1. Describe three types of molecules that are exceptions to the octet rule.
2. Draw Lewis structures for molecules that do not follow the octet rule.

Types of Exceptions. Lewis structures can be drawn for many compounds with the aid of the octet rule; however, structures of some compounds do not follow the rule. The text points out three types of molecules that are exceptions to the octet rule:

• Molecules that have an incomplete octet.
• Molecules with an odd number of electrons.
• Molecules in which the central atom has an expanded octet.

The boron halides BX₃ are well-known examples of molecules with an incomplete octet. They are all planar molecules in which the boron atom has only six valence electrons. The boron atom has incomplete octet. This situation is typical in boron chemistry.

Normally when there is a shortage of electrons, we can draw a double-bonded structure as shown below for BF₃.

However, experiments indicate that each B—F bond is a single bond as shown in the first dot structure. Also, the assignment of formal charges indicates that a structure with a double bond would have adjacent formal charges. As was discussed above, this is unfavorable. In addition, the more electronegative F atom would have a positive charge, rather than the preferred negative charge for fluorine.

Two common oxides of nitrogen, NO and NO₂, have odd numbers of electrons. Since an even number of electrons is required for complete pairing, the octet rule cannot be satisfied. Two additional odd-electron molecules that are known to exist in our atmosphere for very short periods of time are OH (hydroxyl radical) and HO₂ (hydroperoxyl radical).

Molecules exhibiting an expanded octet (having more than eight valence electrons) require the presence of nonmetal atoms from the third period or beyond in the periodic table. Second period elements never exceed the octet rule. Third-period elements are just as likely to exceed the octet rule as they are to follow it. Where for example, PCl₃ obeys the octet rule, gaseous PCl₅ has a phosphorus atom that is joined by single bonds to five chlorine atoms. The phosphorus atom has 10 electrons in its valence shell. This is called an expanded octet.

The central atoms in SF₄ and SF₆, and in the interhalogen compounds ClF₅, BrF₅, and IF₇ exhibit expanded octets.

In Chapter 10, you will need to draw a number of dot structures of molecules exhibiting expanded octets. When the central atom is from the third period or beyond, complete the octets of the surrounding atoms first, and then complete the central atom. If extra electron pairs remain, place them on the central atom. See Example 9.11 below.

These exceptions seem to be telling us that an atom with a completed octet is not necessary for covalent bonding to occur. Within the Lewis framework, it is really the sharing of electron pairs that leads to the covalent bond. A shared pair of electrons acts to attract both atoms.

Draw Lewis structures for:

1. GaI₃

   
   

          Correct!

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2. NO₂ (all bonds are equivalent)

   
   

          Correct!

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3. ClF₃

   
   
   

          Correct!

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1. Gallium is in Group 3A of the periodic table, a group well known for its electron deficient elements. GaI₃ has 3 + 3(7) = 24 valence electrons.

   

   This structure shows 24 electrons, the correct number. Ga, with three electron pairs, is electron deficient.

2. NO₂ has 17 valence electrons. With an odd number of electrons, it cannot obey the octet rule.

   The best we can do is start with 18 valence electrons as in and then remove one from the nitrogen atom (because it is the unique atom). Two contributing structures are necessary.

   

3. ClF₃ has 28 valence electrons. 26 electrons are required to complete the octets of the four atoms. The remaining two electrons are placed on the central Cl atom because chlorine is in the third period, and has vacant 3d orbitals that can hold electrons in addition to an octet. Chlorine is said to have an expanded octet.

   

Complete the following questions to check your understanding of the material. Select the check button to see if you answered correctly.

1. Write Lewis structures for the following molecules:

   1. AlBr₃
   2. SF₄
   3. NO