Exceptions to the Octet Rule

Beryllium and boron form stable compounds with fewer than eight electrons around the central atom. BeCl₂ leaves beryllium with 4 electrons and BF₃ leaves boron with 6. These molecules are strong Lewis acids precisely because the centre is short of electrons, which is why BF₃ reacts eagerly with ammonia.

For BF₃, the budget is 3 + 3(7) = 24 electrons. Three B–F bonds use six; completing the fluorine octets uses eighteen, leaving none for B. Boron therefore has six surrounding electrons in the usual neutral Lewis drawing. It can accept a lone pair from NH₃ to form an adduct, illustrating Lewis acidity. Calling BF₃ electron deficient does not mean you may add an uncounted pair to its original neutral structure.

An odd total cannot be partitioned entirely into pairs. NO has 5 + 6 = 11 valence electrons and NO₂ has 5 + 2(6) = 17, so each requires an odd-electron Lewis description. Electron parity identifies the radical before deciding exactly where the unpaired electron is represented.

For a useful NO drawing, a double bond contains four electrons, two lone pairs on O contain four, and one lone pair plus one unpaired electron on N contain three: eleven total. O has an octet and N has seven surrounding electrons. Adding an electron to finish nitrogen would change neutral NO into a different charged species. Radical placement and delocalization must be assessed from valid structures; the unpaired electron is not universally assigned by electronegativity alone.

Some Lewis drawings for period-3 and heavier central atoms, including PCl₅ and SF₆, assign more than eight electrons around the center. These are electron-bookkeeping models. The historical explanation invoking readily accessible d orbitals is not a required modern account of hypervalent bonding; use delocalized bonding models for that purpose. Second-period C, N, O and F cannot exceed eight electrons in ordinary Lewis structures.

For SF₆ the budget is 6 + 6(7) = 48 electrons. Six S–F bonds use twelve and eighteen fluorine lone pairs use thirty-six, exhausting the budget. The drawing assigns twelve bonding electrons around sulfur. It does not require extra electron pairs invented after counting. Period-2 N, O and F do not have corresponding ordinary ten- or twelve-electron Lewis centers. Period 3 is permission to consider expanded-valence drawings, not an instruction to expand every atom from that period.

An electron shortage at carbon is not the same exception as boron's usual electron deficiency. Complete the count and check formal charges before declaring a proposed structure an exception. Stable species do not all obey the octet rule, but “exception” cannot excuse an incorrect total electron count.