Molecular Geometry(Shapes of Molecules)

http://www.scl.ameslab.gov/MacMolPlt/Surface.JPG

VSEPR Theory

CP Chemistry

VSEPR Theory

• Electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory– because electrons are negatively

charged, they should be most stable when they are separated as much as possible

• The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule

Electrons vs. Molecular Geometry

• The geometry of electron pairs around a central atom is called the electron geometry.

• The arrangement of bonded nuclei around a central atom forms the molecular geometry.

• Lone pair electrons on a central atom will repel other pairs but will not be visible in the molecular geometry (no nuclei)

• If there are lone pairs on the central atom the electron geometry and the molecular geometry will differ.

Examples:

CS2, HCN, BeF2

Two electron pairs on central atom

3 electronpairs on central atom•All are in bonds

•Trigonal Planar Shape

Examples:

SO3, BF3, NO3-, CO3

2-

Other Examples:

•SO2, O3, PbCl2, SnBr2

• Un-bonded electron pair takes up more space and “repels” more.

• The bond angle will change to less than the original 120o to about 117o-115o

About 117o

3 Electron Pairs•2 Bonded •1 Un-Bonded

Bent Shape

Four electron pairs on central atom•All 4 in bonds

Examples:

CH4, SiCl4, SO4

2-, ClO4-

Examples: NH3, PF3, ClO3. H3O+

4 Electron Pairs•3 bonded•1 un-bonded (lone pair)Trigonal Pyramidal Shape

Bond angles are reduced from 109.5o to 107o due to extra repulsion by lone pair

Examples: H2O, OF2, SCl2

4 Electron Pairs2 Bonded2 Un-bonded (lone pairs)BENT SHAPE

Bond angles are reduced a little more due to repulsionTo 104.5o

The steps in determining a molecular shape

Molecular formula

Lewis structure

Electron-group arrangement

(electron geometry) Bond

angles

Molecular geometry

Count all e- pairs around central atom

Note lone pairs and double bonds

Consider bonding e- pairs only

Step 1

Step 2

Step 3

Step 4

Representing 3-Dimensional Shapes on a 2-Dimensional

Surface• One of the problems with drawing molecules is trying to show their dimensionality

• By convention, the central atom is put in the plane of the paper

• Put as many other atoms as possible in the same plane and indicate with a straight line

• For atoms in front of the plane, use a solid wedge

• For atoms behind the plane, use a hashed wedge

Predicting Molecular Shapes with Two, Three, or Four Electron Groups

PROBLEM: Draw the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) PF3

SOLUTION: (a) For PF3 - there are 26 valence electrons, 1 nonbonding pair

PF F

F

The shape is based upon the tetrahedral arrangement.

The F-P-F bond angles should be <109.50 due to the repulsion of the nonbonding electron pair.

The final shape is trigonal pyramidal.

PF F

F

< 109.50

Predicting Molecular Shapes with More Than One Central Atom

SOLUTION:

PROBLEM: Determine the shape around each of the central atoms in acetone, (CH3)2C=O.

Find the shape of one atom at a time after writing the Lewis structure.

C C C

OH

H

H

HH

H

tetrahedral tetrahedral

trigonal planar

C

O

HC

HHH

CH

H>1200

<1200