Lewis Structures for Exceptions to the Octet Rule

The octet rule works well for molecules whose central atom is a second-period element, but it has three well-known families of exception.

1. Molecules with electron-deficient central atoms. Consider boron trifluoride, BF₃. Boron has three valence electrons; each fluorine has seven. Drawing three B-F single bonds and giving each fluorine its full octet (three lone pairs) leaves only SIX electrons around boron (Fig 10.8(a)) -- short of an octet. One fix is to move a lone pair from a fluorine to form an additional B=F bond, completing boron's octet (Fig 10.8(b)) -- but this is actually the LESS favourable structure, because it forces a positive formal charge onto fluorine, the most electronegative atom present (violating guideline 3 above). So the genuinely favoured structure for BF₃ is the one with boron's octet INCOMPLETE. Molecules such as BCl₃ and BeCl₂ are analogous examples with incomplete octets.

2. Molecules containing odd electrons. A few molecules have a central atom with an ODD number of total valence electrons, so no arrangement of dots can pair every electron and give every atom a full octet. Nitric oxide (NO) and nitrogen dioxide (NO₂) are the textbook examples -- in both, not every atom achieves a complete octet, and one electron is left unpaired (Fig 10.9). …