The apparent charge assigned to an atom in a molecule is termed formal charge. It is a representation of charge distribution in a molecule assuming that all electrons are equidistant from bonding atoms with no concept of electronegativity or polarity.
Since the formal charge is a bygone concept relating to the Lewis structures. The best Lewis structure of a molecule is supposed to be the one with the least formal charge. It means that out of different possible configurations and arrangements of atoms on a molecule, the most stable structure will be the one with a formal charge of either zero or as near to zero as possible.
A formal charge is a fake charge or to be more polite, it is a hypothetical charge because if it were real, the charge would have spread to the whole molecule instead of being contained at a single atom.
Why does the formal charge exist?
A formal charge exists because of the deficiencies in the configurations of atoms leading to the formation of molecules. It is the formal charge which makes a certain arrangement and configuration suitable for a molecule when more than one resonating structure is possible.
Formal Charge formula
The formal charge can be calculated using a formula such as;
Qf = V – N – B/2
where,
Qf = Formal charge
V = Valence electrons on a neutral atom
N = Non-bonding electrons on an atom in the molecule
B = Bonding electrons or shared electrons with other atoms in the molecule
Calculating Formal Charges
Formal Charge = Valence electrons of a free atom – Non-bonding electrons (lone pairs) – ½(Bonding electrons)
Some standards must be kept in mind while determining the formal charge on atoms of molecules
- The formal charge of a neutral molecule must always be zero.
- The formal charge of a molecular ion should be equal to the charge on that ion whether it’s an anion or a cation.
The formal charge is just a hypothetical bookkeeping procedure for the determination of the correct Lewis structure and geometry of molecules. It has nothing to do with the actual charge on atoms or the electronic vicinity of an atom or a molecule.
Examples of formal charge calculation
Formal charge on carbon dioxide (CO2)
Formal charge = V (free atom) – Non-BE (lone pairs) – ½ BE (bond pairs)
Formal charge on carbon atom = 4 – 4 – 0 = 0
Formal charge on oxygen atom (1) = 6 – 4 – 2 = 0
Formal charge on oxygen atom (2) = 6 – 4 – 2 = 0
The total formal charge on carbon dioxide is thus zero.
Formal charge on sulphur dioxide (SO2)
Formal charge = V (free atom) – Non-BE (lone pairs) – ½ BE (bond pairs)
Formal charge on sulphur atom = 6 – 2 – 4 = 0
Formal charge on oxygen atom (1) = 6 – 4 – 2 = 0
Formal charge on oxygen atom (2) = 6 – 4 – 2 = 0
The total formal charge on sulphur dioxide is thus zero.
Formal charge on Iodine tetrachloride ( ICl41- )
Formal charge = V (free atom) – Non-BE (lone pairs) – ½ BE (bond pairs)
Formal charge on iodine atom = 7 – 4 – 4 = -1
Formal charge on chlorine atom (1) = 7 – 6 – 1 = 0
Formal charge on chlorine atom (2) = 7 – 6 – 1 = 0
Formal charge on chlorine atom (3) = 7 – 6 – 1 = 0
Formal charge on chlorine atom (4) = 7 – 6 – 1 = 0
The total formal charge on iodine tetrachloride is thus [ -1 + 0 + 0 + 0 + 0 ] = -1
Formal charge on Ozone (O3)
Formal charge = V (free atom) – Non-BE (lone pairs) – ½ BE (bond pairs)
Formal charge on oxygen atom (1) = 6 – 4 – 2 = 0
Formal charge on oxygen atom (2-central) = 6 – 2 – 3 = +1
Formal charge on oxygen atom (3) = 6 – 6 – 1 = -1
The total formal charge on an ozone molecule is thus (0 +1 -1) = zero.
Formal charge on ammonium ion (NH4+)
Formal charge = V (free atom) – Non-BE (lone pairs) – ½ BE (bond pairs)
Formal charge on nitrogen atom = 5 – 0 – 4 = +1
Formal charge on hydrogen atom (1) = 1 – 0 – 1 = 0
Formal charge on hydrogen atom (2) = 1 – 0 – 1 = 0
Formal charge on hydrogen atom (3) = 1 – 0 – 1 = 0
Formal charge on hydrogen atom (4) = 1 – 0 – 1 = 0
The total formal charge on ammonium ion is thus [ +1 + 0 + 0 + 0 + 0 ] = +1
Formal charge on Perchloric acid (HClO4)
Formal charge = V (free atom) – Non-BE (lone pairs) – ½ BE (bond pairs)
Formal charge on chlorine atom = 7 – 0 – 4 = +3
Formal charge on oxygen atom (1) = 6 – 6 – 1 = -1
Formal charge on oxygen atom (2) = 6 – 6 – 1 = -1
Formal charge on oxygen atom (3) = 6 – 6 – 1 = -1
Formal charge on oxygen atom (4-hydroxyl) = 6 – 4 – 2 = 0
The total formal charge on perchloric acid is thus zero i.e. [ +3 + (-1) + (-1) + (-1) + 0 ] = 0
Related Topics
Use of formal charges to predict molecular structures
Carbon dioxide
The O=C=O shape for carbon dioxide is preferred because it has the least formal charges for all atoms.
Sulphur dioxide
The O=S=O shape is preferred for sulphur dioxide because it has the least formal charges on all atoms.
Thiocyanate ion (NCS)
All possible structures of a thiocyanate ion possess a total formal charge of zero but N=C=S possesses the least formal charge on all atoms. Thus NCS is the preferred shape.
Perchloric acid (HClO4)
The (O3Cl-OH) shape of perchloric acid is the most stable one because it has an overall formal charge of zero.
