Exceptions to the Octet Rule

Exceptions to the Octet Rule

1. Elements 1-5 (will only attempt to get 2 valence electrons, not 8)

2. Odd number of valence electrons.Ex. ClO2

3. Expanded octet (more than octet)Ex. XeF6

Exceptions to the Octet Rule

1. Elements 1-5 (will only attempt to get 2 valence electrons, not 8)

BH3

B H

H

H

3 e- on B, 1e- on each H = 6 e- or 3 pair.

All 3 pair are accounted, but Octet is not satisfied.

Exceptions to the Octet Rule

2. Odd number of valence electrons.Ex. ClO2

7 e- + 2(6 e-) = 19 e- or 9.5 pair

Cl OO2 pair, what of the other 7.5?

If 6 go on the oxygen?

The remaining 1.5 pair go on the Chlorine.

Exceptions to the Octet Rule

2. Odd number of valence electrons.Ex. ClO2

Cl OO

When there is an odd # of e- the unpaired or odd e- generally goes on the central atom.

If it went on the Oxygen, how would we decide which oxygen atom?

This is why it goes on the central atom.

Exceptions to the Octet Rule

3. Expanded octet (more than octet)Ex. XeF6

8 e- + 6(7 e-) = 50 e- or 25 pair e-

25 pair e-

XeF F

FF

FF

When we place Xe in the center and satisfy the Octet for the 6 F, we only have placed 24 pair. The remaining 1?

Exceptions to the Octet Rule

3. Expanded octet (more than octet)Ex. XeF6 25 pair e-

XeF F

FF

FF

Just like when there is an odd # of e-, the extended octet and any unbonded pair of electrons go on the central atom.

Exceptions to the Octet Rule

Molecular Shapes

As we practice VSEPR drawings, there are problems…

we can only draw in 2-Dimensions.

Real molecules are in 3-D.

This means that when the electrons repel (the R in VSEPR) they will not take the shapes that we see in the Lewis Diagrams.

TetrahedralEx: CCl4Lewis Diagram

Actual Molecule

Tetrahedral Tetra = 4 (Greek Prefix)

4 pointed pyramid

5-bonded atoms

No unbonded e- pair on central atom

Examples:Methane (CH4), Carbon tetrachloride (CCl4)

Trigonal PyramidalEx: NH3

Trigonal PyramidalTri= 3 4 pointed pyramid4 bonded atoms1 unbonded pair of e- on central atom

Example: Ammonia (NH3)

OHH

(Modified tetrahedral as the unbonded pair cause the bonded atoms to bend.)

Another example: O3, ozone

BentEx: H2O

Bent3 bonded atoms

2 pair of unbonded e- on the central atom

Example: Water (H2O)

LinearEx: CO2 and N2

O C O

N N

Linear Straight line

2 or more bonded atoms

No unbonded e- pair on central atom

Often double bond or triple bond

Examples: Dinitride (N2), Carbon dioxide (CO2)

Other shapes