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Chapter 9Chapter 9

Orbitals and Covalent BondsOrbitals and Covalent Bonds pp pp

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9.1 Atomic Orbitals Don’t Work9.1 Atomic Orbitals Don’t Work

They don’t explain They don’t explain molecularmolecular geometry. geometry. In methane, CHIn methane, CH44 , the shape , the shape isis

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

be two in be two in ss, and two in , and two in pp ( (ifif atomicatomic orbitals “worked.”orbitals “worked.”

If so, the If so, the pp orbitals would have to be at orbitals would have to be at right anglesright angles. . ButBut bond angles are 109.5 bond angles are 109.5oo

So, the atomic orbitals So, the atomic orbitals changechange when when making a moleculemaking a molecule

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HybridizationHybridization We must We must blendblend the the ss and and pp orbitals of orbitals of

the valence electrons to get 109.5º the valence electrons to get 109.5º anglesangles

We end up with the tetrahedral We end up with the tetrahedral geometry.geometry.

We We combinecombine one one ss orbital and 3 orbital and 3 pp orbitals to get sporbitals to get sp33 molecularmolecular orbitals. orbitals.

spsp33 hybridization has hybridization has tetrahedraltetrahedral geometry.geometry.

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Figure 9.3 p. 417Figure 9.3 p. 417

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Fig. 9.3 p. 417 - TetrahedralFig. 9.3 p. 417 - Tetrahedral

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Figure 9.3 p. 417Figure 9.3 p. 417The Formation of The Formation of spsp33 Hybrid Orbitals Hybrid Orbitals

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In terms of energy In terms of energy (LD 2:6.65)(LD 2:6.65)In terms of energy In terms of energy (LD 2:6.65)(LD 2:6.65)E

nerg

y

2p

2s

Hybridization sp3

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

the geometry. So, if the geometry requires a tetrahedral

shape, it is sp3 hybridized. This includes bent and trigonal pyramidal

molecules because one or more of the sp3 lobes holds the lone pair(s).

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sp3 geometry

109.5º

This leads to tetrahedral shape.

Every molecule with a total of 4 things coming off the central atom is sp3 hybridized.

Gives us trigonal pyramidal and bent linear shapes also.

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sp2 hybridization When three effective things come off

atom (either a bonding pair or non-bonding pair)

Electron arrangement is (gumdrops) . . . Trigonal planar. Angles are . . . 120º One and one bond (from the extra p

orbital)

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sp2 hybridization C2H4 (you draw Lewis structure . . .)

double bond acts as one “effective” pair Molecular geometry is . . . So, trigonal planar. Have to end up with three blended

(hybridized) orbitals. use one s and two p orbitals to make

sp2 orbitals. leaves one p orbital perpendicular

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Figure 9.8, p. 419Figure 9.8, p. 419

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Fig. 9.8 p. 419Fig. 9.8 p. 419

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Figure 9.8Figure 9.8The Hybridization of the The Hybridization of the ss, , ppxx, and , and ppyy

Atomic OrbitalsAtomic Orbitals

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In terms of energy In terms of energy (fig. 9.9 p. 419)(fig. 9.9 p. 419)In terms of energy In terms of energy (fig. 9.9 p. 419)(fig. 9.9 p. 419)E

nerg

y

2p

2s

sp2Hybridization

2p

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Where is the P orbital?Where is the P orbital? Perpendicular.Perpendicular. The The overlapoverlap of sp of sp22

orbitals makes a orbitals makes a sigmasigma bond ( bond ( bond).bond).

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Two types of Bonds Sigma () bonds from overlap of orbitals They are between the atoms Pi bond ( bond) above and below atoms and Between adjacent p orbitals. A bond makes the first and a bond makes the

second bond of a double bond

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CCH

H

H

H

Fig. 9.13, p. 420Fig. 9.13, p. 420

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What about two When two things come off one s and one p hybridize (leaving two

p orbitals). Get sp hybridization linear

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sp hybridization End up with two lobes 180º

apart. The two remaining p

orbitals are at right angles Makes room for two

bonds. So, get a triple bond or two

double bonds.

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Summary of sigma & pi bonds pp

Single bond - composed of one comprised of overlapping orbitals

Double bond - composed of one and oneis between p orbitals)

Triple bond - composed of one and twobonds.

Know this for the online homework!

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

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 The model predictspredicts that we must use that we must use

the the dd orbitals (ran out of orbitals (ran out of ss and and pp).). Get dspGet dsp33 hybridization hybridization There is some controversy about how There is some controversy about how

involved the involved the dd orbitals are. orbitals are.

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dspdsp33

Trigonal Trigonal bipyrimidalbipyrimidal

can only can only bond. bond. can’t can’t bond bond (no (no

unused unused pp orbitals). orbitals). The basic shape The basic shape

for five “things.”for five “things.”

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PClPCl55

Can’t tell the hybridization of Cl

Assume it’s sp3 to minimize repulsion of electron pairs.

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dd22spsp33

Gets us to six things Gets us to six things around the central atom.around the central atom.

Octahedral.Octahedral.