Significance of Formal Charge
A molecule can exist in many possible structures. The factor that enables it to exist in a free state is the energy content. The possible structure with the lowest energy is likely to exist the most. Now that the energy of molecules cannot be measured directly, there is a concept of formal charge which can predict whether or not the asked structure is high or low in energy as compared to others.
It means that formal charge cannot predict the exact energy content of a molecule, rather it predicts the comparative stability of possible structures by their formal charge contents.
The formal charge being one of the parameters to predict the possible configuration of molecules becomes essential in chemistry. This is because the correct prediction of the most stable configuration of a product in a reaction enables a chemist to alter the production rates and emphasize the output product.
Furthermore, it is the formal charge that induces advanced concepts like the valence bond theory (VBT), and molecular orbital theory (MOT), etc.
Comparison of Formal Charge and Oxidation state
Formal charges and oxidation states are both similar yet alternative ways to look at the distribution of electrons within molecules.
The formal charge emphasizes on the equidistant approach of individual atoms on shared/bonded electrons. It assumes the bonded electrons to be at the exact center of both atoms.
Oxidation state is rather an ionic-nature favoring parameter for the calculation of charges on molecules. It states that the shared/bonded electrons are not present in the exact center of a bond. These electrons lean towards the electronegative atom, making a difference in electron cloud vicinity for bonded atoms.
Oxidation state is an advanced and efficient way of looking at shared electrons in a molecule to predict the charges and geometries whereas formal charge is an old but basic level of looking at shared electrons to take out charges and correct geometries. So far, the formal charge and oxidation states of molecules do not coincide with each other yet they both are different ways to give the same answer.
Concepts Berg
What is the difference between polarity and formal charge?
A formal charge is a hypothetical charge that exists on an atom in a molecule. It assumes that shared electrons are equidistant from the parent atoms which coincides with the concept of polarity.
Polarity is the assumption that electrons are never equidistant from parent atoms unless they are the same atoms e.g. H-H. The variable distance from individual atoms in a bond is explained by electronegativity in polarity which is the ability of an atom to attract the shared pair of electrons.
What does a formal charge tell?
To be exact, a formal charge tells you whether an atom has more electrons or protons within its vicinity. It also points out the most suitable and low-energy state of configuration a molecule adopts while forming geometry.
Why does nitrogen have a negative formal charge in nitrate ions?
In a nitrate ion (NO3-1) there are two N-O single bonds and one N=O double bond. A lone pair exists on the nitrogen atom.
The formal charge on nitrogen atom = V – Non-BE – ½ BE
= [5 – 2 – 4] = -1
Formal charge on nitrogen on nitrate ion = -1
Why is it called “formal” charge?
A formal charge has a name formal because it is a hypothetical charge which doesn’t exist in reality. In the real world, charges are spread across whole molecules. They are never contained on a single atom after the formation of a molecule, even in ionic compounds. That is the reason this charge is called ‘formal’ or ‘formality’.
What is the importance of formal charge?
- Formal charge enables a chemist to predict the correct nature of geometry a molecule has in a stable free state.
- It makes it possible to analyze electronic sharing by sorting out the molecule into individual charges.
- It is the reason Lewis structures exist.
- The formal charge is the basis of modern concepts like VBT, MOT, etc.
What is the formal charge equation?
Formal Charge = Valence electrons(free atoms) – Non-bonding electrons (lone pairs) – ½(Bonding electrons)
Qf = V – N – B/2
How can we determine a formal charge?
We can determine the formal charge on a molecule by the formal charge equation.
Formal Charge = Valence electrons – Nonbonding electrons – ½(Bonding electrons)
For a molecule, the sum of formal charges of individual atoms is the formal charge of the whole molecule.
What is the formal charge on the carbon atom in CO?
Carbon monoxide exists as [C-O]. In this type of configuration,
F.C = V – N – B/2
Formal charge on carbon atom = 4 – 2 – 3 = -1
Formal charge on oxygen atom = 6 – 2 – 3 = +1
Overall, the formal charge on CO is zero. [+1 -1] = 0
What is the formal charge of the oxygen atom in water?
In water, an oxygen atom is bonded to two hydrogens along with two lone pairs of electrons.
Formal charge on oxygen atom = 6 – 4 – 2 = 0
Formal charge on hydrogen atoms = 1 – 0 – 1 = 0
So, the overall formal charge on water is zero.
Which should be chosen, the octet rule or formal charge?
Out of octet and formal charge concepts, the octet rule is obviously preferred because it is the reason the formal charge concept exists. The octet rule is the most essential one because all other concepts i.e. Lewis structures, formal charge, VBT, and MOT are designed based on octet rule.
What is the formal charge on the hydrogen atom in HF?
In a H-F molecule, the formal charges of atoms are as;
Formal charge on hydrogen atom = 1 – 0 – 1 = 0
Formal charge on fluorine atom = 7 – 6 – 1 = 0
Overall, the HF molecule is with a formal charge of zero.
What is the formal charge of an ozone molecule?
Ozone molecules exist in [O=O+– O–] configuration. This is because it is the only possible configuration in which individual atoms have the least formal charge. Overall, the formal charge of ozone molecules becomes zero. [0+1-1] = 0.
What determines the stability of a molecule?
The stability of a molecule is determined by several factors:
- HOMO-LUMO gap in MOT
- Minimal formal charge
- The negative charge contained in electronegative atom and vice versa
- Resonance
- Inductive effect
- Three-dimensional arrangement and configuration of atoms in a molecule, etc.
What is the formal charge rule?
The rule of formal charge is that the number of electrons and protons in an atom must be equal or as near to equal as possible. If by any means there is inequality among protons and electrons, that effect is shown by either a positive or negative formal charge.
