The Octet Rule

The first successful theory of chemical bonding was formulated by G.N.Lewis in 1916. 

Lewis (1875 -1946 right) created the Chemistry Department at the University of California, Berkeley, and made it into one of the world s best. His other notable work included the first isolation of heavy water (D20), the thermodynamics of solutions, and the phosphorescence and magnetic properties of molecules. 

Although Lewis originated the idea of the electron-pair bond, much of the credit for its early acceptance must go to Irving Langmuir, who extended it somewhat and enthusiastically popularized it to the extent that it began to be known as the Lewis-Lang-muir theory, and even as the Langmuir theory  

Irving Langmuir (1881-1967, Nobel Prize 1932) was an industrial scientist employed by the General Electric Co. His most notable work was on the chemistry of surfaces and monomolecular layers. Lewis and Langmuir were probably the two greatest American chemists of the first half of the twentieth century. 

Present-day shared electron-pair theory is based on the premise that the s2p6 octet in the outermost shells of the noble gas elements above helium represents a particularly favorable configuration. This is not because of any mysterious properties of octets (or of noble gas elements) by allowing each nucleus to claim half-ownership of a shared electron, more electrons are effectively seeing more nuclei, leading to increased electro- 

The elements helium, neon, argon, krypton, xenon, and radon—known as the noble gases—occur in nature as monatomic gases. Their atoms are not combined with atoms of other elements or with other atoms like themselves. Prior to 1962, no compounds of these elements were known. (Since 1962, some compounds of krypton, xenon, and radon have been prepared.) Why are these elements so stable, while the elements with atomic numbers 1 less or 1 more are so reactive The answer lies in the electronic structures of their atoms. The electrons in atoms are arranged in shells, as described in Sec. 3.6 and in greater detail in Chap. 4. 

EXAMPLE 5.3. (a) Arrange the 19 electrons of potassium into shells, (b) Arrange the 18 electrons of argon into shells. 

The first two electrons fill the first shell, the next eight fill the second shell, and the following eight occupy the third. That leaves one electron left in potassium for the fourth shell. 

The electrons involved in chemical bonding are the valence electrons, which, for most atoms, are tho.se in the outermost occupied shell. (Section 6.8) The American chemist G. N. Lewis (1875-1946) suggested a simple way of showing the valence electrons in an atom and tracking them during bond formation, using what are now known as either Lewis electron-dot symbols or simply Lewis symbols. 

The Lewis symbol for an element consists of the element s chemical symbol plus a dot for each valence electron. Sulfur, for example, has the electron configuration [Ne]3s 3p and therefore six valence electrons. Its Lewis symbol is 

The dots are placed on the four sides of the symbol—top, bottom, left, and right— and each side can accommodate up to two electrons. All four sides are equivalent, which means that the choice of on which sides to place two electrons rather than one electron is arbitrary. In general, we spread out the dots as much as possible. In the Lewis symbol for S, for instance, we prefer the dot arrangement shown rather the arrangement having two electrons on three of the sides and none on the fourth. 

The electron configurations and Lewis symbols for the main-group elements of periods 2 and 3 are shown in T TABLE 8.1. Notice that the number of valence electrons in any representative element is the same as the element s group number. For example, the Lewis symbols for oxygen and sulfur, members of group 6A, both show six dots. 

Atoms often gain, lose, or share electrons to achieve the same number of electrons as the noble gas closest to them in the periodic table. The noble gases have very stable electron arrangements, as evidenced by their high ionization energies, low affinity for additional electrons, and general lack of chemical reactivity. (Section 7.8) Because all the noble 

Group Electron Element Configuration Lewis Symbol Electron Element Configuration Lewis Symbol 

Methane is the smallest alkane -alkanes are a family of compounds that contain only C and H atoms linked by single bonds (Section 2.4) 

Drawing organic compounds using full structural formulae and other conventions is discussed in Section 2.5 

To form organic compounds, the carbon atom shares electrons to give a stable full shell electron configuration of eight valence electrons. 

Full structural formula (or Kekule structure) A line = 2 electrons 

A single bond contains two electrons, a double bond contains four electrons and a triple bond contains six electrons. A lone (or non-bonding) pair of electrons is represented by two dots ( ). 

The octet rule works only for atoms with an atomic number less than 20 because at atomic number 21, we enter into the part of the periodic table that houses the transition metals. The transition metals (described in more detail in Chapter 13) are characterized by partially filled d-orbitals, and d-orbitals can hold up to ten electrons each. Each transition metal element has s-orbitals (space for two electrons), p-orbitals (with space for eight electrons), and d-orbitals (with space for ten electrons). Therefore, to make a transition metal stable, it must have a total of 18 electrons to match with the next noble gas configuration. 

Transition metals are electron deficient and try to bind with molecules that have electrons to donate. They need to bind with as many donating groups as they can find to reach a total of 18 electrons in the complex. 

Whereas the octet rule describes the basic philosophy behind the electron counting rules, the 18-electron rule is more useful when dealing with organometallic chemistry. 