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Localized e- model: SummaryLocalized e- model: Summary pppp

Draw the Lewis structure(s) (resonance)Draw the Lewis structure(s) (resonance) Determine arrangement of eDetermine arrangement of e-- pairs using pairs using

the VSEPR model.the VSEPR model. Specify hybrid Specify hybrid molecularmolecular orbitals need orbitals need

to accommodate the eto accommodate the e-- pairs. pairs. Do the steps in this order!Do the steps in this order! http://www.bluffton.edu/~bergerd/classes/CEM222/Handouts/spanimatihttp://www.bluffton.edu/~bergerd/classes/CEM222/Handouts/spanimati

on.htmlon.html

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p. 426 Figure 9.24p. 426 Figure 9.24The Relationship of the The Relationship of the Number of Effective Number of Effective Pairs, Their Spatial Pairs, Their Spatial Arrangement, and the Arrangement, and the Hybrid Orbital Set Hybrid Orbital Set Required (end 9.1)Required (end 9.1)

Effective pairs are also known Effective pairs are also known as as RHEDRHED (Regions of High (Regions of High Electron Density) - use for Electron Density) - use for online HWonline HW

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(Partial) 9.2 Bond Order(Partial) 9.2 Bond Order The number of The number of bondingbonding e e-- pairs shared pairs shared

by by twotwo atoms in a molecule. atoms in a molecule. Can be 1, 2, 3 Can be 1, 2, 3 oror fractional. fractional. E.g.,E.g., CH CH44 = 1, CO = 1, CO22 = 2, N = 2, N22 = 3. = 3.

What about ozone (OWhat about ozone (O33) (wtbd structure)) (wtbd structure) B.O. = B.O. = # shared pairs linking X # shared pairs linking X && Y Y

# X--Y links # X--Y links in the compoundin the compound For ozone, this = 3/2 = 1.5For ozone, this = 3/2 = 1.5 Higher bond order means Higher bond order means shortershorter, ,

strongerstronger bond. bond.

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9.2 Molecular Orbital Model9.2 Molecular Orbital Model The Localized Model we have learned The Localized Model we have learned

explains much about bonding.explains much about bonding. Doesn’t do well with the ideas of Doesn’t do well with the ideas of resonanceresonance, ,

ununpaired electrons, and paired electrons, and bond energybond energy.. The MO model is a parallel of the The MO model is a parallel of the atomicatomic

orbital, using quantum mechanics.orbital, using quantum mechanics. Each MO can hold two electrons with Each MO can hold two electrons with

oppositeopposite spins. spins. Square of wave function tells probability.Square of wave function tells probability. Bond orderBond order is what’s important for AP test (a is what’s important for AP test (a

few MC questions)few MC questions)

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What do you get?What do you get? Solve the two equations for HSolve the two equations for H22 Combine Hydrogen 1s orbitals to get Combine Hydrogen 1s orbitals to get

molecular orbitalsmolecular orbitals HHA A H HBB

Get the possibilityGet the possibility of two of two orbitals.orbitals.

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

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

Which is it? (next slide) Which is it? (next slide)

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The Molecular Orbital ModelThe Molecular Orbital Model

+The molecular orbitals are The molecular orbitals are centered on a line through centered on a line through the nuclei. For:the nuclei. For:

– MOMO22 the greatest the greatest probability is on probability is on either either side ofside of the nuclei the nuclei

– MOMO11 it is it is betweenbetween the the nucleinuclei

– Either type of eEither type of e-- distribution is called a distribution is called a sigmasigma molecular orbital. molecular orbital.

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The Molecular Orbital ModelThe Molecular Orbital Model+In the molecule only the In the molecule only the

molecular orbitals exist, the molecular orbitals exist, the atomicatomic orbitals are orbitals are gonegone..

+MOMO11 is is lowerlower in energy than the in energy than the 1s orbitals it came from.1s orbitals it came from.– So, this favors molecule So, this favors molecule

formation (top figure)formation (top figure)– Called a Called a bondingbonding orbital orbital

MOMO22 is is higherhigher in energy in energy– This goes This goes againstagainst bonding bonding– antiantibonding orbital (bottom bonding orbital (bottom

figure)figure)

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The HThe H22 Molecular Orbital Model Molecular Orbital ModelE

nerg

y

MO2

MO1

ssss

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The Molecular Orbital ModelThe Molecular Orbital Model+ We use labels to indicate shapes and if We use labels to indicate shapes and if

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

– MOMO1 1 = = 1s1s

– MOMO2 2 = = 1s1s** (* indicates (* indicates antiantibonding)bonding)

+ 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, Each MO can hold two electrons, butbut

they must have opposite spins.they must have opposite spins.+ Orbitals are conserved. The number of Orbitals are conserved. The number of

molecularmolecular orbitals orbitals must equalmust equal the the number of number of atomicatomic orbitals that are used orbitals that are used to make them.to make them.

