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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

- 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 (AX2)

BeCl2

Cl Be Cl

2 EP

Cl

Be

Cl

Shape:linear Bond angleCl-Be-Cl = 180º

X-A-X

F B F

CA

Three EP around Central Atom (AX3)3 EP

AX3

BF3

F

B

F

F

Shape:planar triangular Bond angleF-B-F = 120º

X-A-X

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.

C

H

H

H

CA

Four EP around Central Atom (AX4)4 EP

AX4

CH4

H

C

H

H

H

Shape:tetrahedral Bond angleH-C-H = 109.5º

X-A-X

H

H

H

CA

N

H

H

H

Four EP around Central Atom (AX4)AX3E

4 EP

NH3

<

Shape:trigonal pyramidal Bond angleH-N-H = 109.5º

X-A-X

Lone EP occupies a larger space than bonding EP.

H

H

CA

O

H

H

Four EP around Central Atom (AX4)AX2E2

4 EP

H2O

<

Shape:V-shaped Bond angleH-O-H = 109.5º

X-A-X

Lone EP occupies a larger space than bonding EP.

Cl P Cl

Cl Cl

CA

Five EP around Central Atom (AX5)AX5

5 EP

PCl5

Cl

Cl

Cl

P

Cl

Cl

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

X-A-X

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 tetrahedral Bond angleCl-P-Cl = 120º, 90º

X-A-X

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-shaped Bond angleCl-P-Cl = 120º, 90º

X-A-X

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:linear Bond angleCl-I-Cl = 180º

X-A-X

F S F

F F

CA

Six EP around Central Atom (AX6)SF6

6 EP

AX6

F

F

F

S

F

F

F

Shape:octahedral Bond angleF-S-F = 90º

X-A-X

Cl I

Cl Cl

CA

Six EP around Central Atom (AX6)ICl5

6 EP

AX5E

Cl

Cl

Cl

I

Cl

Cl

Shape:square pyramidal Bond angleCl-I-Cl = 90º

X-A-X

Xe

F F

CA

Six EP around Central Atom (AX6)XeF4

6 EP

AX4E2

F

F

Xe

F

F

Shape:square planar Bond angleF-Xe-F= 90º

X-A-X

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

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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