valence bond theory: chemical bonding ii: molecular … 2 predicting molecular... · valence bond...

Chapter 10p

Chemical Bonding II:Molecular Geometry

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Predicting Molecular GeometryThree Models:

VSEPR:Valence electron pairs in the Lewis structure mutually repel each other.

Valence Bond Theory:Addresses molecular shape through overlapping of valence orbitalsAddresses molecular shape through overlapping of valence orbitals of each atom for bonding. Electron pairs are localized between the

two bonding atoms. The two types of the overlapping is thetwo bonding atoms. The two types of the overlapping is the formation of sigma and pi bonds.

M l l O bit l ThMolecular Orbital Theory:Electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the wholeare treated as moving under the influence of the nuclei in the whole

molecule by assuming the formation of molecular orbitals from combination of atomic orbitals (This solves the problem of resonance

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combination of atomic orbitals. (This solves the problem of resonance structure in the VB theory by delocalizing electrons).

Valence shell electron pair repulsion (VSEPR) model:p p ( )Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairsrepulsions between the electron (bonding and nonbonding) pairs.Minimize repulsion amongst bonds and electron lone pairs, or largest separation angles between themlargest separation angles between them.

# of atoms # lone

Classbonded to

central atom pairs on

central atomArrangement ofelectron pairs

MolecularGeometry

AB2 2 0 linear linear

B B

3

0 lone pairs on central atom

Cl ClBe

2 atoms bonded to central atom4

2 atoms bonded to central atom

VSEPR (General Overview)

# of atomsbonded to

# lonepairs on Arrangement of Molecular


Class central atom central atom electron pairs Geometry

2

AB3 3 0 trigonal planar

trigonal planarplanar planar

5 6

VSEPR

# of atomsbonded to




2



AB4 4 0 tetrahedral tetrahedral

7 8

VSEPR

# of atomsbonded to




2




AB5 5 0 trigonalbipyramidal

trigonalbipyramidalbipyramidal bipyramidal

9 10

VSEPR

# of atomsbonded to




2






AB6 6 0 octahedraloctahedral

11 12

Case of no e- lone pair summary:

Arrangement of electron pairs about a central atom (A) in a molecule and geometry ofmolecule and geometry of

some simple molecules and ions in which central atom has

no lone pairs

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Electron lone pair vs. electron bonding pair repulsion

bonding-pair vs bonding lone-pair vs lone pairlone-pair vs bondingbonding pair vs. bondingpair repulsion

lone pair vs. lone pairrepulsion

lone pair vs. bondingpair repulsion< <

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VSEPR

# of atomsbonded to



AB 3 0 trigonal trigonal

bent

AB3 3 0 gplanar

gplanar

AB E 2 1 trigonal bentAB2E 2 1 gplanar

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VSEPR

# of atomsbonded to



AB 4 0 tetrahedral tetrahedraltrigonal

id lAB3E 3 1


tetrahedral pyramidal3 tetrahedral

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VSEPR

# of atomsbonded to



AB 4 0 t t h d l t t h d lAB4 4 0 tetrahedral tetrahedral

AB E 3 1 tetrahedral trigonalAB3E 3 1 tetrahedral gpyramidal

AB E 2 2 t t h d l bentAB2E2 2 2 tetrahedral bent

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VSEPR

# of atomsbonded to



AB 5 0 trigonal trigonalAB5 5 0 bipyramidal bipyramidal

AB E 4 1 trigonal distorted AB4E 4 1 gbipyramidal tetrahedron

SF4

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SF4

VSEPR

# of atomsbonded to



AB 5 0 trigonal trigonalAB5 5 0 bipyramidal bipyramidal

AB E 4 1 trigonal distorted AB4E 4 1 gbipyramidal tetrahedron

AB E 3 2 trigonal T h dAB3E2 3 2 trigonalbipyramidal T-shaped

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# of atoms # loneVSEPR

Class

# of atomsbonded to

central atom

# lonepairs on

central atomArrangement ofelectron pairs

MolecularGeometry



AB4E 4 1 trigonalbipyramidal

distorted tetrahedronpy

AB3E2 3 2 trigonalbipyramidal T-shapedpy

AB2E3 2 3 trigonalbipyramidal

linearbipyramidal

20

VSEPR

# of atomsbonded to



AB6 6 0 octahedraloctahedralAB6 6 0 octahedraloctahedral

AB5E 5 1 octahedral square id l5 pyramidal

21

VSEPR

# of atomsbonded to



AB6 6 0 octahedraloctahedralAB6 6 0 octahedraloctahedral

AB5E 5 1 octahedral square id l5 pyramidal

AB4E2 4 2 octahedral square l4 2 planar

22

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Predicting Molecular Geometry using VSEPR

