Formal Charge

Formal Charge

Introduction

A hydrogen atom is made up of one proton and one electron. The formal charge of the atom, the sum of the charge of the proton and the charge of the electron, is zero. The formal charge on any atom is zero when the number of protons (the atomic number) and the number of electrons that "belong" to that atom are equal. We have seen that it requires 13.6 kcal/mol to separate an electron from a hydrogen atom. The resulting hydrogen nucleus, the proton, has a formal charge of +1.

Assigning formal charges to isolated atoms and ions is easy. So is assigning formal charges to atoms that are covalently bonded within molecules.

Rule 2: Calculating Formal Charges

To determine the formal charge of an atom within a molecule, separate the atom from its bonding partner(s), dividing all bonding electrons equally between the bonded atoms. Then compare the number of electrons that "belong" to each atom to the atomic number of that atom. Figure 1uses color coding to illustrate the procedure for methane, CH4.

Figure 1

Assigning Electrons I

Each hydrogen is assigned one of the two electrons it shares with the central carbon atom; the formal charge on each hydrogen atom in methane is zero. The central carbon is assigned one of the two electrons it shares with each of the four hydrogens. These are its four valence electrons. But the carbon atom also has two inner shell electrons to consider. The total number of electrons assigned to the carbon is six; this is the same as the atomic number of carbon, and the formal charge on the carbon atom is zero.

Exercise 1 Calculate the formal charge on each of the atoms in ethane, CH3CH3.

a. The formal charge on carbon is

b. The formal charge on hydrogen is

Exercise 2 Calculate the formal charge on each of the atoms in propane, CH3CH2CH3.

a. The formal charge on C1 or C3 is

b. The formal charge on C2 is

c. The formal charge on hydrogen is

Figure 2 extends the procedure demonstrated in Figure 1 to methanol, CH3OH, where the oxygen atom has two lone pairs of electrons.