Chemical Bonding II Bonding II-student.pdf · Chemical Bonding II Molecular Geometry Valence Bond...

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Chemical Bonding II

Molecular Geometry

Valence Bond Theory

Phys./Chem. Properties

Quantum Mechanics

Sigma & Pi bonds

Hybridization

MO theory

2

Molecular Geometry

3-D arrangement of atoms

3

VSEPR

Valence-shell electron-pair

repulsion theory.

All valence shell e- pairs

(or e- “domains”) repel each other.

(Coulomb’s law as applied to the

repulsion of valence electrons.)

Determines the geometry of

e- domains around central atom.

4

VSEPR Theory: Rules

1.Single, double, triple bonds and

lone pairs are treated as one e-

domain (approximation)

2.Apply VSEPR to any one

resonance structure

3.Geometry– move e- domains as

far apart as possible in 3-D space.

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AB2 (with no lone pairs)

Beryllium chloride BeCl2

— Be—

Cl—Be—Cl

180o

linear geometry

e-

atoms

6

AB3 (with no lone pairs)

Boron trifluoride BF3

B

F

FF

Trigonal planar

120o

7

C

AB4 (with no lone pairs)

Methane CH4

tetrahedron

109.5o

8

AB5 (with no lone pairs)

Phosphorus Pentachloride PCl5

trigonal bipyramid

equatorial

axial90o

120o

9

SF

F

F

F

F

F

AB6 (with no lone pairs)

Sulfur hexafluoride SF6

octahedral

All angles

90o & 180o

10

Quiz: Name That Shape!

AB2

AB3

AB4

AB5

AB6

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VSEPR: Lone Pairs

Molecules in which the

central atom has lone pair(s)

ABxEy

central atom

surrounding atoms

lone pairs on A

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ABxEy

Geometry is similar to ABx, but

nonbonding pairs are treated like

bonding pairs to determine

geometry of the e- domains.

VSEPR

13

Bonding e- take up less space

than nonbonding e-

Example: water

Bonding e- are

“focused” between

the nuclei.

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VSEPR

lone pair lone pair bonding pair

vs vs vs

lone pair bonding pair bonding pair

repulsion repulsion repulsion

Decreasing e- pair repulsion

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AB2E

Sulfur dioxide SO2

O S—O

OSO angle < 120o

Draw Lewis structure

S

O O3 e- domains

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Shape: e- Pairs vs. Atoms

S

O O

e- domains are trigonal planar

S

O O

molecule is bent linear

(always state molecular shape)

17e- tetrahedral trigonal pyramidal

AB3E

Ammonia H—N—H

H

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AB2E2

Water H—O—H

e- tetrahedral molecule: bent linear

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Comparison

AB4 AB3E AB2E2

Methane ammonia water

109.5 107.3 104.5

CH

H

H

H

NH

H

HO

H

H

20

AB4E

Sulfur tetrafluoride SF4

S

F F

F FPredict: trigonal bipyramid

for e- domains.

Draw Lewis structure

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AB4E: SF4

trigonal bipyramid: 2 choices

F

F

F

F

F

F

F

F

distorted tetrahedron or seesaw

22

AB3E2

ClF3 F

F

F

molecule: T-shaped

A little weird.

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AB2E3Three possibilities for I3

-

Want nonbonding e-

domains farthest apart.

24

AB2E3

I3-

molecule: linear

I

I

-

Lone pairs always go on

equatorial position(s)

25

AB5E

BrF5

molecule: square pyramidal

FF

FF

F

26

AB4E2: 2 Possibilities

Want

nonbonding

e- pairs

farthest

apart.

1

2

XeF4

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AB4E2

XeF4

molecule: square planar

F

FF

F

28

You Try It

Predict geometry and

approximate bond angles for:

AlCl4-

XeF2

XeOF2

Text: Tables 10.1 and 10.2.

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Bond Polarity

FH

Shift of e- density toward F,

Thus HF is polar.

d+ d-

F is more

electronegative

than H.

