbitals.Sigma bonds from overlap of orbitals. Between the atoms.Between the atoms. Pi bond (Pi bond ( bond) above and below atoms bond) above and below atoms Between adjacent p orbitals.Between adjacent p orbitals. The two bonds of a The two bonds of a

double bond.double bond.

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CCH

H

H

H

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spsp22 hybridization hybridization When three things come off atom.When three things come off atom. trigonal planartrigonal planar 120º120º One One bond, bond, lp =3 lp =3

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What about twoWhat about two When two things come off.When two things come off. One s and one p hybridize.One s and one p hybridize. linearlinear

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sp hybridizationsp hybridization End up with two lobes 180º End up with two lobes 180º

apart.apart. p orbitals are at right p orbitals are at right

anglesangles Makes room for two Makes room for two

bonds and two sigma bonds and two sigma bonds.bonds.

A triple bond or two double A triple bond or two double bonds.bonds.

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nerg

y

2p

2s

Hybridizationsp

2p

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COCO22

C can make two C can make two and two and two O can make one O can make one and one and one

CCOO OO

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NN22

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NN22

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Breaking the octetBreaking the octet PClPCl55 The model predicts that we must use The model predicts that we must use

the d orbitals.the d orbitals. dspdsp33 hybridization hybridization There is some controversy about how There is some controversy about how

involved the d orbitals are.involved the d orbitals are.

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dspdsp33

Trigonal Trigonal bipyrimidalbipyrimidal

can only can only bond. bond. can’t can’t bond. bond. basic shape for basic shape for

five things.five things.

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PClPCl55

Can’t tell the hybridization of Cl

Assume sp3 to minimize repulsion of electron pairs.

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dd22spsp33

gets us to six things gets us to six things aroundaround

OctahedralOctahedral Only Only σσ bond bond

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Molecular Orbital ModelMolecular Orbital Model Localized Model we have learned explains Localized Model we have learned explains

much about bonding.much about bonding. It doesn’t deal well with the ideal of It doesn’t deal well with the ideal of

resonance, unpaired electrons, and bond resonance, unpaired electrons, and bond energy.energy.

The MO model is a parallel of the atomic The MO model is a parallel of the atomic orbital, using quantum mechanics.orbital, using quantum mechanics.

Each MO can hold two electrons with Each MO can hold two electrons with opposite spinsopposite spins

Square of wave function tells probabilitySquare of wave function tells probability

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What do you get?What do you get? Solve the equations for HSolve the equations for H22

HHA A H HBB

get two orbitalsget two orbitals

MOMO22 = 1s = 1sAA - 1s - 1sBB

MOMO11 = 1s = 1sAA + 1s + 1sBB

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The Molecular Orbital ModelThe Molecular Orbital Model• The molecular orbitals are centered on The molecular orbitals are centered on

a line through the nucleia line through the nuclei

– MOMO11 the greatest probability is the greatest probability is

between the nucleibetween the nuclei

– MOMO22 it is on either side of the nuclei it is on either side of the nuclei

– this shape is called a sigma molecular this shape is called a sigma molecular orbitalorbital

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The Molecular Orbital ModelThe Molecular Orbital Model• In the molecule only the molecular orbitals In the molecule only the molecular orbitals

exist, the atomic orbitals are goneexist, the atomic orbitals are gone

• MOMO11 is lower in energy than the 1s orbitals is lower in energy than the 1s orbitals they came from.they came from.– This favors molecule formationThis favors molecule formation– Called an bonding orbitalCalled an bonding orbital

• MOMO22 is higher in energy is higher in energy– This goes against bondingThis goes against bonding– antibonding orbital antibonding orbital

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The Molecular Orbital ModelThe Molecular Orbital ModelE

nerg

y

MO2

MO1

ssss

H2

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The Molecular Orbital ModelThe Molecular Orbital Model• We use labels to indicate shapes, and We use labels to indicate shapes, and

whether the MO’s are bonding or whether the MO’s are bonding or antibonding.antibonding.

– MOMO1 1 = = 1s1s

– MOMO2 2 = = 1s1s* (* indicates antibonding)* (* indicates antibonding)

• Can write them the same way as atomic Can write them the same way as atomic orbitalsorbitals

– HH22 = = 1s1s22

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The Molecular Orbital ModelThe Molecular Orbital Model• Each MO can hold two electrons, but Each MO can hold two electrons, but

they must have opposite spinsthey must have opposite spins

• Orbitals are conserved. Orbitals are conserved.

• The number of molecular orbitals The number of molecular orbitals must equal the number atomic must equal the number atomic orbitals that are used to make them.orbitals that are used to make them.

