Learning the concept of calculating formal charges is essential because we can use formal charges to identify the most reasonable Lewis structures for a given molecule. Moreover, it can explain the concept of resonance and draw Lewis structures representing resonance forms for a given molecule. The formal charge usually “de-emphasize” the bond polarity by assuming that all electrons are shared equally. An atom in a molecule is given a formal charge (FC) based on the assumption that electrons in all chemical bonds are shared equally among atoms, regardless of relative electronegativity.
Formal charges, also known as fake charges, are helpful for coordinate covalent bonding where one atom (the donor) gives both electrons to the acceptor atom. The charge distribution in a Lewis structure can be computed by formal charge formula. The aggregate formal charges on the atoms within a molecule or an ion must be equal to the overall charge on the molecule or ion. A formal charge does not represent an actual charge on an atom in a covalent bond but is used to predict the most likely structure when a compound has more than one valid Lewis’s structure.
This article will emphasize what a formal charge is, how to find the formal charge and its significance in practical applications.
Formal Charge Formula
The formal charge can be calculated by excluding the number of electrons in the lone pairs and the number of bonds from the total number of valence electrons. The following equation can calculate the formal charge of an atom in a molecule:
FC = V – N – B/2
Where;
V; the number of valence electrons in the ground state of the atom
N is the number of non-bonding valence electrons on this atom
B is the total number of electrons participating in bonding with other atoms in the molecule.
What is Formal Charge
The formal charge definition can be elaborated as the hypothetical charge on the atom if the electrons in the bonds are distributed evenly between the atoms. If put in another way, when we take the number of valence electrons of a neutral atom, subtract the nonbonding electrons, and subtract the number of bonds connected to that atom in the Lewis structure, a formal charge on the atom is obtained. It is the difference between the valence electron of that atom in the elemental state and the number of electrons assigned to that atom in the Lewis structure.
It is a theoretical charge over a single atom of an ion since the real charge over a polyatomic molecule or ion is dispersed throughout the entire ion rather than a single atom. Because bonding e– is shared by two atoms, a multiplier of 1/2 is applied to the number of bonding e– when calculating formal charge analytically. The sum of formal charges of all atoms in a molecule must equal to zero. Furthermore, the sum of the formal charges in an ion should equal the charge of the ion. The calculated formal charge is not the actual charge of the atom. The formal charge is only a helpful bookkeeping procedure; it does not indicate actual charges.
Significance of Calculating Formal Charge
In exceptional cases, more than one valid Lewis structure for molecules and polyatomic ions is discovered. When more than one plausible Lewis’ structure exists, the idea of formal charges can be utilized to determine the most acceptable Lewis’ structure. The relationship between the amount of bonding electrons and how many are technically “owned” by the atom is explicitly stated in this idea. The formal charge being a theoretical concept does not indicate any real charge segregation in the molecule.
- Formal charges help select the most suitable structure, which is the lowest energy structure from several alternatives for a given category.
- Knowledge of the lowest energy structure helps predict the primary product of a reaction and describes a lot of phenomena.
- Generally, the lowest energy structure is the one with the most nominal formal charges on the atoms and the most distributed charge.
How to Calculate Formal Charge
Formal charge equation formally compares the number of valence electrons in an isolated neutral atom (which is determined from the older style group number of the periodic table) with the number of valence electrons around the atom in the molecule. The sum of the formal charges for the entire structure can be used to double-check the formal charge calculations.
Step by Step Calculation
- The first step for calculating the formal charge is drawing the Lewis structure of a molecule.
- Divide the electron pairs in bonds equally for all the bonds.
- Assign electrons in lone pairs to their atoms.
- Apply the formula and subtract the number of unbonded electrons and bonds from the number of valence electrons for the atom.
Formal charge = valence e− − (nonbonding e− + bonding e–/2)
Tips for Calculating Formal Charge
Electrons are assigned to individual atoms according to these rules:
- Non-bonding electrons are assigned to the atom which they are a part of.
- The bonding electrons are distributed evenly among the bound atoms.
Example 1
To illustrate this method, let us compute the formal charge on the atoms in an ammonia molecule (NH3). It has the following Lewis electron structure
:
The valence electrons of a neutral nitrogen atom are five (it is in group 15). In ammonia, each nitrogen atom has one lone pair and shares three bonding pairs with hydrogen atoms, giving nitrogen a total of five electrons [2 nonbonding e + (6 bonding e/2)]. When we substitute into the formula, we get:
Formal charge (N) = 5 valence e−−(2 non−bonding e− + 6 bonding e−/2) = 0
A neutral hydrogen atom has one valence electron. Each hydrogen atom in the molecule shares one pair of bonding electrons and is assigned one electron [0 nonbonding e− + (2 bonding e−/2)]. Using the formula to calculate the formal charge on hydrogen, we obtain:
Formal charge (H) = 1 valence e−−(0 non−bonding e− + 2 bonding e−/2) = 0
The formal charges when added together should give us the overall charge on the molecule or ion. In this example, the nitrogen and each hydrogen have a formal charge of zero. When summed, the overall charge is zero, consistent with the overall charge on the NH3 molecule.
Calculating Formal Charge in Excel
The Lewis structure of SO42- is as follows:
Identify all the atoms present in the Lewis Structure and write the number of valence electrons present in each atom as shown in Column B below:
Assign the lone pairs and hence, the number of nonbonding electrons to their atoms shown in Column C:
Divide the bonding pairs for all the bonds present in the molecule as shown in Column D:
All the required data has been collected and the formula has to be applied to calculate formal charge as shown in cell E2:
= B2 – C2 – (D2/2)
Copy down the formula in Column E:
FAQs
| Sr. | Questions | Answers |
| 1 | What is the difference between a partial charge and a formal charge? | A formal charge is a charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms regardless of relative electronegativity. In some situations, a molecule can appear to have a partial positive charge on one side and a partial negative charge. |
| 2 | How does formal charge help in identifying the stability of molecular structures? | A molecular structure with all formal charges equal to zero is preferred over the one in which some formal charges might not be zero. |
| 3 | How does formal charge help in selecting the suitable Lewis structure? | When choosing from several Lewis structures with similar distributions of formal charges, the structure with the negative formal charges on the more electronegative atoms is preferable. |
| 4 | Differentiate between formal charge and oxidation state? | In the case of formal charge, we assume that electrons present in a bond equally distributed between both the atoms. For the oxidation state, we consider the differences in the electronegativity of the two atoms. |