1.9 Assigning Electrons to Atoms in Molecules: Formal Charge

In an isolated atom or atomic ion, charge is determined by the difference between the atom’s group number and the actual number of valence electrons it possesses. A carbon atom, for example, has zero charge if it possesses four valence electrons—its group number is 4. It carries a charge of –1 if it has five valence electrons and it carries a charge of +1 if it has only three valence electrons.

YOUR TURN 1.8
SHOW ANSWERS

Fill in the table below for a carbon nucleus that has 3, 4, or 5 valence electrons.

Number of valence electrons (total number of electrons, number of protons, charge): 3 (5, 6, + 1), 4 (6, 6, 0), 5 (7, 6, –1)

Number of Valence Electrons

Total Number of Electrons

Number of Protons

Charge

3

4

5

In a molecule or polyatomic ion, we can also assign a charge to an individual atom by computing the difference between the atom’s group number and the number of valence electrons it possesses. But how do we assign electrons to atoms involved in covalent bonds, where electrons are being shared?

One way to do so gives us the atom’s formal charge, according to these two rules:

Connections The formate anion, , is produced in significant amounts in the mitochondria of embryonic liver cells and also in cancer cells.

●  Both electrons of a lone pair are assigned to the atom on which they appear.

●  In a given covalent bond, half the electrons are assigned to each atom involved in the bond.

Solved Problem 1.16 shows how to apply these rules to a Lewis structure of .

Solved Problem 1.16

Determine the formal charge on every atom in the methanoate anion (formate anion, ), shown here.

A condensed structural formula of formate anion. The condensed structure shows a central carbon atom, single bonded to a hydrogen atom and double bonded to two oxygen atoms, one carrying two lone pairs of electrons and the other carrying three lone pairs of electrons. The overall charge is minus one.

Think
SHOW SECTION

How are lone pairs assigned? How are bonding pairs of electrons assigned to atoms?

Solve
SHOW SECTION

The figure below shows how the formal charge rules are applied to assign valence electrons to each atom. Notice that each pair of electrons in a covalent bond is split evenly. The H atom, the C atom, and the double-bonded O atom are assigned formal charges of 0 because they have the same number of valence electrons as their corresponding group numbers. The single-bonded O is assigned seven valence electrons, which is one more than its group number of 6. It is therefore assigned a formal charge of –1.

A condensed structural formula of carboxyl group, labeled with number of electrons and formal charge on its atoms. The condensed structural formula of the formate ion shows a central carbon atom, single bonded to a hydrogen atom and double bonded to two oxygen atoms, one carrying two lone pairs of electrons and the other carrying three lone pairs of electrons. The carbon atom has four electrons with zero formal charge. The hydrogen atom has one electron with zero formal charge. The oxygen atom carrying two lone pairs of electrons has six electrons with zero formal charge. The oxygen atom carrying three lone pairs of electrons has seven electrons with a formal charge of negative 1.

Assigning formal charges to atoms is simply a way of distributing the electrons that are already present within a particular species. Therefore:

The formal charges of all atoms must sum to the total charge of the species.

YOUR TURN 1.9
SHOW ANSWERS

Sum the formal charges assigned in Solved Problem 1.16. What do you notice?

Sum of formal charges: 0 + 0 + 0 + (–1) = –1. Sum equals overall charge on ion, –1.

Connections Oximes are important intermediates in organic synthesis, including the industrial production of nylon-6. Some oximes are used as nerve-agent antidotes, and one is an artificial sweetener.

problem 1.17 Determine the formal charge of each atom in the molecule at the right, which belongs to a class of compounds called oximes. Note: Assume that each atom has a filled valence shell.

A condensed structural formula of a molecule. The condensed structure shows a central carbon atom, single bonded to two methyl groups and double bonded to a nitrogen atom. This is shown further attached to the oxygen of a hydroxyl group by a single bond.