Molecular geometry
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Molecular Geometry. VSEPR model. V alence S hell E lectron P air R epulsion Shape of molecule is determined by the repulsion of electron pairs around the central atom . The repulsion must be minimum , i.e. Electron pairs are farthest from each other ( potential energy minimum ).

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Molecular Geometry

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Molecular geometry

Molecular Geometry

VSEPR model

  • Valence Shell Electron Pair Repulsion

  • Shape of molecule is determined by the repulsion of electron pairs around the central atom.

  • The repulsion must be minimum, i.e. Electron pairs are farthest from each other (potential energy minimum).


Rules

Rules

  • Draw the right Lewis Structure (formal charge!).

  • Determine the number of electron pairs around the central atom.

    • Multiple bonds are treated as if they were one electron pair because these electron pairs can not be separated from each other

    • 2, 3, 4, 5 or 6 EP.


Two ep around central atom ax 2

CA

Two EP around Central Atom (AX2)

BeCl2

ClBeCl

2 EP

Cl

Be

Cl

Shape:linearBond angleCl-Be-Cl = 180º

X-A-X


Three ep around central atom ax 3

F

F B F

CA

Three EP around Central Atom (AX3)

3 EP

AX3

BF3

F

B

F

F

Shape:planar triangularBond angleF-B-F = 120º

X-A-X


Three ep around central atom ax 31

O

S O

CA

Three EP around Central Atom (AX3)

3 EP

AX2E

SO2

S

O

O

<

Shape:V-shaped (bent)Bond angleO-S-O = 120º

X-A-X

Lone EP occupies a larger space than bonding EP.


Four ep around central atom ax 4

H

C

H

H

H

CA

Four EP around Central Atom (AX4)

4 EP

AX4

CH4

H

C

H

H

H

Shape:tetrahedralBond angleH-C-H = 109.5º

X-A-X


Four ep around central atom ax 41

N

H

H

H

CA

N

H

H

H

Four EP around Central Atom (AX4)

AX3E

4 EP

NH3

<

Shape:trigonal pyramidalBond angleH-N-H = 109.5º

X-A-X

Lone EP occupies a larger space than bonding EP.


Four ep around central atom ax 42

O

H

H

CA

O

H

H

Four EP around Central Atom (AX4)

AX2E2

4 EP

H2O

<

Shape:V-shapedBond angleH-O-H = 109.5º

X-A-X

Lone EP occupies a larger space than bonding EP.


Five ep around central atom ax 5

Cl

Cl P Cl

Cl Cl

CA

Five EP around Central Atom (AX5)

AX5

5 EP

PCl5

Cl

Cl

Cl

P

Cl

Cl

Shape:trigonal bipyramidalBond angleCl-P-Cl = 120º, 90º

X-A-X


Five ep around central atom ax 51

F S F

F F

F

CA

F

S

F

F

Five EP around Central Atom (AX5)

AX4E

5 EP

SF4

Lone EP occupies a larger space than bonding EP.

Shape:disordered tetrahedralBond angleCl-P-Cl = 120º, 90º

X-A-X


Five ep around central atom ax 52

Cl I Cl

Cl

Cl

CA

Cl

I

Cl

Five EP around Central Atom (AX5)

ICl3

5 EP

AX3E2

Lone EP occupies a larger space than bonding EP.

Shape:T-shapedBond angleCl-P-Cl = 120º, 90º

X-A-X


Five ep around central atom ax 53

Cl I Cl

Cl

CA

I

Cl

Five EP around Central Atom (AX5)

ICl2-

5 EP

AX2E3

Lone EP occupies a larger space than bonding EP.

Shape:linearBond angleCl-I-Cl = 180º

X-A-X


Six ep around central atom ax 6

FF

FSF

FF

CA

Six EP around Central Atom (AX6)

SF6

6 EP

AX6

F

F

F

S

F

F

F

Shape:octahedralBond angleF-S-F = 90º

X-A-X


Six ep around central atom ax 61

ClCl

ClI

ClCl

CA

Six EP around Central Atom (AX6)

ICl5

6 EP

AX5E

Cl

Cl

Cl

I

Cl

Cl

Shape:square pyramidalBond angleCl-I-Cl = 90º

X-A-X


Six ep around central atom ax 62

F F

Xe

F F

CA

Six EP around Central Atom (AX6)

XeF4

6 EP

AX4E2

F

F

Xe

F

F

Shape:square planarBond angleF-Xe-F= 90º

X-A-X


Polarity of molecules

Polarity of molecules

  • Molecules polar if net dipole moment ≠ 0

  • If molecule is “pure” AXn type

    • No lone EP

    • All surrounding atoms are the same element

      non-polar

m1

m3

m2

m1

m2


Molecular geometry

Usually, if there is a lone EP  polar

NH3

AX3E

H2O

AX2E2


Molecular geometry

Exceptions:

Cl

AX2E3

I

Cl

F

F

AX4E2

Xe

F

F


Molecular geometry

Different ligand atoms

Cl

m3

BF2Cl

m1+m2 > m3

m1

B

F

Net dipole moment ≠ 0

m2

m1+m2

F

XeF2Cl2

F

F

Cl

Cl

Xe

Xe

Cl

Cl

F

F

nonpolar

polar


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