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Overall Molecular Polarity

O C O

Even though CO2 has polar bonds,

it is nonpolar since the individual

bond polarities add to zero.“Dipoles” are a vector quantities.

Symmetrical = nonpolar

“pull test”

(Recall the “Regents” rule about

lone pairs on the central atom.)

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Molecular Polarity

Molecule GeometryDipole

Strength

HF Linear 1.92

HBr Linear 1.08

Water Bent 1.87

SO2 Bent 1.60

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Molecular Polarity

NH

H

H

Which has larger polarity?

NH3 NF3

NF

F

F

m =0.24 Dm = 1.46 D

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Dipole Moments

Predict whether the following

molecules are polar.

IBr

CH2Cl2AlCl3

Remember to distinguish between

bond polarity and molecular polarity.

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Both H2 & F2 have single bonds,

but…

Bond

length

Bond

energy

H2 74 pm 436 kJ

F2 142 pm 151 kJ

Shortfall of VSEPR

35

Quantum Mechanics…

Valence Bond Theory

e- in molecule occupy

blended atomic orbitals

…to the rescue

Molecular Orbital Theory

molecule has

“molecular orbitals”

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Valence Bond Theory

Atomic orbitals (s, p, d …)

of the valence electrons

“hybridize” or mix to form new

orbitals for the molecule.

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Valence Bond Theory

2s 2pcarbon

atomic

orbitalsC

How can carbon form four equal

bonds with four hydrogen atoms

using its atomic ‘s’ and ‘p’ orbitals?

Consider tetrahedral CH4

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CH4: sp3 Hybridization

2s 2p

carbon

hybridized

orbitals

carbon

atomic

orbitalsC

Csp3 (all the same)

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Hybridization Analogy

s p p p

4 sp3 hybrid orbitals

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sp3 Hybridization

It takes energy to form hybrid

orbitals, but this energy is

more than compensated by

bond formation.

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sp3 Orbital Shapes

Each

orbital

can hold

2 e-

tetrahedral

One s & three p orbitals change

to four sp3 hybrid orbitals.

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CH4 Bonding

The 4 sp3 hybrid

orbitals of C bond

(overlap) with the

4 1s atomic orbitals

of the H atoms.

H

HHH

C

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NH3 sp3 Bonding

nitrogen

hybridized

orbitals

nitrogen

atomic

orbitals

2s 2pN

sp3 (all the same)

N

44

sp3

bonding orbitalslone pair

NH3 sp3 Bonding

45

Hybridization

First use VSEPR to predict the

arrangement of e- pairs, then

use hybridization to predict the

type of bonding.

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sp Hybridization

Be atomic

orbitals

2s 2pBe

Be hybrid

orbitals

sp 2p

Be

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sp Hybridization: BeCl2

BeCl Cl

one atomic p

orbital of Cl

two sp hybrid

orbitals of Be

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sp2 Hybridization

B atomic

orbitals

2s 2p

B

B hybrid

orbitals

sp2 2p

B

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sp2 Hybridization: BF3

B

F F

F

one p orbital

of F

three sp2

hybrid orbitals

of B

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Hybridization

Hybridization for 2nd Period

elements (C, N, O, F) explains the

“octet” rule, since there are 4 hybrid

orbitals formed from one s and three

p atomic orbitals.

s p sp3

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Hybridization: Review

1.Not applied to isolated atoms

2.First determine VSEPR geometry

3.Mix nonequivalent atomic orbitals

of central atom to form hybrid

orbitals

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Hybridization: Review

4.Requires energy, but energy is

more than returned by bond

formation

5.Covalent bonds formed by

overlap of hybrid-hybrid and/or

hybrid-unhybridized orbital

53

Hybridization: Try It

Determine hybridization in:

AlBr3PF3

HgCl2

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Let’s Not Forget d Orbitals

For elements in the 3rd Period and

higher, hybridization can also

include ‘d’ orbitals.

d-orbital hybridization is still

being debated!

