The Foundations of Chemistry

FCs are indicated by
and . The
sum of the formal charges in a
polyatomic ion is equal to the charge
on the ion: 1 in NH 4 .

290 CHAPTER 7: Chemical Bonding

For H,

FC(group number)[(number of bonds)(number of unshared e)]

 1 (10)0 (for H)

The formal charges of N and H are both zero in NH 3 , so the sum of the formal charges

is 03(0)0, consistent with rule 3a.

In NH 4 the atom has 4 bonds and no unshared e, and so for N,

FC(group number)[(number of bonds)(number of unshared e)]

 5 (40)1 (for N)

Calculation of the FC for H atoms gives zero, as shown previously. The sum of the formal

charges in NH 4 is (1)4(0) 1 . This is consistent with rule 3b.

Thus, we see the octet rule is obeyed in both NH 3 and NH 4 . The sum of the formal

charges in each case is that predicted by rule 3, even though nitrogen has four covalent

bonds in the NH 4 ion.

This bookkeeping system helps us to choose among various Lewis formulas for a mole-

cule or ion, according to the following guidelines.

a. The most likely formula for a molecule or ion is usually one in which the

formal charge on each atom is zero or as near zero as possible.

b.Negative formal charges are more likely to occur on the more electronega-

tive elements.

c. Lewis formulas in which adjacent atoms have formal charges of the same sign

are usually notaccurate representations (the adjacent charge rule).

Let us now write some Lewis formulas for, and assign formal charges to, the atoms in

nitrosyl chloride, NOCl, a compound often used in organic synthesis. The Cl atom and the

O atom are both bonded to the N atom. Two Lewis formulas that satisfy the octet rule are

(i) (ii)

For Cl, FC 7 (24) 1 For Cl, FC 7 (16) 0

For N, FC 5 (32) 0 For N, FC 5 (32) 0

For O, FC 6 (16) 1 For O, FC 6 (24) 0

We believe that (ii) is a preferable Lewis formula, because it has smaller formal charges than

(i). We see that a double-bonded terminal Cl atom would have its electrons arranged as

/ÅClUX, with the formal charge of Cl equal to 7(24)1. A positive formal charge

on such an electronegative element is quite unlikely, and double bonding to chlorine does

not occur. The same reasoning would apply to the other halogens.

Cl N O Cl N O

 

H

H

HHN





See the Saunders Interactive
General Chemistry CD-ROM,
Screen 9.13, Formal Charge.