Arteleo Chemistry Journal

Limitations Of The Octet Rule For Lewis Formulas

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Recall that representative elements achieve noble gas electron configurations in most of their compounds. But when the octet rule is not applicable, the relationship S = N — A is not valid without modification. The following are general cases for which the procedure in Section 7-5 must be modified—that is, there are four types of limitations of the octet rule.

A. Most covalent compounds of beryllium, Be. Because Be contains only two valence shell electrons, it usually forms only two covalent bonds when it bonds to two other atoms. We therefore use four electrons as the number needed by Be in step 2, Section 7-5. In steps 3 and 4 we use only two pairs of electrons for Be.

B. Most covalent compounds of the Group IIIA elements, especially boron, B. The IIIA elements contain only three valence shell electrons, so they often form three covalent bonds when they bond to three other atoms. We therefore use six electrons as the number needed by the IIIA elements in step 2; and in steps 3 and 4 we use only three pairs of electrons for the IIIA elements.

C. Compounds or ions containing an odd number of electrons. Examples are NO, with 11 valence shell electrons, and NO2, with 17 valence shell electrons.

D. Compounds or ions in which the central element needs a share in more than eight valence shell electrons to hold all the available electrons, A. Extra rules are added to steps 2 and 4 of the procedure in Section 7-5 when this is encountered.

Step 2 a: If S, the number of electrons shared, is less than the number needed to bond all atoms to the central atom, then S is increased to the number of electrons needed.

Step 4a: If S must be increased in step 2 a, then the octets of all the atoms might be satisfied before all of the electrons (A) have been added. Place the extra electrons on the central element.

Many species that violate the octet rule are quite reactive. For instance, compounds containing atoms with only four valence shell electrons (limitation type A above) or six valence shell electrons (limitation type B above) frequently react with other species that supply electron pairs. Compounds such as these that accept a share in a pair of electrons are called Lewis acids; a Lewis base is a species that makes available a share in a pair of electrons. (This kind of behavior will be discussed in detail in Section 10-10.) Molecules with an odd number of electrons often dimerize (combine in pairs) to give products that do satisfy the octet rule. Examples are the dimerization of NO to form N2O2 (Section 24-15) and NO2 to form N2O4 (Section 24-15). Examples 7-5 through 7-9 illustrate some limitations and show how such Lewis formulas are constructed.

See the Saunders Interactive

V_J General Chemistry CD-ROM,

Screen 9.8, Free Radicals—Exceptions to the Octet Rule.

Lewis formulas are not normally written for compounds containing d- and f-transition metals. The d- and /-transition metals utilize d or f orbitals (or both) in bonding as well as s and p orbitals. Thus, they can accommodate more than eight valence electrons.

Example 7-5 Limitations of the Octet Rule

Write the Lewis formula for gaseous beryllium chloride, BeC^, a covalent compound. Plan

This is an example of limitation type A. So, as we follow the steps in writing the Lewis formula, we must remember to use four electrons as the number needed by Be in step 2. Steps 3 and 4 should show only two pairs of electrons for Be.

Solution

Step 1: The skeleton is Cl Be Cl see limitation type A

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