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Chapter 10p
Chemical Bonding II:Molecular Geometry
1Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
2
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
AB2 2 0 linear linear
Class central atom central atom electron pairs Geometry
2
AB3 3 0 trigonal planar
trigonal planarplanar planar
5 6
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
AB2 2 0 linear linear
Class central atom central atom electron pairs Geometry
2
AB3 3 0 trigonal planar
trigonal planarplanar planar
AB4 4 0 tetrahedral tetrahedral
7 8
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
AB2 2 0 linear linear
Class central atom central atom electron pairs Geometry
2
AB3 3 0 trigonal planar
trigonal planarplanar planar
AB4 4 0 tetrahedral tetrahedral
AB5 5 0 trigonalbipyramidal
trigonalbipyramidalbipyramidal bipyramidal
9 10
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
AB2 2 0 linear linear
Class central atom central atom electron pairs Geometry
2
AB3 3 0 trigonal planar
trigonal planarplanar planar
AB4 4 0 tetrahedral tetrahedral
AB5 5 0 trigonalbipyramidal
trigonalbipyramidalbipyramidal bipyramidal
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
13
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< <
14
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
Class central atom central atom electron pairs Geometry
AB 3 0 trigonal trigonal
bent
AB3 3 0 gplanar
gplanar
AB E 2 1 trigonal bentAB2E 2 1 gplanar
15
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
Class central atom central atom electron pairs Geometry
AB 4 0 tetrahedral tetrahedraltrigonal
id lAB3E 3 1
AB4 4 0 tetrahedral tetrahedral
tetrahedral pyramidal3 tetrahedral
16
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
Class central atom central atom electron pairs Geometry
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
17
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
Class central atom central atom electron pairs Geometry
AB 5 0 trigonal trigonalAB5 5 0 bipyramidal bipyramidal
AB E 4 1 trigonal distorted AB4E 4 1 gbipyramidal tetrahedron
SF4
18
SF4
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
Class central atom central atom electron pairs Geometry
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
19
# of atoms # loneVSEPR
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement ofelectron pairs
MolecularGeometry
AB5 5 0 trigonalbipyramidal
trigonalbipyramidalbipyramidal bipyramidal
AB4E 4 1 trigonalbipyramidal
distorted tetrahedronpy
AB3E2 3 2 trigonalbipyramidal T-shapedpy
AB2E3 2 3 trigonalbipyramidal
linearbipyramidal
20
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
Class central atom central atom electron pairs Geometry
AB6 6 0 octahedraloctahedralAB6 6 0 octahedraloctahedral
AB5E 5 1 octahedral square id l5 pyramidal
21
VSEPR
# of atomsbonded to
# lonepairs on Arrangement of Molecular
Class central atom central atom electron pairs Geometry
AB6 6 0 octahedraloctahedralAB6 6 0 octahedraloctahedral
AB5E 5 1 octahedral square id l5 pyramidal
AB4E2 4 2 octahedral square l4 2 planar
22
23
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
24
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
28
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
31
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
41
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
62
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
63
theory.