Chemical bonding and the octet rule

Lewis summarized much of his theory of chemical bonding with the octet rule. According to the octet rule, atoms will lose, gain, or share electrons in order to achieve a noble gas electron configuration. This rule enables us to predict many of the formulas for compounds consisting of specific elements. The octet rule holds for nearly all the compounds made up of second period elements and is therefore especially important in the smdy of organic compounds, which contain mostly C, N, and O atoms. 

The roots of VB theory in chemistry can be traced to the famous paper of Lewis The Atom and The Molecule / which introduces the notions of electron-pair bonding and the octet rule. Lewis was seeking an understanding of weak and strong electrolytes in solution, and this interest led him to formulate the concept of the chemical bond as an intrinsic property of the molecule that varies between the covalent (shared-pair) and ionic situations. Lewis paper predated the introduction of quantum mechanics by 11 years, and constitutes... 

LEWIS SYMBOLS AND THE OCTET RULE We begin with descriptions of the three main types of chemical bonds ionic, covaient, and metaiiic. In evaluating bonding, Lewis symbois provide a useful shorthand for keeping track of valence electrons. 

CHEMICAL BONDS, LEWIS SYMBOLS, AND THE OCTET RULE (INTRODUCTION AND SECTION 8.1) In this chapter we have focused on the interactions that lead to the formation of chemical bonds. We classify these bonds into three broad groups ionic bonds, which result from the electrostatic forces that exist between ions of opposite charge covalent bonds, which result from the sharing of electrons by two atoms and metallic bonds, which result from a delocalized sharing of electrons in metals. The formation of bonds involves interactions of the outermost electrons of atoms, their valence electrons. The valence electrons of an atom can be represented by electron-dot symbols, called Lewis symbols. The tendencies of atoms to gain, lose, or share their valence electrons often follow the octet rule, which says that the atoms in molecules or ions (usually) have eight valence electrons. 

The discussion above has indicated some of the limitations of the original Lewis/ Kossel descriptiOTi of chemical bonding and the manner in which it has been adapted to assimilate the multitude of new compounds being reported from chemical laboratories during the last century. Central to the model is the definition of the chemical bond as a pair of electrons and the adherence to the octet rule. 

Gillespie RJ, Robinson EA (2006) Gilbert N Lewis and the chemical bond the electron pair and the octet rule from 1916 to the present day. J Comput Chem 28 87-97... 

Chemical Bonds, Lewis Symbols, and the Octet Rule... 

Several articles dealing with the work of C. N. Lewis and the nature of the chemical bond may be found in the March 1984 issue of the Journal of Chemical Education. These articles include the following Anthony N. Stranges, "Reflections on the Electron Theory of the Chemical Bond 1900-1925,"/. Chem. Educ, Vol. 61, 1984,185-190. William B. Jensen, "Abegg, Lewis, Langmuir, and the Octet Rule," J. Chem. Educ., Vol. 61, 1984,191-200. Linus Pauling, "G. N. Lewis and the Chemical Bond," J. Chem. Educ,... 

Lewis structure (Section 1 3) A chemical formula in which electrons are represented by dots Two dots (or a line) be tween two atoms represent a covalent bond in a Lewis structure Unshared electrons are explicitly shown and sta ble Lewis structures are those in which the octet rule is sat isfied... 

Lewis s interest in chemical bonding and structure dated from 1902. In attempting to explain "valence" to a class at Harvard, he devised an atomic model to rationalize the octet rule. His model was deficient in many respects for one thing, Lewis visualized cubic atoms with electrons located at the corners. Perhaps this explains why his ideas of atomic structure were not published until 1916. In that year, Lewis conceived of the... 

In 1923. Lewis published a classic book (later reprinted by Dover Publications) titled Valence and the Structure of Atoms and Molecules. Here, in Lewis s characteristically lucid style, we find many of the basic principles of covalent bonding discussed in this chapter. Included are electron-dot structures, the octet rule, and the concept of electronegativity. Here too is the Lewis definition of acids and bases (Chapter 15). That same year, Lewis published with Merle Randall a text called Thermodynamics and the Free Energy of Chemical Substances. Today, a revised edition of that text is still used in graduate courses in chemistry. 

Atoms of these elements have empty J-orbitals in the valence shell. Another factor—possibly the main factor—in determining whether more atoms than allowed by the octet rule can bond to a central atom is the size of that atom. A P atom is big enough for as many as six Cl atoms to fit comfortably around it, and PC15 is a common laboratory chemical. An N atom, though, is too small, and NC15 is unknown. A compound that contains an atom with more atoms attached to it than is permitted by the octet rule is called a hypcrvalent compound. This name leaves open the question of whether the additional bonds are due to valence-shell expansion or simply to the size of the central atom. 

There are several important chemical species that consist of four atoms and have a total of 24 valence-shell electrons. Some of the most common isoelectronic species of this type are C032-, N03 , S03, and P() j (known as the metaphosphate ion). Because four atoms would require a total of 32 electrons for each to have an octet, we conclude that eight electrons must be shared in four bonds. With four bonds to the central atom, there can be no unshared pairs on that atom if the octet rule is to be obeyed. Therefore, we can draw the structure for CO, 2 showing one double C=0 bond and two single C-O bonds as... 

Quantum chemists have developed considerable experience over the years in inventing new molecules by quantum chemical methods, which in some cases have been subsequently characterized by experimentalists (see, for example, Refs. 3 and 4). The general philosophy is to explore the Periodic Table and to attempt to understand the analogies between the behavior of different elements. It is known that for first row atoms chemical bonding usually follows the octet rule. In transition metals, this rule is replaced by the 18-electron rule. Upon going to lanthanides and actinides, the valence f shells are expected to play a role. In lanthanide chemistry, the 4f shell is contracted and usually does not directly participate in the chemical bonding. In actinide chemistry, on the other hand, the 5f shell is more diffuse and participates actively in the bonding. 

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. 

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