Chemical bonding octet rule, exceptions

The octet rule is one of the cornerstones of chemical bonding theory. While the vast majority of molecules conform, apparent exceptions occur for molecules incorporating second-row (and heavier) main-group elements. Apparent refers to the fact that molecules such as dimethylsulfoxide and dimethylsulfone may either be represented in terms of structures with ten and twelve valence electrons, respectively, surrounding sulfur, or as zwitterions with the normal complement of eight valence electrons (see also discussions in Chapters 5 and 16). 

There are many exceptions to the octet rule—after all, it s called the octet rule, not the octet law—but it is nevertheless useful for making predictions and for providing insights about chemical bonding. 

The octet rule works mainly for elements in the second period of the periodic table. These elements have only 2s and 2p subsheUs, which can hold a total of eight electrons. When an atom of one of these elements forms a covalent compound, it can attain the noble gas electron configuration [Ne] by sharing electrons with other atoms in the same compound. Later, we will discuss a number of important exceptions to the octet rule that give us further insight into the nature of chemical bonding. 

The octet rule for chemical bonding was mentioned briefly in Chapter 3. At this point, it will be useful to examine this rule in more detafl. Although there are many exceptions to it, the octet rule remains a valuable concept for an introduction to chemical bonding. 

Exceptions to the Octet Rule The octet rule applies mainly to the second-period elements. The three cat ories of exceptions to the octet rule are the incomplete octet, in which an atom in a molecule has fewer than eight valence electrons, the odd-electron molecules, which have an odd number of valence electrons, and the expanded octet, in which an atom has more than eight valence electrons. These exceptions can be explained by more refined theories of chemical bonding. 

Hybridization of Atomic Orbitals Hybridization is the quantum mechanical description of chemical bonding. Atomic orbitals are hybridized, or mixed, to form hybrid orbitals. These orbitals then interact with other atomic orbitals to form chemical bonds. Various molecular geometries can be generated by different hybridizations. The hybridization concept accounts for the exception to the octet rule and also explains the formation of double and triple bonds. 

Atoms with eight valence electrons are particularly stable and are said to have an octet. Atoms such as hydrogen, helium, lithium, and beryllium are exceptions to the octet rule as they achieve stability when their outermost shell contains two electrons—a duet. A chemical bond is the sharing or transfer of electrons to attain stable electron configurations among the bonding atoms. 

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