Breaking the Octet Rule

Octet Rule Exceptions

Atoms that can break the octet rule:

• Generally atoms under row two of the periodic table.
  • Examples: Phosphorus (P), Bromine (Br), Iodine (I).
  • Some noble gases can also break the octet rule.

Example 1: PO4^3-

• Phosphorus (P) has 5 valence electrons.
• Initial bonding:
  • Phosphorus forms three single bonds with three oxygen atoms.
• Breaking a lone pair:
  • Phosphorus breaks a lone pair to form a double bond with one oxygen atom.
• Current Lewis structure (incomplete):
  • One double bond to an oxygen.
  • Three single bonds to three other oxygen atoms.
• Considering the 3- charge:
  • Add three extra electrons, one to each of the singly-bonded oxygen atoms.
  • Each of these oxygen atoms now has a negative charge.
• Final Lewis structure:
  • [O=P(O^-)_3] (with appropriate lone pairs on oxygen atoms)
• Valence electron count around phosphorus:
  • 2 (from double bond) + 3 * 2 (from single bonds) = 10 valence electrons
  • Phosphorus violates the octet rule in this case.

Example 2: Chlorine

• Central atom selection:
  • Chlorine (Cl) is the central atom because it is the bigger atom.
• Valence electron count around Chlorine:
  • Chlorine forms five bonds with five Fluorine atoms.
• Breaking lone pairs:
  • Chlorine breaks apart that last lone pair with fluorine to rearrange bonds.
• Lewis structure:
  • ClF5
• Valence electron count around the chlorine:
  • There is a total of 10 electrons around the central atom, which means the octet rule is violated.
  • Chlorine is violating the octet rule because it's under row two on the periodic table, which is allowed.