What is Formal Charge?
A formal charge (F.C. or q) is the charge assigned to an atom in a molecule in the covalent view of bonding, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity.
The formal charge is the difference between an atom's number of valence electrons in its neutral free state and the number allocated to that atom in a Lewis structure.
When choosing the optimum Lewis structure (or predominant resonance structure) for a molecule, it is important to keep the formal charge on each of the atoms as low as feasible.
The following equation can be used to compute the formal charge of an atom in a molecule:
F = V - L - \[\frac{B}{2}\]
Where,
F = Formal Charge
V = Valence Electron of the neutral atom in isolation
L = Number of non-bonding valence electrons on this atom in the molecule
B = Total number of electrons shared in bonds with other atoms in the molecule
Formula, Calculation, Importance, and Example
The formula for computing a formal charge is:
(Number of valency electrons in neutral atom)-(electrons in lone pairs + 1/2 the number of bonding electrons)
The number of bonding electrons divided by two equals the number of bonds that surround the atom, hence this expression can be reduced to:
Formal Charge = (number of valence electrons in neutral atom)-(non-bonded electrons + number of bonds)
Example 1:
Take the compound BH4 or tetrahydrdoborate.
Boron (B) possesses three valence electrons, zero non-bonded electrons, and four bonds around it.
This changes the formula to 3-(0+4), yielding a result of -1.
Let us now examine the hydrogen atoms in BH4. One valence electron, zero non-bonded electrons, and one bond make up hydrogen.
In BH4, the formal charge of hydrogen is 1-(0+1), resulting in a formal charge of 0.
Example 2:
Calculate the formal charge on the following:
O atoms of O3
Cl atom in HClO4- ion
S atom in HSO4- ion
Ans: We are showing how to find a formal charge of the species mentioned.
Formal charge on O1: 6 – 6/2 – 2 = +1
Formal charge on O2: 6 – 4/2 – 4 = 0
Formal charge on O3: 6 – 2/2 – 6 = -1
Formal charge on Cl atom of HClO4 ion: 7 – 8/2 – 0 = 3
Formal charge on S atom of HSO4- ion: 6 – 8/2 – 0 = 2
Significance
Molecular Structure
An atom in a molecule should have a formal charge of zero to have the lowest energy and hence the most stable state. If there are numerous alternatives for a molecule's structure, this gives us a hint: the one with the least/lowest formal charges is the ideal structure.
Resonance
While formal charge can indicate a molecule's preferred structure, the problem becomes more complicated when numerous equally preferred structures exist. This condition could point to resonance structures, especially if the structures have the same atom arrangement but different types of arrangements of bonds.
Reactivity
The formal charge of a molecule can indicate how it will behave during a process. A negative formal charge indicates that an atom is more likely to be the source of electrons in a reaction (a nucleophile). If it has a positive one, on the other hand, it is more likely to take electrons (an electrophile), and that atom is more likely to be the reaction's site.
It's also worth noting that an atom's formal charge differs from its actual charge. Formal charge ignores electronegativity and assumes that electrons in a bond are uniformly distributed.
It's only a courtesy that's utilized to make molecular structures and reaction mechanisms more understandable. The actual charge, on the other hand, is based on the electronegativities of the atoms and the polarity of the bonds and looks at the actual electron density.
Importance Of Formal Charge
Now that we know what is the formal charge and we are familiar with the process for calculating a formal charge, we will learn about its importance.
The formal charge is a theoretical concept, useful when studying the molecule minutely. It does not indicate any real charge separation in the molecule. This concept and the knowledge of ‘what is formal charge' is vital.
The formal charge is crucial in deciding the lowest energy configuration among several possible Lewis structures for the given molecule. Therefore, calculating formal charges becomes essential.
Knowing the lowest energy structure is critical in pointing out the primary product of a reaction. This knowledge is also useful in describing several phenomena.
The structure of least energy is usually the one with minimal formal charge and most distributed real charge.
Besides knowing what is a formal charge, we now also know its significance.
Fun Facts On Formal Charge
In organic chemistry, convention governs that formal charge is essential for depicting a complete and correct Lewis-Kekulé structure. However, the same does not apply to inorganic chemistry.
The structure variation of a molecule having the least amount of charge is the most superior.
The differences between formal charge and oxidation state led to the now widely followed and much more accurate valence bond theory of Slater and the molecular orbital theory of Mulliken.
