Lewis Structures for Exceptions to the Octet Rule
(a) Electron-Deficient Molecules: gaseous molecules containing
either Be or B as the central atom; have fewer than 8 electrons
around the Be or B (4 e- around Be and 6 e- around B) (BF3).
(b) Odd-Electron Molecules: have an odd number of valence
electrons; examples include free radicals, which contain a lone
(unpaired) electron and are paramagnetic (use formal charges to
locate the lone electron) (NO2).
(c) Expanded Valence Shells: for molecules that have more than
8 electrons around the central atom; use empty outer d orbitals;
occurs only with a central atom from Period 3 or higher (SF6, PCl5).
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