Octet theory

There is no question that, indirectly or directly, Kirrmann and Prevost were influenced by Lowry s theories for explanation of reaction mechanisms. Another important influence was Dupont, with whom they talked at length in the laboratory and who published a paper in 1927 in which he attempted to combine the electron octet theory of valence and Bohr s hydrogen electron model with classical concepts of stereochemistry. Dupont also adopted without reservation Lowry s application of ionic radicals in hydrocarbon chemistry. 66... 

Irving Langmuir, "The Structure of Molecules," BAAS Rep. Edinburgh. 1921 (1922) 468469. G. N. Lewis. "The Atom and the Molecule," 762785 Irving Langmuir, "The Structure of Atoms and the Octet Theory of Valence." Proc. NAS 5 (1919) 252259, and JACS "The Arrangement of Electrons in Atoms and Molecules,"... 

G. N. Lewis s first sketch of the octet theory of valence electrons. 

Structure.—Many possible structures may be assigned to the molecule. Whichever of these is adopted the phosphorus must be quinque-valent according to the old representation, or quadrivalent with a semipolar bond according to the octet theory. Thus... 

Although valency strokes have been customary in chemical formulae for a century, one could not until recently attach to them any real notion about their true nature. On the patient paper one operated with them as with hooks which were undone, rotated etc. at will. Even the Rutherford-Bohr theory of the atom furnished no explanation, not even for the bonding of two hydrogen atoms to form a hydrogen molecule. The successful octet theory and the Lewis and Langmuir theory of the electron-pair bond associated with it was also still purely formal, but later was seen to be essentially correct. 

The above statements formed the basis of the descriptive theory of G. N. Lewis and I. Langmuir on atomic bonding by the formation of electron pairs, in which each atom at the same time forms as far as possible an octet of eight electrons (only hydrogen merely two, He configuration) in connection with the stability of the inert gas configuration (Octet theory). 

In general each atom will be surrounded by four electron pairs (octet theory), either bonding or lone-pair (hydrogen by two electrons). A bond stroke in the formulae always represents a common electron pair while the lone pairs are likewise represented by transverse strokes. The older representation by dots is, however, still largely employed. 

Molecular Properties. Corresponding to its nuclear charge number, the nitrogen atom possesses seven shell electrons. One electron pair in in the ground state 1 s(K shell), and five electrons are distributed over the four orbitals with the principal quantum number 2 (L shell). Of these, one electron pair occupies the 2 s level and three unpaired electrons, respectively, a half of the remaining three levels, 2 px, 2 pv, 2 pz. The unpaired electrons can enter into electron-pair bonds with the 1 s electron of three hydrogen atoms. Thus, the three half occupied orbitals of the L shell become about fully accupied (formation of an octet of the neon type in accordance with the octet theory of Lewis-Langmuir). 

In terms of the classical octet theory originated by G.N. Lewis [9] in 1916, the electronic configuration in pyramidal and tetrahedral phosphorus compounds is completed by an outer shell of eight electrons as indicated in (3.7). 

Langmuir I (1919) The structure of atoms and octet theory of valence. Proc Natl Acad Sci U S A 5 252-259... 

Kohler R (1974) Irving Langmuir and the Octet theory of valence. Hist Stud Phys Sci 4 39-87... 

Langmuir I (1919) Isomorphism, isosterism and covalence. J Am Chem Soc 41 1543-1559 Langmuir I (1920) The octet theory of valence and its applications with special reference to organic nitrogen compounds. J Am Chem Soc 42 274-292... 

In extreme exception to the octet rule, xenon forms, for example, the compounds XeF XeF, XeF, XeOj and XeO. In terms of the expansion of its valence shell this means the promotion of a 5p electron to a 5d orbital to form the divalent state of Xe, the promotion of two 5 p electrons to 5d orbitals to form the four-valent state, the promotion of three 5p electrons to 5d orbitals to give the six-valent state, and the extra uncoupling and promotion of a 5s electron to a fourth 5d orbital to form the eight-valent state. Such electronic juggling does explain the bonding in these exceptional compounds in terms of electron-pair bonds, but there is an alternative approach via molecular orbital theory which does not require these promotions in order to explain the bonding in hypervalent compounds. Tliis is left to more specialist books, except to remark that all hypervalent compounds have bonds to the central atom which are weaker than those in compounds which are consistent with the octet theory, implying that the bonds are not truly electron-pair bonds of the conventional kind. 

E. C. Crocker, Application of the octet theory to single-ring aromatic compounds, J. Am. Chem. Soc. 44 (1922) 1618-1630. 

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