1.5 Lewis Dot Structures and the Octet Rule

To understand a molecule’s chemical behavior, it is necessary to know its connectivity—that is, which atoms are bonded together, and by what types of bonds (single, double, or triple). It is also useful to know which valence electrons participate in bonding and which do not. Lewis dot structures (or, Lewis structures) are a convenient way to convey this information. Let’s review some basic conventions of Lewis structures.

● Lewis structures take into account only valence electrons. In a complete Lewis structure, such as the following for isolated C and Cl atoms, all valence electrons are shown:

● Bonding and nonbonding electrons are clearly shown.

●  Single, double, and triple bonds are indicated by one, two, or three lines (i.e. —, ═, or ≡), respectively, which represent the sharing of two, four, or six electrons, respectively. Thus, each line represents a shared pair, or bonding pair, of electrons.

●  Nonbonding electrons are indicated by dots, and are usually paired (:). These are called lone pairs of electrons. In some species, nonbonding electrons are unpaired, and are represented by single dots. These species are called free radicals and are discussed in greater detail in Chapter 25.

● Atoms in Lewis structures obey the duet rule and the octet rule. Atoms are especially stable when they have complete valence shells: two electrons (a duet) for hydrogen and helium, and eight electrons (an octet) for atoms in the second row of the periodic table. These duets and octets can be achieved through the formation of covalent bonds, where valence electrons are shared between atoms. Examples are shown in Figure 1-14.

YOUR TURN 1.5

SHOW ANSWERS

Using Figure 1-14 as your guide, circle the electrons in the Lewis structure of CH₃OH that represent carbon’s octet, oxygen’s octet, and each hydrogen’s duet.

Because of their widespread use in organic chemistry, you must be able to draw Lewis structures quickly and accurately. The following steps allow you to do so in a systematic way.

Connections Methanol (CH₃OH, Your Turn 1.5), also called methyl alcohol or wood alcohol, is used industrially as a feedstock to produce formaldehyde, which is integral in the production of some plastics, paints, and explosives. Methanol is also the primary fuel used in many types of racing vehicles.

Solved Problem 1.7

Draw a Lewis structure of , where carbon is the central atom.

First, count the total number of valence electrons.

Carbon is the central atom, so six electrons must be used to connect C to the other three atoms. This leaves 12 more that can be placed as lone pairs around the O atoms to achieve octets, as shown at the right. To give C its octet, one of those lone pairs is converted to a C═O double bond.

problem 1.8 Draw a Lewis structure for C₂H₃N. One carbon is bonded to three hydrogen atoms and to the second carbon. The nitrogen atom is bonded only to a carbon atom.

In some molecular species, not all atoms have a complete valence shell. In borane (BH₃, Fig. 1-15a), for example, the B atom has a share of only six valence electrons. It is two electrons short of an octet because there are not enough valence electrons available to achieve a complete valence shell for all atoms in the molecule. Similarly, in thionyl chloride (SOCl₂, Fig. 1-15b), the S atom has a share of 10 electrons but, being in the third row of the periodic table, its valence shell can contain up to 18 electrons. There are even some examples of molecular species in which hydrogen has a share of fewer than two electrons, but these are somewhat rare. (Such examples will be discussed as necessary.)

Because of the emphasis that organic chemistry has on atoms from the second row of the periodic table, we often talk about atoms from the third row and below in terms of an octet. In SOCl₂, for example, both Cl atoms have a share of eight valence electrons, so we say that each Cl atom has an octet. The S atom, on the other hand, has what is called an expanded octet, given its share of 10 electrons. Be careful, however, when using this terminology, because only atoms in the third row and below can have an expanded octet.

Connections Borane (BH₃) and thionyl chloride (SOCl₂), shown in Figure 1-15, are important reagents in organic synthesis. BH₃ is involved in hydroboration (Section 12.6). SOCl₂ is commonly used to replace an –OH group with –Cl in certain types of molecules (Section 21.4). SOCl₂ is also used in lithium–thionyl chloride batteries, where it acts as the positive electrode.

problem 1.9 For each structure below, determine whether it is a legitimate Lewis structure. If not, explain why not.