Completing octet

Multiple bonds are very common m organic chemistry Ethylene (C2H4) contains a carbon-carbon double bond m its most stable Lewis structure and each carbon has a completed octet The most stable Lewis structure for acetylene (C2H2) contains a carbon-carbon triple bond Here again the octet rule is satisfied... 

For example, nitrogen ( N ) has five valence electrons and needs three more electrons to complete its octet. Chlorine (-CL) has seven valence electrons and needs one more electron to complete its octet. Argon OArO already has a complete octet and has no tendency to share any more electrons. Hydrogen (H-) needs one more electron to reach its helium-like duplet. Because hydrogen completes its duplet by sharing one pair of electrons, we say that it has a valence of 1 in all its compounds. In general, the valence of an element is the number of bonds that its atoms can form. 

Model A cannot be eliminated definitely by the photographs there are, however, some points which make this model improbable. From the curve for this model the first minimum would be expected to be at least as well pronounced as the second minimum, whereas on the photographs the first minimum is not very well defined. That the qualitative appearance of the photographs supports model C rather than model A is further shown by the fact that the photographs resemble those of methyl nitrate more closely than those of carbon tetrafluoride. Some evidence is also provided by the radial distribution curve (Fig. 1), the first peak being displaced by 0.15 A. from the position expected for it for model A. For these reasons and the additional reason that it is difficult to correlate the tetrahedral configuration with an electronic structure involving only completed octets, we consider model A not to be satisfactory.7... 

There are four types of organic species in which a carbon atom has a valence of only 2 or 3/ They are usually very short lived, and most exist only as intermediates that are quickly converted to more stable molecules. However, some are more stable than others and fairly stable examples have been prepared of three of the four types. The four types of species are carhocations (A), free radicals (B), carbanions (C), and carbenes (D). Of the four, only carbanions have a complete octet around the carbon. There are many other organic ions and radicals with charges and unpaired electrons on atoms other than carbon, but we will discuss only nitrenes (E), the nitrogen analogs of carbenes. 

This rule, which is an extension of the (8-N) rule for the elements (17), was developed on an empirical basis (18—21) and later shown (22) to be a mathematical formulation of the requirement that there be complete octets on all A s. Moreover, the rule can be extended (22) to allow configurations other than octets ... 

In the BeCl2 monomer, there are only two bonds to Be, which gives only four electrons around the atom. In the bridged structure, unshared pairs of electrons on Cl are donated to complete octets around Be atoms. 

Four bonding pairs of electrons surround the central boron atom in this structure. This arrangement gives the boron atom a complete octet and a formal charge of 1-By virtue of being surrounded by three lone pairs and one bonding electron pair, each fluorine achieves a full octet. 

Oxygen is moved closer to a complete octet by placing the remaining pair of electrons on oxygen as a lone pair. 

At this stage, the valence shells for the two oxygen atoms are closed, but the sulfur atom is two electrons short of a complete octet. If we complete the octet for sulfur by converting a lone pair of electrons on the right hand side oxygen atom into a sulfur-to-oxygen n-bond, we end up generating the resonance contributor (A) shown below ... 

At this stage the valence shells for the oxygen atoms are filled, but the nitrogen atom is two electrons short of a complete octet. If we complete the octet for nitrogen by... 

Included in the "alternative strategy" is the assumption that the central atom does not form double bonds with any of the terminal atoms. This means that in many instances, the central atom does not possess a complete octet. The presence or absence of an octet is, however, of no consequence to the VSEPR method because, according to the tenets of this theory, the shape adopted by the molecule is determined solely by the number and types of electron pairs on the central atom. Examples follow on the next two pages. 

The outer shell of the helium atom is full and complete the shell can only accept two electrons and, indeed, is occupied by two electrons. Similarly, argon has a complete octet of electrons in its outer shell. Further reaction would increase the number of electrons if argon were to undergo a covalent bond or become an anion, or would decrease the number of electrons below the perfect eight if a cation were to form. There is no impetus for reaction because the monatomic argon is already at its position of lowest energy, and we recall that bonds form in order to decrease the energy. 

The reaction of magnesium, with two valence electrons, and chlorine, with seven valence electrons will produce magnesium chloride. The magnesium must donate one valence electron to each of two chlorine atoms. This leaves a magnesium ion and two chloride ions. All the ions have a complete octet. The ions form the ionic compound magnesium chloride, MgCl2. 

Now we will apply this formal-charge concept to the cyanate ion OCN We chose this example because many students incorrectly write the formula as CNO , and then try to use this as the atomic arrangement in the Lewis structure. Based on the number of electrons needed, the carbon should be the central atom. We will work this example using both the incorrect atom arrangement and the correct atom arrangement. Notice that in both structures all atoms have a complete octet. 

Atoms are generally most stable when they have a complete octet (eight electrons). 

Pyrrole is more reactive than pyridine because its intermediate is more stable. For both compounds the intermediate has a on N. However, the pyrrole intermediate is relatively stable because every atom has a complete octet, while the pyridine intermediate is very unstable because N has only six electrons. 

In a molecule formed by two elements, complete octets can also result from bond formation between two equal atoms besides the compounds given above there exist others... 

It is somewhat more difficult to see that the third nitrogen— hydrogen compound, azoic acid N3H, also has only complete octets. By the study of the solid azides, e.g. KN3, it has been shown that the... 

It will be shown below that these compounds are able to react with certain other compounds in such a way that the sextet is graduated to an octet. Boron, alone, has not enough electrons to form a complete octet with halogen atoms. 

There will be a large resonance energy that gives an appreciable contribution to the stability of the molecule. The situation is different in two molecules that probably are stabilized by resonance, as well, e.g. NaO and X3H. For the first, three electronic formulae with complete octets can be wTritten... 

The third group contains the elements that still form strongly covalent bonds, which are, however, always single in so far as this can be reconciled with the formation of complete octets. 

Besides the two complexes derived from sulphur, mentioned above, there are a large number of others, in which two or more sulphur atoms are bound together the ions in these compounds again can all be formulated by assuming that the S—O bonds are semipolar, and the sulphur atoms have a complete octet. 

In order that the nitrogen atom in trimethylamine oxide, (CH3)3NO, might be assigned the neon structure with a completed octet of valence... 

Simple algebraic equations for calculating the number of shared electrons for structures with completed octets and other completed electron shells were given by I. Langmuir (J.A.C.S. 41, 868 [1919]. These equations usually need not be called upon, since electronic formulas of the sort desired can be written easily with a little practice. 

---