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HH221-1- ion ion

Ene

rgy

1s1s

1s1s**

ssss

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Molecular Orbital SummaryMolecular Orbital Summary pppp

1. Molecular orbitals are 1. Molecular orbitals are eithereither bonding bonding oror antibonding;antibonding;

2. Bonding orbitals are lower in energy than 2. Bonding orbitals are lower in energy than corresponding atomic orbitals while corresponding atomic orbitals while antibonding orbitals are higher;antibonding orbitals are higher;

3. MO theory helps explain paramagnetism 3. MO theory helps explain paramagnetism & diamagnetism; & diamagnetism;

4. MO theory obviates "resonance", 4. MO theory obviates "resonance", replacing it with delocalized pi bonding.replacing it with delocalized pi bonding.

http://www.wwnorton.com/chemistry/tutorials/ch6.htm

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MagnetismMagnetism pppp

Magnetism has to do with electrons.Magnetism has to do with electrons. Paramagnetism Paramagnetism attractedattracted by a magnet. by a magnet. Associated with Associated with ununpaired electrons paired electrons (i.e., (i.e.,

any lone electrons means “para”)any lone electrons means “para”).. Diamagnetism Diamagnetism repelledrepelled by a magnet. by a magnet. Associated with Associated with pairedpaired electrons electrons (i.e., (i.e.,

all the electrons are in pairs)all the electrons are in pairs).... BB22 is diamagnetic. is diamagnetic.

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Bond OrderBond Order pppp

The difference between the number of The difference between the number of bondingbonding electrons and the number of electrons and the number of antiantibonding electrons bonding electrons divideddivided by two. by two.

†

Bond Order = # bonding-#antibonding

2

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Bond OrderBond Order pppp

The The differencedifference between the number of between the number of bondingbonding electrons and the number of electrons and the number of antiantibonding electrons bonding electrons divideddivided by two. by two.

E.g.,E.g., H H22 has 2 has 2 bondingbonding electrons (the electrons (the covalent bond) and 0 covalent bond) and 0 antiantibonding (no bonding (no other electrons).other electrons).

B.O. = (2 - 0) ÷ 2 = 1B.O. = (2 - 0) ÷ 2 = 1 HH22

1-1- ion has 2 bonding + 1 antibonding ion has 2 bonding + 1 antibonding B.O. = (2 - 1) ÷ 2 = 1/2B.O. = (2 - 1) ÷ 2 = 1/2 The non-ion is more stable because it The non-ion is more stable because it

has the has the higherhigher bond order. bond order.

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Bond OrderBond Order pppp

Use bond order to explain why He is Use bond order to explain why He is monoatamic monoatamic vs.vs. diatomic. diatomic.

He bond order = (2 - 0) ÷ 2 = 1He bond order = (2 - 0) ÷ 2 = 1 HeHe22 has 2 bonding and 2 anti-bonding has 2 bonding and 2 anti-bonding (2 - 2) ÷ 2 = 0(2 - 2) ÷ 2 = 0 So He is monoatomic is more stable So He is monoatomic is more stable

because it has the higher bond order.because it has the higher bond order. (End of our part of 9.2, but go on)(End of our part of 9.2, but go on)

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Molecular Orbitals Molecular Orbitals

Part 2Part 2

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More on M.O. TheoryMore on M.O. Theory The 1s orbital is much smaller than the The 1s orbital is much smaller than the

2s orbital2s orbital When When onlyonly the 2 the 2 ss orbitals orbitals

are involved in bonding are involved in bonding Don’t use the Don’t use the 1s1s or or 1s1ss*s* for for

HeHe-2-2 (added 2 e (added 2 e--s)s)

HeHe-2-2 ( (22ss))2 2 ((22ss*)*)22 In order to participate in bonds the In order to participate in bonds the

orbitals must orbitals must overlapoverlap in space. in space.

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9.3 Bonding in Homonuclear 9.3 Bonding in Homonuclear Diatomic MoleculesDiatomic Molecules

Need to use Homonuclear so that we now Need to use Homonuclear so that we now can compare the relative energies.can compare the relative energies.

E.g., E.g., LiLi22 ((2s2s))2 2 ((2s2s*)*)22

What about the What about the pp orbitals? orbitals? How do they form orbitals? How do they form orbitals? Remember that orbitals must be Remember that orbitals must be

conserved. 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 overlap.Hard 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 Paramagnetism attractedattracted by a magnet. by a magnet. Associated with Associated with ununpaired electrons paired electrons (i.e., (i.e.,

any lone electrons means “para”)any lone electrons means “para”).. Diamagnetism Diamagnetism repelledrepelled by a magnet. by a magnet. Associated with Associated with pairedpaired electrons electrons (i.e., (i.e.,

all the electrons are in pairs)all the electrons are in pairs).... BB22 is diamagnetic. is diamagnetic.

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

are reversed by s-p mixing.are reversed by s-p mixing. 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

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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.

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ExamplesExamples CC22

NN22

OO22

FF22

PP22

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Finishing Chapter 9Finishing Chapter 9

Details, details, detailsDetails, details, details

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9.4 Heteronuclear Diatomic Species9.4 Heteronuclear 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. 2p much higher than 1 s orbital.2p much higher than 1 s orbital.

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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

electroegativity. electroegativity.

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9.5 Names9.5 Names 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--