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Molecular Geometry

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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).

- 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.

CA

BeCl2

ClBeCl

2 EP

Cl

Be

Cl

Shape:linearBond angleCl-Be-Cl = 180º

X-A-X

F

F B F

CA

3 EP

AX3

BF3

F

B

F

F

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

X-A-X

O

S O

CA

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.

H

C

H

H

H

CA

4 EP

AX4

CH4

H

C

H

H

H

Shape:tetrahedralBond angleH-C-H = 109.5º

X-A-X

N

H

H

H

CA

N

H

H

H

AX3E

4 EP

NH3

<

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

X-A-X

Lone EP occupies a larger space than bonding EP.

O

H

H

CA

O

H

H

AX2E2

4 EP

H2O

<

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

X-A-X

Lone EP occupies a larger space than bonding EP.

Cl

Cl P Cl

Cl Cl

CA

AX5

5 EP

PCl5

Cl

Cl

Cl

P

Cl

Cl

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

X-A-X

F S F

F F

F

CA

F

S

F

F

AX4E

5 EP

SF4

Lone EP occupies a larger space than bonding EP.

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

X-A-X

Cl I Cl

Cl

Cl

CA

Cl

I

Cl

ICl3

5 EP

AX3E2

Lone EP occupies a larger space than bonding EP.

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

X-A-X

Cl I Cl

Cl

CA

I

Cl

ICl2-

5 EP

AX2E3

Lone EP occupies a larger space than bonding EP.

Shape:linearBond angleCl-I-Cl = 180º

X-A-X

FF

FSF

FF

CA

SF6

6 EP

AX6

F

F

F

S

F

F

F

Shape:octahedralBond angleF-S-F = 90º

X-A-X

ClCl

ClI

ClCl

CA

ICl5

6 EP

AX5E

Cl

Cl

Cl

I

Cl

Cl

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

X-A-X

F F

Xe

F F

CA

XeF4

6 EP

AX4E2

F

F

Xe

F

F

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

X-A-X

- 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

Usually, if there is a lone EP polar

NH3

AX3E

H2O

AX2E2

Exceptions:

Cl

AX2E3

I

Cl

F

F

AX4E2

Xe

F

F

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