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HH22--

Ene

rgy

1s1s

1s1s**

ssss

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Bond OrderBond Order The difference between the number of The difference between the number of

bonding electrons and the number of bonding electrons and the number of antibonding electrons divided by twoantibonding electrons divided by two

Bond Order = # bonding-#antibonding

2

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Only outer orbitals bondOnly outer orbitals bond The 1s orbital is much smaller than the The 1s orbital is much smaller than the

2s orbital2s orbital When only the 2s orbitalsWhen only the 2s orbitals are are

involved in bondinginvolved in bonding Don’t use the Don’t use the 1s1s or or 1s1s** for Lifor Li22

LiLi22 = ( = (2s2s))22

In order to participate in bonds the In order to participate in bonds the orbitals must overlap in space.orbitals must overlap in space.

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Bonding in Homonuclear Diatomic Bonding in Homonuclear Diatomic MoleculesMolecules

Need to use Need to use HomonuclearHomonuclear so that we know the so that we know the relative energies.relative energies.

LiLi22--

((2s2s))2 2 ((2s2s*)*)11

BeBe22

((2s2s))2 2 ((2s2s*)*)22

What about the p orbitals? How do they form What about the p orbitals? How do they form orbitals? orbitals?

Remember that orbitals must be conserved. Remember that orbitals must be conserved.

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BB22

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BB22

2p*

2p

2p*

2p

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Expected Energy DiagramExpected Energy DiagramE

nerg

y

2s 2s

2p2p

2s

2p*

2p

2s*

2p* 2p*2p2p

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BB22E

nerg

y

2s 2s

2p2p

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BB22

((2s2s))22((2s2s*)*)2 2 ((2p2p))22

Bond order = (4-2) / 2Bond order = (4-2) / 2 Should be stable.Should be stable. This assumes there is no interaction This assumes there is no interaction

between the s and p orbitals.between the s and p orbitals. Hard to believe since they overlapHard to believe since they overlap proof comes from magnetism.proof comes from magnetism.

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MagnetismMagnetism Magnetism has to do with electrons.Magnetism has to do with electrons. Paramagnetism attracted by a magnet.Paramagnetism attracted by a magnet.

– associated with unpaired electrons.associated with unpaired electrons. Diamagnetism attracted by a magnet.Diamagnetism attracted by a magnet.

– associated with paired electrons.associated with paired electrons. BB22 is paramagnetic. is paramagnetic.

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MagnetismMagnetism The energies of of the The energies of of the 2p2p and the and the 2p2p

are reversed by p and s interactingare reversed by p and s interacting The The 2s2s and the and the 2s2s* are no longer * are no longer

equally spaced.equally spaced. Here’s what it looks like.Here’s what it looks like.

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Correct energy diagramCorrect energy diagram

2s 2s

2p2p

2s

2p*

2p

2s*

2p* 2p*

2p2p

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BB22

2s 2s

2p2p

2s

2p*

2p

2s*

2p*

2p

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PatternsPatterns As bond order increases, bond energy As bond order increases, bond energy

increases.increases. As bond order increases, bond length As bond order increases, bond length

decreases.decreases. Supports basis of MO model.Supports basis of MO model. There is not a direct correlation of bond There is not a direct correlation of bond

order to bond energy.order to bond energy. OO22 is known to be paramagnetic. is known to be paramagnetic. Movie. .

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MagnetismMagnetism Ferromagnetic strongly attractedFerromagnetic strongly attracted Paramagnetic weakly attractedParamagnetic weakly attracted

– Liquid Oxygen Diamagnetic weakly repelledDiamagnetic weakly repelled

– Graphite

– Water Frog Frog

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ExamplesExamples CC22

NN22

OO22

FF22

PP22

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Heteronuclear Diatomic SpeciesHeteronuclear Diatomic Species Simple type has them in the same Simple type has them in the same

energy level, so can use the orbitals we energy level, so can use the orbitals we already know.already know.

Slight energy differences.Slight energy differences. NONO

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NONO

2s2s

2p2p

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You tryYou try NONO++

CNCN--

What if they come from completely What if they come from completely different orbitals and energy?different orbitals and energy?

HFHF Simplify first by assuming that F only Simplify first by assuming that F only

uses one if its 2p orbitals.uses one if its 2p orbitals. F holds onto its electrons, so they have F holds onto its electrons, so they have

low energylow energy

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1s

2p

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ConsequencesConsequences ParamagneticParamagnetic Since 2p is lower in energy, favored by Since 2p is lower in energy, favored by

electrons.electrons. Electrons spend time closer to fluorine.Electrons spend time closer to fluorine. Compatible with polarity and Compatible with polarity and

electronegativity. electronegativity.

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NamesNames sp orbitals are called the sp orbitals are called the Localized Localized

electron modelelectron model and and olecular orbital modelolecular orbital model Localized is good for geometry, doesn’t Localized is good for geometry, doesn’t

deal well with resonance.deal well with resonance. seeing seeing bonds as localized works well bonds as localized works well It is the It is the bonds in the resonance bonds in the resonance

structures that can move. structures that can move.

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delocalized bondingdelocalized bonding CC66HH66

H

H

H

HH

H

H

H

H

HH

H

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CC22HH66

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NONO33--

1 chapter 9 orbitals and covalent bond. 2 molecular orbitals n the overlap of atomic orbitals from...

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