5 Steps to a 5 AP Chemistry 2019

152 ❯ STEP 4. Review the Knowledge You Need to Score High

eight electrons—bonding or nonbonding, shared or not—for every atom except hydrogen,

which gets two. Figure 11.4 shows the Lewis structural formula of CH 4 O.

Lewis structures may also be written for polyatomic anions or cations. The N – A = S rule can

be used, but if the ion is an anion, extra electrons equal to the magnitude of the negative charge

must be added to the electrons available. If the ion is a cation, electrons must be subtracted.

As we have mentioned previously, there are many exceptions to the octet rule. In these

cases, the N – A = S rule does not apply, as illustrated by the following example.

Draw the Lewis structure for XeF 4.

Answer:

Each of the fluorines will have an additional three pairs of electrons. Only the four

fluorine atoms have their octets.

H

H

H

C O H

Figure 11.4 Lewis structure of CH 4 O.

Xe

F

F F

F

This process will usually result in the correct Lewis structure. However, there will be

cases when more than one structure may seem to be reasonable. One way to eliminate

inappropriate structures is by using the formal charge.

There is a formal charge associated with each atom in a Lewis structure. To deter-

mine the formal charge for an atom, enter the number of electrons for each atom into

the following relationship:

Formal charge = (number of valence electrons) – (number of nonbonding

electrons + 1/2 number of bonding electrons)

A formal charge of zero for each atom in a molecule is a very common result for a

favorable Lewis structure. In other cases, a favorable Lewis structure will follow these

rules: The formal charges are:

1. Small numbers, preferably 0.


2. No like charges are adjacent to each other, but unlike charges are close together.


3. The more electronegative element(s), the lower the formal charge(s) will be.


4. The total of the formal charges equals the charge on the ion.
   Now we will apply this formal-charge concept to the cyanate ion OCN-. We
   chose this example because many students incorrectly write the formula as CNO-,
   and then try to use this as the atomic arrangement in the Lewis structure. Based on

ENRICHMENT