Chemical bonding incomplete octets

Empirically measured parameters are additional solvent properties, which have been developed through the efforts of physical chemists and physical organic chemists in somewhat different, but to some extent related, directions. They have been based largely on the Lewis acid base concept, which was defined by G. N. Lewis. The concept originally involved the theory of chemical bonding which stated that a chemical bond must involve a shared electron pair. Thus, an atom in a molecule or ion which had an incomplete octet in the early theory, or a vacant orbital in quantum mechanical terms, would act as an electron pair acceptor (an acid) from an atom in a molecule or ion which had a complete octet or a lone pair of electrons (a base). Further developments have included the concepts of partial electron transfer and a continuum of bonding from the purely electrostatic bonds of ion-ion interactions to the purely covalent bonds of atoms and molecules. The development of the concept has been extensively described (see Ref. 11 for details). 

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. 

I. As elements, both sodium and chlorine are very reactive due to their electron configurations. Their reactivity makes them harmful to biological systems. Sodium has one valence electron, while chlorine has seven valence electrons. Thus, both elements have incomplete octets. Sodium will lose one electron and chlorine will gain one electron to form a chemical bond. Both atoms are now stable due to octets in their outer shell. Therefore, sodium and chloride when bonded together are relatively harmless. 

In view of its molecular properties and chemical behavior, the best representation of BF3 appears to be a resonance hybrid of structures (10.20,10.21, and 10.22), with perhaps the most important contribution made by the structure with an incomplete octet (10.20). Whichever BF3 structure we choose to emphasize, an important characteristic of BF3 is its strong tendency to form a coordinate covalent bond with a species capable of donating an electron pair to the B atom. This can be seen in the formation of the BF4 ion. 

Many chemical models are known to be incorrect, for example, the Lewis theory of bonding, or an incomplete description of chemical phenomena, for example, the octet rule (Chapter 4). For the purposes of teaching chemistry it is often preferable to use simple approximate bonding models that give correct predictions for the majority of cases, than to use a more accurate but complicated model, such as the quantum mechanical model (Chapter 12), which is based on the electron s wave properties. 

---