Exceptions to octet rule

To check on the validity of a Lewis structure, verify that each atom has an octet or a duplet. As we shall see in Section 2.10, a common exception to this rule arises when the central atom is an atom of an element in Period 3 or higher. Such an atom can accommodate more than eight electrons in its valence shell. Consequently, the most stable Lewis structure may be one in which the central atom has more than eight electrons. 

Of course, now that you re getting the hang of it, you need to know about those ever-present exceptions to the rule. There are some cases in which the octet rule is not obeyed. You should be familiar with the examples, and there are some patterns you can learn to help you memorize this. 

Note that the sodium ion has the same electron configuration as neon (ls 2s 2p ), a noble gas. This observation leads to one of the most important principles in chemistry, the octet rule. The octet rule states that atoms tend to gain, lose, or share electrons in order to acquire a full set of eight valence electrons. This reinforces what you learned earlier that the electron configuration of filled s and p orbitals of the same energy level (consisting of eight valence electrons) is unusually stable. Note that the first period elements are an exception to the rule, as they are complete with only two valence electrons. 

Lone-pair nucleophiles are by far the most enthusiastic participants in Sn2 substitution reactions. Sigma-bond nucleophiles may also participate in Sn2 reactions, but they do not do so as often as lone-pair nucleophiles. By contrast, 7r-bond nucleophiles do not usually have sufficiently high energy to react with an atom that already has an octet. The major exceptions to this rule are the enam-ines (R2N-CR=CR2 <—> R2N=CR-CR2), which are sufficiently nucleophilic at the j8 position to attack particularly reactive alkyl halides such as CH3I and al-lylic and benzylic bromides, and enolates (0-CR=CR2 <—> 0=CR-CR2), which react with many alkyl halides. 

The other exceptions to this rule involve compounds of boron and beryllium, which form compounds like BeHj and BF3, in which there are four and six electrons, respectively, in their completed valence shells. However, the octet rule applies to all of the other elements in the first and second periods of the Periodic Table, on whose compounds we will now focus our attention. 

By analogy with the octet rule, it has been proposed that a transition metal tends to be surrounded by the number of valence electrons equal to that of the following rare gas (electron configuration nd °(n + l)s (n + l)p ). One thereby obtains the 18-electron rule, for which we shall provide a first theoretical justification in this chapter ( 1.6.3). However, in light of the examples given above, one must note that there are many exceptions to this rule we shall analyse them in greater detail in the following chapters. 

Although many simple molecules fulfill the octet rule, some common molecules are exceptions to this rule. Give three examples of molecules whose Lewis structures are exceptions to the octet rule. 

We recognize that the octet rule is more of a guideline than an absolute rule. Exceptions to the rule include molecules with an odd number of electrons, molecules where large differences in electronegativity prevent an atom from completing its octet, and molecules where an element from period 3 or below in the periodic table attains more than an octet of electrons. 

The Lewis model of covalent bonding focuses on valence electrons and the necessity for each atom other than H participating in a covalent bond to have a completed valence shell of eight electrons. Although most molecules formed by main-group elements (Groups 1A-7A) have structures that satisfy the octet rule, there are important exceptions to this rule. 

One notable exception to this rule involves compounds containing the nitro group (—NO2), which have resonance structures with more than two charges. Why We saw earlier that the structure of the nitro group must be drawn with charge separation in order to avoid violating the octet rule ... 

As was the case for Cl in HCl, the O atom in the Lewis structure of H2O and in CI2O is surrounded by eight electrons (when the bond-pair electrons are double counted). In attaining these eight electrons, the O atom conforms to the octet rule—a requirement of eight valence-shell electrons for the atoms in a Lewis structure. Note, however, that the H atom is an exception to this rule. The H atom can accommodate only two valence-shell electrons. 

These examples illustrate the principle that atoms in covalently bonded species tend to have noble-gas electronic structures. This generalization is often referred to as the octet rule. Nonmetals, except for hydrogen, achieve a noble-gas structure by sharing in an octet of electrons (eight). Hydrogen atoms, in molecules or polyatomic ions, are surrounded by a duet of electrons (two). 

Reality Check Whenever you write a Lewis structure, check to see if it follows the octet rule. The structures written for OCl and C2H6 do just that each atom except H is surrounded by eight electrons. 

Exceptions to the Octet Rule Electron-Deficient Molecules... 

Octet rule The principle that bonded atoms (except H) tend to have a share in eight valence electrons, 166-171 exceptions to, 172-176 molecular geometry and, 181t molecular orbitals and, 650 Octyl acetate, 596t Open-pit copper mine, 540 Oppenheimer, J. Robert, 523 Optical isomer Isomer which rotates the... 

The octet rule accounts for the valences of many of the elements and the structures of many compounds. Carbon, nitrogen, oxygen, and fluorine obey the octet rule rigorously, provided there are enough electrons to go around. However, some compounds have an odd number of electrons. In addition, an atom of phosphorus, sulfur, chlorine, or another nonmetal in Period 3 and subsequent periods can accommodate more than eight electrons in its valence shell. The following two sections show how to recognize exceptions to the octet rule. 

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