Molecular Geometry2-2

Ch. 6 – Molecular shape

• Molecules are three-dimensional objects that occupy a three-dimensional world; In general, only the smallest molecules can be said to have a fixed geometrical shape; the icosahedral C60 “soccer ball” is a rare exception.

VSEPR Theory

• Valence Shell Electron Pair Repulsion Theory- focuses on the bonding and nonbonding electron

pairs present in the outermost (“valence”) shell of an atom to which are connected two or more other atoms.

• Electron pairs orient themselves in order to minimize repulsive forces.

VSEPR Theory

• Types of e- Pairs– Bonding pairs - form bonds– Lone pairs - nonbonding e-

Lone pairs repel

more strongly than

bonding pairs!!!

• Draw the Lewis Diagram.

• Shape is determined by the # of bonding pairs and lone pairs.

Know the 5 common shapes !

Determining Molecular Shape

1. Common Molecular Shapes

2 bond

0 lone

LINEAR180°AB2

Linear molecules: AB2

• Ex: BeCl2 and CO2.

• -If you write out the electron dot formula for carbon dioxide, you will see that the C-O bonds are double bonds.

Double bonds are treated

like single bonds when

shape is determined!!!!!!

Be

180°

180°

Be90°

270°

The shape of BeCl2 is linear.

BeCl2

TWO ELECTRON PAIRS AROUND BERYLLIUM ATOM

Cl Be Cl

THE MOLECULAR SHAPE IS BASED ON THE POSITION OF THE ATOMS!

• CO2

O C O2 bond

0 loneLINEAR

180°

3 bond

0 lone

TRIGONAL PLANAR

120°

AB3

2. Common Molecular Shapes

Trigonal planar: AB3

• In the molecule BF3, there are three regions of electron density extending out from the central boron atom. The repulsion between these will be at a minimum when the angle between any two is 120°. This requires that all four atoms be in the same plane; the resulting shape is called trigonal planar.

B

120°

120°

120°

F F

F

B

THE SHAPE OF BF3 IS TRIGONAL PLANAR.

BF3 NO OCTET ON B !

THREE ELECTRON PAIRS AROUND THE BORON ATOM.

MOLECULAR SHAPE

F B F

F

ATTACH FLOURINES TO EPG.

3. Common Molecular Shapes

2 bond

1 lone

BENT

<120°

AB2E

AB2E2

Shape with lone pairs: AB2E2

2 bonding electrons and 2 lone pairs

or AB2E , 2 bonding electrons and 1

lone pair • Bent:

• The nonbonding electrons are also in orbitals that occupy space and repel the other orbitals.

O Se OSeO2

SeO2 IS V-SHAPED, OR BENT

Se

O O

VSEPR treats double bonds like a single bond

Se

O O

Se

O O

RESONANCE!

2 bond

2 lone

BENT

104.5°

H2O

Common Molecular Shapes

4 bond

0 lone

TETRAHEDRAL

109.5°

AB4

4. Common Molecular Shapes

Tetrahedral: AB4

• Methane, CH4, contains a carbon atom to which are connected four hydrogens. Consequently, the four equivalent bonds will point in four equivalent directions in three dimensions.

CH4

H C H

H

HThere are four

electron pairs

around the

carbon atom.

90°

C

90° 90°

90°

BUT……….

There is a better arrangement for four electron pairs:

109.5°

C

TETRAHEDRAL

The angle is…..

Put on the H-atoms…….

There is a better arrangement for four electron pairs:

TETRAHEDRAL

4 electron pairs tetrahedral EPG

C

H

HH

H

The shape of CH4 is tetrahedral.

3 bond

1 lone

TRIGONAL PYRAMIDAL

107°

AB3E

5. Common Molecular Shapes

• PF3

3 bond

1 lone

TRIGONAL PYRAMIDAL

107°

F P FF

Polyatomic ions

• The charge will dictate how many less or extra electrons will be added to the lewis dot diagrams.

• NO3-

• the negative charge tells you that you need to add one more electron to the diagram.

5 + 3(6) + 1 = 24

Polyatomic ions

Nitrate anion

N

O O

O

VSEPR treats multiple bonds as effective single electron pairs.Trigonal planar is the shape