Considering the tremendous variety in properties of elements and compounds in the periodic system, it is asking a great deal to expect a rule as simple as Lewis’ octet theory to be able to predict all formulas or to account for all molecular structures involving covalent bonds. Lewis’ theory concentrates on resemblances to noble-gas ns2np6 valence octets. Therefore it is most successful in accounting for formulas of compounds of the representative elements, whose distinguishing electrons are also s and p electrons. The octet rule is much less useful in dealing with compounds of the transition elements or inner transition elements, most of which involve some participation of d or f orbitals in bonding.

Even among the representative elements there are some exceptions to the Lewis theory. These fall mainly into three categories:

Some stable molecules simply do not have enough electrons to achieve octets around all atoms. This usually occurs in compounds containing Be or B. Elements in the third period and below can accommodate more than an octet of electrons. Although elements such as Si, P, S, Cl, Br, and I obey the octet rule in many cases, under other circumstances they form more bonds than the rule allows. Free Radicals


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Electron Deficient Species

Good examples of the first type of exception are provided by BeCl2 and BCl3. Beryllium dichloride, BeCl2, is a covalent rather than an ionic substance. Solid BeCl2 has a relatively Complex structure at room temperature, but when it is heated to 750°C, a vapor which consists of separate BeCl2 molecules is obtained. Since Cl atoms do not readily form multiple bonds, we expect the Be atom to be joined to each Cl atom by a single bond. The structure is

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Instead of an octet the valence shell of Be contains only two electron pairs. Similar arguments can be applied to boron trichloride, BCl3, which is a stable gas at room temperature. We are forced to write its structure as

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in which the valence shell of boron has only three pairs of electrons. Molecules such as BeCl2 and BCl3 are referred to as electron deficient because some atoms do not have complete octets. Electron-deficient molecules typically react with species containing lone pairs, acquiring octets by formation of coordinate covalent bonds. Thus BeCl2 reacts with Cl– ions to form BeCl4–;

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BCl3 reacts with NH3 in the following way:

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Instead of an octet the phosphorus atom has 10 electrons in its valence shell. Sulfur hexafluoride (also a gas) consists of SF6 molecules. Its structure is

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, contain at least one unpaired electron, a clear violation of the octet rule.

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Free radicals play many important roles a wide range of applied historicsweetsballroom.comistry fields, including biology, medicine, and astrohistoricsweetsballroom.comistry.

Three well-known examples of such molecules are nitrogen (II) oxide, nitrogen(IV) oxide, and chlorine dioxide. The most plausible Lewis structures for these molecules are