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7 VSEPR Model

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					Localized electron model
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

A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms The model has three parts: – Description of valence electron arrangement (Lewis structures) – previous lecture – Prediction of geometry (VSEPR model) – Description of atomic orbital types used to share electron pairs or to contain lone pairs localized on a single atom

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VSEPR Theory

VSEPR = Valence Shell Electron “Pair” Repulsion (Theory)

- Basic idea: The region surrounding a central atom is crowded.
- Electrons will get as far from one another while maintaining their basic groupings Electron groups: Lone pairs (2 electrons) = 1 group Single bonds (2 electrons) = 1 group Double bonds (4 electrons) = 1 group Triple bonds (6 electrons) = 1 group Note: To justify the word “pair” in the name of the theory, some texts call double bonds and triple bond effective pairs.

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Basic Electron Group Arrangements (1)

1) Two Groups = linear arrangement of electron groups
Atom

Angle between groups: 180o
Electron groups

2) Three Groups = trigonal (planar triangle) arrangement of groups

Atom

Angle between groups: 120o

Electron groups
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Basic Electron Group Arrangements (2)

3) Four Groups = tetrahedral arrangement of groups
Forward pointing electron groups Atom

Angle between groups: 109.54o

Backward pointing electron groups

4) Five Groups = trigonal bipyramid arrangement of groups
Equatorial (E) groups Atom Axial (A) groups Top View

Angles between groups: A-A 180o (1) E-E 120o (3) A-E 90o (6)
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Basic Electron Group Arrangements (3)

5) Six Groups = octahedral arrangement of groups
z

Angle between groups: 90o
y
x

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Molecular Structures

The structure of the molecule is defined by the positions of the atomic nuclei. - Lone pairs are ignored. Three steps: 1) Write the Lewis Structures. 2) Identify the basic arrangement of the electron groups. 3) “Erase” the lone pairs and characterize what’s left. Note: It takes three atoms to have a molecular “structure.”

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Molecular Structures (1)

Electron Arrangement # Lone Pairs Molecular Structure Linear 0 Linear

Bond Angle 180o

CO2, HCN
Trigonal 0 1 Trigonal Bent 120o 120o

BF3 SO3 SOCl2
No lone pairs One lone pair

SO2

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Models of a linear molecule

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Models of a trigonal planar molecule

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Impact of a double bond on bond angle
10_8

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Molecular Structures (2)

Electron Arrangement # Lone Pairs Molecular Structure

Bond Angle

Tetrahedral

0 1 2

Tetrahedral Trigonal Pyramid Bent

109o <109o <<109o

No lone pairs

One lone pair

Two lone pairs

CCl4 CH3Cl SiH4

NH3 (107o) PCl3

H2O (104o) OF2
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Models of a tetrahedral molecule

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8-15

Applying VSEPR to NH3

Lone pair

N
H

N
H H

(a)

(b)

(c)

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8-16

Applying VSEPR to H2O

Lone pair Bonding pair Bonding pair

O
H

O
H Lone pair

(a)

(b)

(c)

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Distortions from the tetrahedral bond angle
10_7

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Molecular Structures (3)

Electron Arrangement # Lone Pairs Molecular Structure

Bond Angle

Trigonal Bipyramid

0 1 2 3

Trigonal Bipyramid Seesaw T-shape Linear

90o, 120o, 180o 90o, 120o, 180o 90o, 180o 180o

NOTE: All lone pairs are placed in equatorial positions.

No lone pairs

One lone pair

Two lone pairs

Three lone pairs

PCl5

SF4

ClF3

XeF2
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The trigonal bipyramidal molecular structure
10_10

A

90º E E

120º

E

A
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8-20

Appling VSEPR to I3-

I
90º

I

I I
90º

I I
90º

I I I

120º

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Molecular Structures (4)

Electron Arrangement # Lone Pairs Molecular Structure

Bond Angle

Octahedral

0 1 2

Octahedral Square Pyramid Square Planar

90o 90o 90o

d d d d d d
No lone pairs

d d
d

d d

d d

d d

One lone pair

Two lone pairs

XeF6

BrF5

XeF4

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P398

Applying VSEPR to XeF4 (1)

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Applying VSEPR to XeF4 (2)
8-19

F

F

F Xe F F
90º leads to the structure

F Xe F F

F Xe F

F
180º leads to the structure

F Xe F

F F

F

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How Well Does VSEPR Work?

