Formal Charge and Its Properties

Introduction

A formal charge is defined or described as the charge (q) assigned (provided) to an atom (a) in a molecule (m), assuming that the electrons (e) in all bonds or chemical bonds are shared (divided) equally (=) between (b/w) the atoms (a), irrespective of relative (respective) electronegativity. Or the formal (f) charge can also be defined as the difference (d) between (b/w) the number (no.) of valence electrons (e) of an atom (a) in a neutral type free state (s) and the number (no) provided to that particular atom in a structure named Lewis Structure. The equation or formula for the formal charge is − F=V-L-B/2, the formal charge is important as it is a theoretical concept, it does not indicate any real charge separation in the molecule.

What is a Formal Charge?

Formal charge denoted by F.C. or q is the charge (q) provided to an atom (a) in a molecule (m), considering that the electrons (e) in all the bonds (chemical) are distributed equally (=) between (b/w) the atoms (a), irrespective of related electronegativity. Simply, we can define formal (f) charge as the difference (d) between (b/w) the number (no.) of valence (v) electrons (e) of an atom (a) in a neutral (n) free state (s) and the number (no.) given to that atom (a) in a lewis structure. When we have to determine the best (b) lewis structure (st) for a molecule (m), the structure (st) is selected such that (in order) the formal charge (f.c) on individual atoms (a) is closer to 0 (zero) as much as possible. A Formal charge can be considered a tool (measure) for determining the distribution (division) of electric charge (q) within a molecule.

Formula

The formal (f) charge (q) of any individual atom (a) in a molecule (mol) can be given by the below equation −

$$\mathrm{F=V-L-B/2, }$$

where, F= formal charge (q), V= number (no.) of valence (v) electrons (e) of the (n) neutral type atom (a) in isolation (or in the ground level or state), and L is the no. of not bonding valence electrons (e) on the atom (a) in the molecule, and B denotes the total (t) number (no) of electrons (e) shared in the bonds (b) with other atoms (a) in the molecule.

Calculation

As we know formal charge (q)= [no. of valence (v) electrons (e) on atom (a)]-[non-bonded (n.b) electrons (e) + number of bonds].

• Let’s calculate the formal charge (q) of B in BH₄, here the number (no.) of valence (v) electrons (e) for boron (B) is +3, here the number (no) of not-bonded electrons (e) is zero (0), and the number (no) of bonds (b) around boron (B) is 4. So, the formal charge (q) = 3- (0+4) = 3 - 4 = -1.

• Let’s calculate the formal charge (q) of C in :CH₃, here the number (no) of valence electrons (e) for carbon (C) is +4, here the number of non-bonded electrons is 0 (zero), and the number (no) of bonds (b) around carbon (C) is 3. So, the formal charge (q) = 4 - (0+3) = 4 - 3= +1.

Importance

Some importance of formal charge are as follows −

• The Formal charge (q) is a theoretical concept, so it does not involve or indicate any real charge (q) separation in the molecule.

• The Formal charge also plays a very crucial role in determining the lowest energy (e) configuration among several (multiple) possible lewis structures for the given molecule.

• Also, knowing the lowest possible energy (e) structure is critical in pointing (taking) out the primary product (p) of a reaction.

• The structure of least or lowest energy (e) is usually the one with the minimal (least) formal charge (q) and most distributed real charge.

Example

Let us take an example for calculating formal charges.

1. Calculate the formal charge on the following −

• O atoms of O₃

• Cl atoms in HClO₄^- ion

F (formal charge)=V(valence electrons)-L(non-bonding electrons)-B( t.e)/2

• Formal charge on O1: 6-2-6/2=+1.

  Formal (f) charge on O2: 6-4-4/2=0.

  Formal (f) charge on O3: 6-6-2/2=-1.

• Formal (f) charge on Cl atoms in HClO₄^- ion: 7-0-8/2=+3.

Significance

• An atom (a) in a molecule must have a formal charge (q) of zero to have the lowest (l) energy and hence the most stable (st) state.

• As we know formal charge (q) is used to indicate a molecule’s preferred structure (s), but the problem becomes more difficult when numerous or several equally preferred (important) structures exist. This is a condition of resonance structures.

• The formal charge (q) of a molecule can easily indicate how it will act or behave during a process (p). And a negative (-ve) formal charge (q) indicates that the atom (a) is more likely to be the source (s) of electrons (e) in a reaction.

Fun Facts On Formal Charge

• According to some conventions, formal charge (q) is important for depicting or representing a complete and right Lewis-Kekule structure in organic chemistry, but the same is not applicable for inorganic chemistry.

• The variations in the structure (st) of a molecule having the lowest or least amount (quantity) of charge (q) are the most superior.

Conclusion

A formal charge, denoted by q is defined or described as the charge (q) provided to an atom (a) in a molecule (mol), expecting that the electrons (e) in all bonds or chemical bonds are distributed equally (=) between (b/w) the atoms (a), regardless of related (respective) electronegativity. Or the formal (f) charge can also be defined as the difference (d) between (b/w) the number (no.) of valence electrons (e) of an atom (a) in a neutral (n) free state (s) and the number (no.) assigned to that particular atom (a) in a Lewis Structure. The equation or formula for the formal charge (q) is- F=V-L-B/2. It is also used for selecting the lowest (lower) energy structure from all possible structures.

FAQs

1. Define Electronegativity.

Electronegativity is defined as the ability of an atom (a) of a chemical (given) element (e) to attract shared electrons (e) when forming a bond (chemical).

2. What do you mean by the neutral state?

When an object or material is neither positively nor negatively charged then it is considered a neutral state.

3. Describe Lewis Structure.

Lewis structure also called lewis dot structure or electron dot structure, are the diagrams or pictorial representation to show the bonding (b) between the atoms (a) of a molecule, also the depiction of lone pairs (l.p) of electrons (e) that may present in that molecule.

4. The Formula for calculating formal charge.

The equation or formula for the formal charge (q) is- F=V-L-B/2.

5. The Formal charge is used for?

A Formal charge can be considered or used as a tool (measure) for determining the distribution (division)of electric charge (q) within a molecule.