The Foundations of Chemistry

7-6 Resonance 289

Formal Charges

An experimental determination of the structure of a molecule or polyatomic ion is neces-
sary to establish unequivocally its correct structure. We often do not have these results
available, however. Formal chargeis the hypothetical charge on an atom in a molecule or
polyatomic ion;to find the formal charge, we count bonding electrons as though they were
equally shared between the two bonded atoms. The concept of formal charges helps us to
write correct Lewis formulas in most cases. The most energetically favorable formula for a
molecule is usually one in which the formal charge on each atom is zero or as near zero as
possible.
Consider the reaction of NH 3 with hydrogen ion, H, to form the ammonium ion,
NH 4 .

The previously unshared pair of electrons on the N atom in the NH 3 molecule is shared
with the Hion to form the NH 4 ion, in which the N atom has four covalent bonds.
Because N is a Group VA element, we expect it to form three covalent bonds to complete
its octet. How can we describe the fact that N has four covalent bonds in species like NH 4 ?
The answer is obtained by calculating the formal chargeon each atom in NH 4 by the
following rules:

Rules for Assigning Formal Charges to Atoms of A Group Elements

1.The formal charge, abbreviated FC, on an atom in a Lewis formula is given by

the relationship

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

Formal charges are represented by 
and to distinguish them from real charges

on ions.

2.In a Lewis formula, an atom that has the same number of bonds as its periodic group

number has a formal charge of zero.

3.a. In a molecule, the sum of the formal charges is zero.

b. In a polyatomic ion, the sum of the formal charges is equal to the charge.

Let us apply these rules to the ammonia molecule, NH 3 , and to the ammonium ion, NH 4 .
Because N is a Group VA element, its group number is 5.

In NH 3 the N atom has 3 bonds and 2 unshared e, and so for N,

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

 5 (32)0 (for N)

H

H

HHN



H

HHN

H

H

HHN



H

HHHN  

E

nrichment

The Hion has a vacant orbital,

which accepts a share in the lone pair

on nitrogen. The formation of a

covalent bond by the sharing of an

electron pair that is provided by one

atom is called coordinate covalent bond

formation.This type of bond formation

is discussed again in Chapters 10 and

25.