Nearly all molecules with second row elements have bond angles that are consistent with VSEPR theory: H2O (105o) NH3 (107o) CH2O (120o) CH4 (109.5o) For molecules whose central atom is third row or below, some obey VSEPR theory: PCl5 (90o, 120o, 180o) Others do not: H2S (92o) PH3 (92o) Reason: VSEPR reflects the crowding of electron groups around a central atom. When the central atom is large and the other atoms are small, there is less crowding and VSEPR does not always apply. PH4+ (109.5o) ClO4- (109.5o)

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VSEPR Summary (1)
10_3

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10_4a

VSEPR Summary (2)

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10_4b

VSEPR Summary (3)

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VSEPR Summary (4)
10_9a

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VSEPR Summary (5)
10_9b

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Bond Formation

   

Covalent bonds form by creating new orbitals from existing atomic orbitals. The new orbitals are not centered about one atomic nucleus. They describe a condition in which electrons are between the nuclei. We envision these bond orbitals as arising from overlap of the atomic orbitals of the atoms involved in the bond.
- The new orbitals are actually mathematical combinations of the functions that describe the original orbitals of the atom.

(Orbital on atom A) + (Orbital on atom B)  (A-B bond)

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9_9

A reminder on the definition of a covalent bond

0

H+H

Potential energy

Bond dissociation energy

H2 Bond length 0 74 pm Distance between nuclei

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Examples

HCl:

H

+

Cl

H Cl

1s

3p

s bond

Cl2:

Cl
3p

+

Cl
3p

Cl Cl
s bond

Electrons mostly in between

When the bond is characterized by electrons shared directly between the nuclei, it is called a sigma (s) bond.
There must be exactly one s bond between a pair of bonded atoms in a molecule.
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The Three 2p Orbitals (Reminder of orbital geometry)

The three different values of ml result in three p orbitals. l = 1 (ml = -1, 0 1)
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Bond Angles form Atomic Orbitals

If the atoms use the ordinary atomic orbitals to form bonds, the resulting bond angles should be similar to those between the atomic orbitals.

H

+

S + H

H S H
s bond 3px

3py

Exercise:

s bond

1s

PH3 also has bond angles near 90o. Which orbitals does P use?

92o
Pretty good!

1s
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Need for Hybridized Orbitals
10_7

The molecular shapes and bond angles that are observed (or predicted by VSEPR) do not match the shapes of s and p orbitals

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Hybridization



The use of ordinary p orbitals explains how the exceptions to VSEPR occur. How do the atoms form bonds with the angles that arise when VSEPR theory applies? Solution: The atom create new atomic orbitals from the “normal” s p and d orbitals in a process called hybridization.
–





These new hybrid orbitals point in exactly the right directions to form the VSEPR structures.



The process of hybridization increases the energy of the central atom.
– –

However, energy is released in the bonding process (bond energies). When VSEPR theory works, the bonding energy more than compensates for the increased energy associated with the central atom.
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sp3 Hybridization
sp3 orbital

The number of hybrid orbitals formed is always exactly the same as the number of ordinary atomic orbitals used. sp3 hybrid orbitals


The s orbital and the three p orbitals in the valence shell of the central atom combine to form four new sp3 orbitals. - Four orbitals in  four orbitals out - These orbitals point in exactly the tetrahedral directions. - 109.5o bond angles - They can be used to form fours bonds. - sp3 orbitals are always involved in 4-group VSEPR structures. The elements C, N and O often create sp3 hybrid orbitals: CH4 NH3 H2 O



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9-3

sp3 hybridization of orbitals
z x z x y px z x y pz y Hybridization y z x y sp3 gives a tetrahedral arrangement sp3 sp3 z x y z x z x

y

s z x

y

py

sp3

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Comparison of H2S and H2O

H2S- No hybrid orbitals
3p
y

H2O-sp3 hybridization
2sp3

3px
1s

2sp3

1s
s bond 1s s bond
90o

1s s bond

92o
expected

s bond

105o
expected 109.5o
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sp3 hybridization of O in the water molecule
10_25

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sp2 Hybridization
sp2 orbital

sp2 hybrid orbitals The s orbital and two p orbitals in the valence shell of the central atom combine to form three new sp2 orbitals. - Three orbitals in  three orbitals out - These three orbitals lie in a plane and point toward the corners of an equilateral triangle - Bond angles: 120o - They can be used to form threes bonds. - One p orbital remains behind (unhybridized). - sp2 orbitals are always involved in 3-group VSEPR structures.


