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SO2 Bent shape Bond angle of ~119° • Tetrahedral, Lecture notes of Chemistry

Trigonal planar electron-group arrangement with one lone pair. →Two atoms attached to the central atom + one lone pair (AX2E) → Bent shape.

Typology: Lecture notes

2021/2022

Uploaded on 09/12/2022

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Trigonal planar electron-group arrangement
with one lone pair
Two atoms attached to the central atom + one lone
pair (AX2E)Bent shape
The lone pair is bulkier and repels the bonding
pairs stronger bond angle is less than 120°
Example: SO2
Bent shape
Bond angle of ~119°
Tetrahedral electron-group arrangement with
one lone pair
Three atoms attached to the central atom + one lone
pair (AX3E)Trigonal pyramidal shape
The lone pair is bulkier and repels the bonding
pairs stronger bond angles are less than 109.5°
Example: NH3
Trigonal pyramidal
shape
Bond angle of 107.3°
Tetrahedral electron-group arrangement with
two lone pairs
Two atoms attached to the central atom + two lone
pairs (AX2E2)Bent shape
The two lone pairs have even greater repelling
effect bond angles are less than 109.5°
Example: H2O
Bent shape
Bond angle of 104.5°
(lower than in NH3)
Strengths of electron group repulsions
lone pair-lone pair > lone pair-bonding pair >
bonding pair-bonding pair
In the electron arrangement, lone pairs occupy
positions as far from one another and from
bonding pairs as possible
Trigonal bipyramidal electron-group
arrangement with 1, 2 or 3 lone pairs
VSEPR classes AX4E, AX3E2and AX2E3
–The lone pairs occupy equatorial positions
(provides more space for the lone pairs and
minimizes the repulsion)
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Trigonal planar

electron-group arrangement

with

one lone pair

Two

atoms attached to the central atom +

one

lone

pair (

AX

E 2

)^

Bent shape

The

lone pair is bulkier

and repels the bonding

pairs stronger

bond angle is less than 120

Example: SO

2

Bent shapeBond angle of ~

Tetrahedral

electron-group arrangement with

one lone pair

Three

atoms attached to the central atom +

one

lone

pair (

AX

E 3

)^

Trigonal pyramidal shape

The

lone pair is bulkier

and repels the bonding

pairs stronger

bond angles are less than 109.

Example: NH

3

Trigonal pyramidalshapeBond angle of 107.

Tetrahedral

electron-group arrangement with

two lone pairs

Two

atoms attached to the central atom +

two

lone

pairs (

AX

2

E

Bent shape

The

two lone pairs

have even greater repelling

effect

bond angles are less than 109.

Example: H

2

O

Bent shapeBond angle of 104.

(lower than in NH

3

  • Strengths of electron group repulsions

lone pair-lone pair > lone pair-bonding pair >bonding pair-bonding pair^ ⇒

In the electron arrangement, lone pairs occupypositions as far from one another and frombonding pairs as possible

Trigonal bipyramidal

electron-group

arrangement with

,^

or

3 lone pairs

  • VSEPR classes

AX

E 4

,^

AX

E 3

2

and

AX

E 2

3

  • The

lone pairs occupy

equatorial

positions

(provides more space for the lone pairs andminimizes the repulsion)

Octahedral

electron-group arrangement with

or

2 lone pairs

  • VSEPR classes

AX

E 5

, and

AX

E 4

2

  • The

lone pairs occupy positions opposite to

each other

(provides the lowest repulsion)

Bent

e-groups = 3# lone pairs = 1

Trigonal pyramidal

e-groups = 4# lone pairs = 1

See-saw

e-groups= 5# lone pairs = 1

Square pyramidal

e-groups = 6# lone pairs = 1

~

o

~

o

~

o (^) ~

o

~

o

~

o

Bent

e-groups = 4# lone pairs = 2

~

o

T-shaped

e-groups = 5# lone pairs = 2

~

o

Linear

e-groups= 5# lone pairs = 3

Square planar# e-groups = 6# lone pairs = 2

90

o

Summary of molecular shapes for central atomswith lone pairs

Steps

in determining molecular shapes using

the VSEPR model:– Write the Lewis structure– Determine the electron-group arrangement, ideal

bond angles and VSEPR class

  • Place the surrounding atoms and lone pairs in

appropriate positions around the central atom andpredict any deviations from the ideal bond angles

  • Name the molecular shape
    • For molecules with

more than one central

atom

, find the electron-group arrangement

and corresponding shape around each centralatom (one central atom at a time)

Polar

molecules – nonzero dipole moment

  • Heteronuclear diatomic molecules (

HF

,^

CO

  • Polyatomic molecules where the bond dipoles do

not cancel each other

H

O 2

is a polar

molecule because thebond dipoles of theO-H bonds do notcancel due to the bentmolecular shape

H

O

H

H

O

δ+H

The polarity of molecules depends on both thepolarity of the bonds and the moleculargeometry which must be known

Example:

The isomers 1,2-dichlorobenzene and

1,4-dichlorobenzenehave the sameformula, C

6

H

4

Cl

2

but differentphysical properties

B.p. 180

oC

Very soluble

in EtOH

Cl

Cl

Cl

Cl

B.p. 174

o^ C

Slightly soluble

in EtOH

δ−

δ−

δ−

δ−

Polar

Non-polar

  • Highly symmetric molecules are normally

nonpolar–

AX

n

molecules (n=2, 3, 4, 5, 6) where

X

are

atoms of the same element

  • Molecules with symmetrically positioned lone

pairs (

AX

E 2

AX

E 4

  • Molecules with asymmetrically positioned

lone pairs or different atoms attached to thecentral atom are normally polar–

AX

E 2

,^

AX

E 2

2

,^

AX

3

E

,^

AX

3

E

AX

E 4

,^

AX

E 5

CF

H 3

,^

CF

2

H

2

,^

SO

(bent), ... 2

Example:

Is

PCl

2

F

3

a polar molecule?

The Lewis structure is similar to

PCl

5

(five atoms

bonded to the

P

atom, no lone pairs)

trigonal bipyramidal shape

The

Cl

atoms are larger and take two of the

equatorial positions; the

F

atoms are smaller and

take the two axial and one of the equatorial positions

The

P–F

dipoles are larger than the

P–Cl

dipoles (

EN

is larger for

P

and

F

The molecule is

polar

  • the bond

dipoles don’t cancel (asymmetricarrangement)

Linear (non-polar)

Trigonal planar

(non-polar)

Tetrahedral(non-polar)

Trigonal bipyramidal

(non-polar)

Octahedral(non-polar)

180

o

120

o

o

90

o^120

o^

90

o

90

o

Summary of molecular polarities for moleculeshaving central atoms without lone pairs

Bent(polar)

Trigonal pyramidal

(polar)

See-saw(polar)

Square pyramidal

(polar)

~

o

~

o

~

o (^) ~

o

~

o

~

o

Bent(polar)

~

o

~90 T-shaped(polar)

o

Linear (non-polar)

Square planar (non-polar)

90

o

Summary of molecular polarities for moleculeshaving central atoms with lone pairs