1 D L i t t f l l1. Draw Lewis structure for molecule.

2. Count number of lone pairs on the central atom and pnumber of atoms bonded to the central atom.

3 Use VSEPR to predict the geometry of the molecule3. Use VSEPR to predict the geometry of the molecule.

What are the molecular geometries of SO2 and SF4?g 2 4

SO O F AB ESO O

AB2E SF F

AB4E

di t t dbent

F

distortedtetrahedron

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Example

Give the number of lone pairs around the central atom and the molecular geometry of SCl2.

25Ans: 2 lone pairs, bent

Example

What is the predicted arrangement of the electrons and the molecular geometry,respectively, in the central atom of a carbon disulfide molecule?

2626Ans: linear, linear (no lone pair left on C)

Predict using VSEPR theory, ground-state geometry of PF3

Example

27Ans: Tri-gonal pyramidal

Insufficiency of VSEPR theorySharing of two electrons between the two atoms.

Bond Enthalpy Bond Length

H 436 4 kJ/ l 4

Overlap Of

2 1H2

F

436.4 kJ/mol

150 6 kJ/ l

74 pm

142

2 1s

2 2F2 150.6 kJ/mol 142 pm 2 2p

How does Lewis theory explain the bonds strength in H and F ?How does Lewis theory explain the bonds strength in H2 and F2?

Not possible the VSEPR does not provide this kind of infoNot possible, the VSEPR does not provide this kind of info. Need quantum mechanics treatment VB theory

Valence bond theory – bonds are formed by sharing of e- from overlapping atomic orbitals

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of e from overlapping atomic orbitals.

Change in Potential Energy of Two Hydrogen Atomsas a Function of Their Distance of Separationas a Function of Their Distance of Separation

Th VB th h l di ti t b d l th29

The VB theory can help predicting correct bond length

Change in electron density as two hydrogen atoms approach each otherapproach each other.

30

In VB theory, the atomic orbitals for bonding can mix with others

Hybridization – mixing of two or more atomic orbitals to form a new set of “hybrid” orbitalsorbitals to form a new set of hybrid orbitals.

1. Mix at least 2 nonequivalent atomic orbitals (e.g. s1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitalsfrom original atomic orbitals.

2. Number of resulting hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process.y p

3. Covalent bonds are then formed by:O fa. Overlap of hybrid orbitals with atomic orbitals

b. Overlap of hybrid orbitals with other hybrid

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b. Overlap of hybrid orbitals with other hybrid orbitals

Formation of sp Hybrid Orbitals

Beryllium chloride32

Beryllium chloride

Formation of sp2 Hybrid Orbitals

Boron trifluoride

33

Formation of sp3 Hybrid Orbitals

34

Formation of sp3-hybridized bonds in CH4

35

In NH3 also sp3-hybridized bonds of the N atom

Hybridization theory can also predict correct bond angle

36

So how do we predict which hybridization of the central atom?

1. Draw the Lewis structure of the molecule.2. Count the number of lone pairs AND the number of Cou t t e u be o o e pa s t e u be o

atoms bonded to the central atom

# of Lone Pairs+

# f B d d At H b idi ti E l# of Bonded Atoms Hybridization Examples

2 sp BeCl22

3

sp

sp2

BeCl2

BF3

4

p

sp3

3

CH4, NH3, H2O

5 sp3d PCl5

376 sp3d2 SF6 38

sp2 Hybridization of Carbonsp Hybridization of Carbonand the left over unhybridized pi-orbital

39

Th h b idi d 2 bit l ( ti l) iThe unhybridized 2pz orbital (gray or vertical) is perpendicular to the plane of the hybrid (green) orbitals.

The unhybridized yorbitals can also participate in bonding

usually pi bonding

40

Bonding in Ethylene, C2H4

Sigma bond (σ): Head-to-head overlap electron density g ( ) p ybetween the 2 atoms

Pi bond (π): Side-by-side overlap electron density above and

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Pi bond (π): Side by side overlap electron density above and below plane of nuclei of the bonding atoms

Another View of π Bonding in Ethylene, C2H4

42

sp Hybridization of Carbonsp Hybridization of Carbon

43

Bonding in Acetylene, C2H2, (one sigma and two pi bonds)

(2 bonding atoms and zero lone pair sp-hybridization)The 2 electrons left are distributed in the two p-orbitals forming two pi bonds

The sp-hybridized forms the sigma gbonds

The two p orbitals poverlap sideway toform the two pi bonds.

