Lewis Structures The Octet Rule

The idea of the covalent bond was first suggested by the American physical chemist Gilbert Newton Lewis (1875-1946) in 1916. He pointed out that the electron configuration of the noble gases appears to be a particularly stable one. Noble-gas atoms are themselves extremely unreactive. Moreover, as pointed out in Chapter 6, a great many monatomic ions have noble-gas structures. Lewis suggested that nonmetal atoms, by sharing electrons to form an electron-pairbond, can acquire a stable noble-gas structure. Consider, 

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Noble-gas structures are stable in molecules, as they are in atoms and ions. 

At internuclear distances less than 0.074 nm, the energy of interaction rises rapidly because of repulsion between the hydrogen nuclei. 

This idea is readily extended to simple molecules of compounds formed by nonmetal atoms. An example is the HF molecule. You will recall that a fluorine atom has the electron configuration ls22s22p5. ft has seven electrons in its outermost principal energy level (n = 2). These are referred to as valence electrons, in contrast to the core electrons filling the principal level, n = 1. If the valence electrons are shown as dots around the symbol of the element, the fluorine atom can be represented as 

The combination of a hydrogen with a fluorine atom leads to 

Lewis Structures of Atoms Commonly Forming Covalent Bonds 

This is why we put the second digit of the group number in bold type. 

As you can see, the fluorine atom owns six valence electrons outright and shares two others. Putting it another way, the F atom is surrounded by eight valence electrons its electron configuration has become ls22s22p6, which is that of the noble gas neon. This, according to Lewis, explains why the HF molecule is stable in contrast to species such as H2F, H3F. none of which exist. 

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The relative stability of resonance structures can be judged by the same rules that were previously introduced to judge the stability of Lewis structures the octet rule, the number and location of formal charges, and the interactions between charges in the structure. 

The three steps in writing the Lewis structure. The octet rule has to be satisfiedfor all the atoms in the molecule. Thus the lone pair is placed on the nitrogen atom. 

Use an example to illustrate each of the following terms lone pair, Lewis structure, the octet rule, bond length. 

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)... 

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. 

The use of resonance structures such as 7 and 8 to describe bond polarity led to a subtle change in the meaning of the octet rule, namely, that an atom obeys the octet rule if it does not have more than eight electrons in its valence shell. As a result, resonance structures such as 7 and 8 are considered to be consistent with the octet rule. However, this is not the sense in which Lewis used the octet rule. According to Lewis, a structure such as 7 would not obey the octet rule because there are only three pairs of electrons in the valence shell of carbon, just as BF3 does not obey the octet rule for the same reason. Clearly the octet rule as defined by Lewis is not valid for hypervalent molecules, which do, indeed, have more than four pairs of shared electrons in the valence shell of the central atom. 

Resonance forms must he valid Lewis structures and obey normal rules of valency. A resonance form is like any other structure The octet rule still applies.. For example, one of the following structures for the acetate ion is noV a valid resoriance fViv because the carbon atom has five bonds and ten valence electrons ... 

SAMPLE PROBLEM 10.5 Writing Lewis Structures for Octet-Rule Exceptions Problem Write Lewis structures for (a) H3PO4 (pick the most likely structure) (b) BFCI2. Plan We write each Lewis structure and examine it for exceptions to the octet rule. In (a), the central atom is P, which is in Period 3, so it can use d orbitals to have more than an octet. Therefore, we can write more than one Lewis structure. We use formal charges to decide if one resonance form is more important. In (b), the central atom is B, which can have fewer than an octet of electrons. 

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... 

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

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... 

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... 

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. 

The concept of formal charge has a much wider applicability than this short discussion might imply. In particular, it can be used to predict situations in which conventional Lewis structures, written in accordance with the octet rule, may be incorrect (Table 7.2). 

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