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SF

F

F

F

F

F

d Hybridization

SF6 VSEPR predicts

octahedral geometry

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Hybridization: SF6

S atomic orbitals

S hybridized sp3d2 orbitals

3s 3p 3d

sp3d2 3d

57

Hybridization

You Try It .

What is the VSEPR geometry

and the hybridization in PBr5?

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Summary: Hybrid Orbitals

2 e- pairs (sp) linear

3 e- pairs (sp2)

trigonal planar

4 e- pairs (sp3)

tetrahedral

e- domains (bonding & nonbonding pairs)

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Summary: Hybrid Orbitals

5 e- pairs (sp3d)

trigonal bipyramid

6 e- pairs (sp3d2)

octahedral

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Multiple Bonds

In VSEPR, no distinction was made

among single, double, triple bond or lone

pair. All were counted as an “e- domain”.

Each carbon is trigonal planar

C2H4 C CH

H

H

H

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Double Bond: C2H4

2s 2pC atomic

orbitals

C hybrid

orbitals

sp2 pz

+

unhybridized

p orbital

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Double Bond: C2H4

C hybrid

orbitals

sp2 pz

+

trigonal planar dumb bell

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Double Bonds: C2H4

C C

H

HH

H

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Double Bond: C2H4

Sigma (s) and Pi (p) Bonds

(model)

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Pi & Sigma Bonds

Pi (p) bond: covalent bond formed

by sideways e- overlap above and

below the plane connecting atoms

(weaker than s)

Sigma (s) bond: covalent bond

formed by e- overlap along the

axis connecting atoms

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Double Bond: C2H4

The double bond is one sigma and one

pi bond between the carbon atoms

C CH

H

H

H

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Triple Bond: H-C=C-H

2s 2pC atomic

orbitals

C hybrid

orbitals

sp py pz

+unhybridized

p orbitals

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Triple Bond: C2H2

Hybridize s bonds only

hybridized

sp orbitals

unhybridized

p orbitals

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Hybridizing Shortcuts

1.Single bonds: sigma2.Double bonds: one sigma, one pi3.Triple bonds: one sigma, two pi4.Hybridize: add the number of

sigma bonds plus lone pairs

s p d

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You Try It

Describe the bonding and

hybridization for each atom in:

•formaldehyde, CH2O

•hydrogen cyanide, HCN

(assign formal charges)

71

Delocalized Molecular Orbitals

An explanation of resonance.

O OOO O O

ozone

Ozone is a blend of the two

resonance structures.

Delocalized Molecular Orbitals

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The p component of the double bond

is delocalized over the molecule.

sp2

sp2

sp2

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Delocalized “MOs”

e- in s bonds– localized

e- in p bonds-- delocalized

H

H

H

H

H

H

H

H

H

H

H

H

benzene

74

Benzene

sp2

sp2

C

C

C C

C

C

Sigma bonds

(localized)

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Benzene

p bonds

(delocalized)

sp2 pz

+

76

Benzene

Bond order 1.5

H

H

H

H

H H

“Conjugated” double bonds are

more stable (less reactive)

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Carbonate Ion

+ resonance

structuresO—CO

O

2-

C: sp2 fors bonds (localized)

plus a 2pz for p bond (delocalized)

O: each has a 2pz orbital (delocalized)

78

Carbonate Ion

o

o

oC

Additional stability due to

delocalization of the pi bond.o

o

oC

79

You Try It

Would you predict the NO3- anion

to have additional stability due to

delocalization of the p electrons?

80

Polyatomic Ions

Many common polyatomic ions

have delocalized pi bonds,

partially accounting for their

additional stability in chemical

reactions.

CO3-2, NO3

- , ClO3- etc.

81

Ionic & Covalent

CaCO3

C

O

OOCa

2-

2+

Trigonal planar, sp2, 120o

Ionic compounds containing polyatomic

ions have both ionic and covalent bonds!