Essentially, this rule allows you to predict what formulations create stable organometallic complexes. In short, when you know the number of valence 

The nucleus of an atom is surrounded by a series of an ever increasing number of orbitals around it. The first orbital holds two electrons and is then full and stable. The second and subsequent orbitals hold up to eight electrons each. When thinking about the stability of the organometallic compounds, think of the whole complex as a system that acts like a nucleus when trying to achieve stability. 

C is in group 14 and hence has 4 valence electrons H is in group 1 and hence has 1 valence electron 

0 ) often exist only in environments in which electrostatic interaction energies compensate for the energies needed to form the ions from atoms. 

An atom obeys the octet rule when it gains, loses or shares electrons to give an outer shell containing eight electrons with the configuration ns np.  

We have already noted the exception of He, but for n 3, there is the possibility of apparently expanding the octet (see p. 104). Although the octet rule is still useful at an elementary level, we must bear in mind that it is restricted to a relatively small number of elements. Its extension to the 18-electron rule, which takes into account the filling of ns, np and nd sub-levels, is discussed in Sections 20.4 and 23.3. 

Worked example 1.9 The octet rule and the apparent expansion of the octet 

In which of the following covalent compounds is the central atom obeying the octet rule (a) CH4 (b) H2S (c) CIF3  

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In the example in Figure 2-19, the oxygen atom 3 has 2- 4 (row) + 2 + 4 (column) - 4 (diagonal element) = 8 electrons. This shows that the ox> gen atom obeys the octet rule. 

Figure 2-19. The BE-matriK of ethanal allows one to determine tine number of valence electrons (the sum of each row) on the atoms and to validate the octet rule,...

Protonated methane (CH ) does not violate the octet rule of carbon. A bonding electron pair (responsible for covalent bonding between C and H atoms) is forced into sharing with the proton, resulting in 2 electron-3 center bonding (2e-3c) (see Chapter 10). Higher alkanes are protonated similarly. 

Lewis s concept of shared electron parr bonds allows for four electron double bonds and SIX electron triple bonds Carbon dioxide (CO2) has two carbon-oxygen double bonds and the octet rule is satisfied for both carbon and oxygen Similarly the most stable Lewis structure for hydrogen cyanide (HCN) has a carbon-nitrogen triple bond... 

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... 

It will always be true that a nitrogen with four covalent bonds has a formal charge of + 1 (A nitrogen with four co valent bonds cannot have unshared pairs because of the octet rule)... 

Lewis structures in which second row elements own or share more than eight valence electrons are especially unstable and make no contribution to the true structure (The octet rule may be ex ceeded for elements beyond the second row)... 

When two or more structures satisfy the octet rule the most stable one is the one with the smallest separation of oppositely charged atoms... 

The two Lewis structures D and E of methyl nitrite satisfy the octet rule... 

Among structural formulas in which the octet rule IS satisfied for all atoms and one or more of these atoms bears a formal charge the most stable reso nance form is the one in which negative charge re sides on the most electronegative atom... 

Section 1 3 The most common kind of bonding involving carbon is covalent bond ing A covalent bond is the sharing of a pair of electrons between two atoms Lewis structures are written on the basis of the octet rule, which limits second row elements to no more than eight electrons m their valence shells In most of its compounds carbon has four bonds... 

There is ample evidence from a variety of sources that carbocations are mterme diates m some chemical reactions but they are almost always too unstable to isolate The simplest reason for the instability of carbocations is that the positively charged car bon has only six electrons m its valence shell—the octet rule is not satisfied for the pos itively charged carbon... 

Free radicals are species that contain unpaired electrons The octet rule notwithstand mg not all compounds have all of their electrons paired Oxygen (O2) is the most famil lar example of a compound with unpaired electrons it has two of them Compounds that have an odd number of electrons such as nitrogen dioxide (NO2) must have at least one unpaired electron... 

Of the two resonance forms A and B A has only six electrons around its positively charged carbon B satisfies the octet rule for both carbon and oxygen It is more stable than A and more stable than a carbocation formed by protonation of a typical alkene... 

Many transition metal complexes including Ni(CO)4 obey the 18 electron rule, which IS to transition metal complexes as the octet rule is to mam group elements like carbon and oxygen It states that... 

Isomtriles are stable often naturally occumng compounds that contain a divalent carbon An example is axisonitnle 3 which can be isolated from a species of sponge and possesses anti malanal activity Write a resonance form for axisonitnle 3 that satisfies the octet rule Don t for get to include formal charges... 

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... 

Note that these compounds are covalently bonded compounds containing only hydrogen and carbon. The differences in their strucmral formulas are apparent the alkanes have only single bonds in their structural formulas, while the alkenes have one (and only one) double bond in their structural formulas. There are different numbers of hydrogen atoms in the two analogous series. This difference is due to the octet rule that carbon must satisfy. Since one pair of carbon atoms shares a double bond, this fact reduces the number of electrons the carbons need (collectively) by two, so there are two fewer hydrogen atoms in the alkene than in the corresponding alkane. 

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