# Exceptions to the Octet Rule

There are three general ways in which the octet rule breaks down:

1. Molecules with an odd number of electrons
2. Molecules in which an atom has less than an octet
3. Molecules in which an atom has more than an octet

### Odd number of electrons

Draw the Lewis structure for the molecule nitrous oxide (NO):

1. Total electrons: 6+5=11

2. Bonding structure:

3. Octet on "outer" element:

4. Remainder of electrons (11-8 = 3) on "central" atom:

5. There are currently 5 valence electrons around the nitrogen. A double bond would place 7 around the nitrogen, and a triple bond would place 9 around the nitrogen.

We appear unable to get an octet around each atom

### Less than an Octet

This is most often encountered with elements of Boron and Beryllium.

Draw the Lewis structure for boron trifluoride (BF3):

1. Add electrons (3*7) + 3 = 24

2. Draw connectivities:

3. Add octets to outer atoms:

4. Add extra electrons (24-24=0) to central atom:

5. Does central electron have octet?

• NO. It has 6 electrons
• Add a multiple bond (double bond) to see if central atom can achieve an octet:

6. The central Boron now has an octet (there would be three resonance Lewis structures)

However...

• In this structure with a double bond the fluorine atom is sharing extra electrons with the boron.
• The fluorine would have a '+' partial charge, and the boron a '-' partial charge, this is inconsistent with the electronegativities of fluorine and boron.
• Thus, the structure of BF3, with single bonds, and 6 valence electrons around the central boron is the most likely structure

BF3 reacts strongly with compounds which have an unshared pair of electrons which can be used to form a bond with the boron:

### More than an octet

This is most common example of exceptions to the octet rule.

PCl5 is a legitimate compound, whereas NCl5 is not.

Expanded valence shells are observed only for elements in period 3 (i.e. n=3) and beyond

• The 'octet' rule is based upon available ns and np orbitals for valence electrons (2 electrons in the s orbitals, and 6 in the p orbitals)
• Beginning with the n=3 principal quantum number, the d orbitals become available (l=2)

The orbital diagram for the valence shell of phosphorous is:

Third period elements occasionally exceed the octet rule by using their empty d orbitals to accommodate additional electrons

Size is also an important consideration:

• The larger the central atom, the larger the number of electrons which can surround it
• Expanded valence shells occur most often when the central atom is bonded to small electronegative atoms, such as F, Cl and O.

Draw the Lewis structure for ICl4-

1. Count up the valence electrons: 7+(4*7)+1 = 36 electrons

2. Draw the connectivities:

3. Add octet of electrons to outer atoms:

4. Add extra electrons (36-32=4) to central atom:

5. The ICl4- ion thus has 12 valence electrons around the central Iodine (in the 5d orbitals)