44Three bonds between the two carbon atoms triple bonding

Another View of the Triple Bonding in Acetylene, C2H2

45

What is the type of hybridization bonding in CH2O ?Example

yp y g 2

HC

HO

H

C has 3 bonded atoms, 0 lone pairs2 fi ti f C=> sp2 configuration of C

46

Single, Double, and Triple Bonds

Single bond 1 sigma bondSingle bond 1 sigma bond

Double bond 1 sigma bond and 1 pi bondDouble bond g p

Triple bond 1 sigma bond and 2 pi bondsp g p

How many σ and π bonds are in the acetic acid (vinegar) y ( g )molecule CH3COOH?

H

O

σ bonds = 6 + 1 = 7C CH O H

π bonds = 1

47

H

The number of pi bonds in the molecule below isExample

p

48Ans: 3

Dipole Moments and Polar Molecules

electron richregionelectron poor

region

H Fregion

δ+ δ−δ+ δ

μ = Q x rQ is the charge r is the distance between chargesQ is the charge, r is the distance between charges

Unit of molecular dipole moment: Debye (D)

491 D = 3.36 x 10-30 C m

Behavior of Polar Molecules

Field off Field on

E

Molecules have random orientation Molecules re-orient to have their dipoles aligned with the fi ld di ti

50

field direction

Bond moments and resultant dipole moments in NH3 and NF3.

51

Which of the following molecules have a dipole moment?Example

H2O, CO2, SO2, and CF4

O S

dipole momentpolar molecule

dipole momentpolar moleculepolar molecule polar molecule

F

C

F

CO O

di l t

C

FFF

no dipole momentnonpolar molecule

Fno dipole momentnonpolar molecule

52

nonpolar molecule

Does BF3 have a dipole moment?Example

3 p

53 54

Which one of the following molecules has a zero dipole moment?Self-practice

A) CO

B) CH2Cl2B) CH2Cl2C) SO3

D) SOD) SO2

E) NH3

55Ans: SO3

Consider molecules composed of elements X and Y. Self-practice

Which of the following molecules must be non-polar regardless of the identity of X and Y?

A) XYB) XY2C) X2D) none of the above

56Ans: C

Molecular Orbital (MO) Theory – bonds are formed o ecu a O b ta ( O) eo y bo ds a e o edfrom interaction of atomic orbitals to form molecularorbitals (no longer orbitals of individual atoms)

1 The number of molecular orbitals (MOs) formed is always

orbitals (no longer orbitals of individual atoms).

1. The number of molecular orbitals (MOs) formed is always equal to the number of atomic orbitals combined.

2 Filli l t i t th MO d f l t hi h2. Filling electrons into the MOs proceeds from low to high energies.

3. Each MO can accommodate up to two electrons.

4 Use Hund’s rule when adding electrons to MOs of the same4. Use Hund s rule when adding electrons to MOs of the same energy, i.e. maximize parallel spins in MOs of same energy.

5. The number of electrons in the MOs is equal to the sum of all the electrons on the bonding atoms.

57

Energy levels of bonding and antibonding molecularorbitals in hydrogen (H2).

A bonding molecular orbital has lower energy and greater stability than the atomic orbitals from which it was formed.An antibonding molecular orbital has higher energy and lower stability than the atomic orbitals from which it was

58

yformed.

Constructive and Destructive Interference

If wave 1 & 2

If wave 1 & wave 2 arewave 2 are

in phase t ti

wave 2 are out of phase

destructiveconstructiveinterference

destructiveinterference

59

Two Possible Interactions Between Two Equivalent p Orbitals

60

General molecular orbital energy level diagram for the d i d h l di t i l l Li B Bsecond-period homonuclear diatomic molecules Li2, Be2, B2,

C2, and N2.

61

Delocalized molecular orbitals are not confined (or localized) between two adjacent bonding atoms butlocalized) between two adjacent bonding atoms, but actually extend (delocalized) over three or more atoms.

Example: Benzene, C6H6

Delocalized π orbitals

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Example: Electron density delocalized in Carbonate Ion, CO32-

Recalling in VB theory, where we consider only dot structure, CO3

2- has 3 equivalent structures of 1 double bond, and 2 single bonds on the 3 C-O bonds.

But in MO theory this ambiguity is eliminated andBut in MO theory, this ambiguity is eliminated, and resonance structure in VB becomes delocalization in MO theory

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theory.

valence bond theory: chemical bonding ii: molecular … 2 predicting molecular... · valence bond...

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