Structures for Exceptions to the Octet Rule

The octet rule is a useful guide for most molecules with Period 2 central atoms, but not for every one. Also, many molecules have central atoms from higher periods. As you ll see, some central atoms have fewer than eight electrons around them, and others have more. The most significant octet rule exceptions are for molecules containing electron-deficient atoms, odd-electron atoms, and especially atoms with expanded valence shells. 

Electron-Deficient Molecules Gaseous molecules containing either beryllium or boron as the central atom are often electron deficient that is, they have/ewer 

There are only four electrons around beryllium and six around boron. Why don t lone pairs from the surrounding halogen atoms form multiple bonds to the central atoms, thereby satisfying the octet rule Halogens are much more electronegative than beryllium or boron, and formal charges show that the following are unlikely structures  

Odd-Electron Molecules A few molecules contain a central atom with an odd number of valence electrons, so they cannot possibly have all their electrons in pairs. Such species, called free radicals, contain a lone (unpaired) electron, which makes them paramagnetic (Section 8.5) and extremely reactive. Let s use formal charges to decide where the lone electron resides. Most odd-electron molecules have a central atom from an odd-numbered group, such as N [Group 5A(15)] or Cl [Group 7A(17)]. 

Consider nitrogen dioxide (NO2) as an example. A major contributor to urban smog is formed when the NO in auto exhaust is oxidized. NO2 has several resonance forms. Two involve the O atom that is doubly bonded, as in the case of ozone. Several others involve the location of the lone electron. Two of these resonance forms are shown below. The form with the lone electron on the singly bonded O has zero formal charges (right)  

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Formal Charge Selecting the More Important Resonance Structure 308 Lewis Structures for Exceptions to the Octet Rule 309... 

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