The elements C, amd N can create sp2 hybrid orbitals: CH2O NO3-

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sp2 Hybridization
9-8

z x y y z

z

z x

z

x
Hybridization

x y

s

px

y z

x y py y

x

z
gives a trigonal planar arrangement

x 120º y

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 bonds



What do the unhybridized p orbitals do? 1) They can be entirely unused: BH3 2) They can form a new kind of bond by overlapping side to side: One  bond

    

The electrons in a pi bond do not lie directly between the nuclei. A  bond never forms in the absence of a s bond. Every double bond has one  bond and one sbond. Every triple bond has two  bonds and one s bond . Examples: CH2O (C-O  bond) CO2 (Two C-O  bonds)
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Examples of Multiple bonds

One double bond:

H2C=O

(One C-O s bond and one

 bond )
(Two C-O s bonds and two C-O  bonds)

Two double bonds: O=C=O

Triple bond: H-CN

(One C-N s bond and two CN  bonds)

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sp2 Hybridization
9-10

p orbital sp2 orbital sp2 orbital

sp2 orbital

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Use of sp2 hybridization in C2H4
9-11

H1s sp 2 sp 2

H1 s

C sp 2 H1s sp 2 sp 2

C

sp 2 H1 s

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Remaining p orbital will form a pi bond
9-12

p orbital

p orbital

C

C

pi bond

sigma bond
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Pi bonding
03_04

p C sp2 + sp2

p C Two sp2-hybridized carbons with orbitals parallel

p C

p

s

C

The s bond is formed by two electrons in overlapping sp2 orbitals


sp2 sp2 C

s
1.34Å

C

sp2 sp2



The  bond is formed by two electrons in overlapping parallel p orbitals

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sp Hybridization
sp orbital

sp hybrid orbitals

The s orbital and one p orbital in the valence shell of the central atom combine to form two new sp orbitals. - Two orbitals in  two orbitals out - These two orbitals lie on a straight line and point in opposite directions - Bond angles: 180o - They can be used to form twos bonds. - Two p orbital remains behind (unhybridized). - sp orbitals are always involved in 2-group VSEPR structures.


The elements C, and N can create sp hybrid orbitals: CO2 HCN
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sp hybridization
9-14

z

z Hybridization x x

z

z gives a linear arrangement x x y y 180 °

z

x

y

s

y

px

y

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Hybridization in CO2
9-19

sigma bond (1 pair of electrons) pi bond (1 pair of electrons)

O pi bond (1 pair of electrons) (a)

C

O

O
(b)

C

O

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Hybridization in N2
9-20

p

sp

N p

sp

(a)

lone pair sigma bond lone pair N sp sp sp N sp

(b)

N

N

(c)

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Hybridization in C2H2
03_14

H sp

p Two sp-hybridized carbons aligned for bonding sp + sp p H s 1 s 2 s H The resulting carbon-carbon triple bond, with a hydrogen atom attached to each remaining sp bond. (The orbitals involved in the CH bonds are omitted for clarity.) sp H p

p

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sp3d Hybridization

sp3d hybrid orbitals

The s orbital, all three p orbitals and one d orbital in the valence shell of the central atom combine to form five new sp3d orbitals. - Five orbitals in  five orbitals out - Can only occur when d orbitals are available on the central atom. - Third row and lower in periodic table. - These orbitals point are distributed in a trigonal bipyramid structure - Bond angles: 90o, 120o, 180o - They can be used to form fives bonds. - The remaining d orbitals are not used. - sp3d orbitals are always involved in 5-group VSEPR structures.

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sp3d2 Hybridization

sp3d2 hybrid orbitals

The s orbital, all three p orbitals and two d orbital in the valence shell of the central atom combine to form six new sp3d2 orbitals. - Six orbitals in  six orbitals out - Can only occur when d orbitals are available on the central atom (third row and lower) - These orbitals point are distributed in an octahedral structure - Bond angles: 90o, 180o - They can be used to sixs bonds. - The remaining d orbitals are not used. - sp3d2 orbitals are always involved in 6-group VSEPR structures.

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Summary: LEM
9-24

Number of Effective Pairs

Arrangement of Pairs

Hybridization Required 180°

LEM 1. Draw the Lewis structure of the molecule Determine the arrangement of electron pairs using the VSEPR model Specify the hybrid orbitals required to accommodate the electron pairs Review the molecular geometry study sheet

2

Linear

sp

3

Trigonal planar

sp2 120° 109.5°

2.

4

Tetrahedral

sp3

3.

90° 5 Trigonal bipyramidal dsp3 120°

4.

90° 6 Octahedral d2sp3 90°

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Description: Lawrence Technological University - Chemistry 1 CHM1221 Fall 06