Lewis structures with extra electrons

Sometimes, as with PCI5 and SFg, it is clear from the formula that the central atom has an expanded octet. Often, however, it is by no means obvious that this is the case. At first glance, formulas such as CIF3 or XeF4 look completely straightforward. However, when you try to draw the Lewis structure it becomes clear that an expanded octet is involved. The number of electrons available after the skeleton is drawn is greater than the number required to give each atom an octet. When that happens, distribute the extra electrons (two or four) around the central atom as unshared pairs. 

Students sometimes assume (mistakenly) that chemical bonding is completed once the electrons are maximally paired up in a closed-shell species of valid Lewis structural form. The error of this assumption was recognized nearly a century ago with discovery of numerous complexes that defied Lewis structural formulation, unless written as two (or more) distinct species. Such complexes therefore appear to violate the valence rules that usually govern chemical stmcture and reactivity, apparently involving some type of extra-valence Nebenvalenz, in the phrase of German inorganic chemist Alfred Werner) that demands significant extension of Lewis structural concepts. Nowadays, the term hypervalency is commonly used to describe species that have too many bonds for conventional Lewis structural depiction, or seem to require chemical association mechanisms beyond those of closed-shell Lewis structure formation. 

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