“Formal Charge” Explained

Molecules or ions in Chemistry are presented as a structure called Lewis Structure, for any molecule/ion this structure can be drawn by using simple rules. I’ve put together another article, listing all rules which can be used for drawing Lewis Structures of different molecules/ions.

But in this article here, I will be explaining the concept of “Formal Charge” of an electron in a molecule/ion. Below is the mathematical formula which can be used for calculating Formal Charge.

Formal Charge = (number of valence electrons in free atom) – (number of valence electrons in bonded atoms) – 1/2 X (number of bonded electrons)

Below are some examples showing steps for calculating the Formal Charge of an atom.

Example 1 – Calculating Formal Charge of Oxygen Atom in Hydroxide Anion OH

Lewis Structure of Hydroxide Anion showing single bond between hydrogen and oxygen atom with 6 electrons, a negative charge around it

Step 1
Number of valence electrons in free Oxygen atom = 6
(Electronic configuration of Oxygen is 1s22s22p4 so the number of electrons in valence orbit are 6)

Step 2
Number of valence electrons in bonded Oxygen atom in Hydroxide Anion = 6
(One little dot around the Oxygen atom denotes 1 electron)

Step 3
Number of bonded electrons around Oxygen atom = 2
(As Oxygen atom is bonded with only one single bond, which contains 2 electrons to hydrogen)

Step 4
Putting all the numbers together in the mathematical formula for figuring out Formal Charge.
Formal Charge of oxygen atom in Hydroxide Anion = 6 – 6 – 1/2 X 2 = 6 – 6 – 1 = -1
So in the Hydroxide Anion, Formal Charge of Oxygen will be -1 which is represented as little minus symbol on the top right of oxygen atom in Lewis Structure of Hydroxide Anion.

Example 2 – Calculating Formal Charge of Nitrogen Atom in Hydrogen Cyanide

Hydrogen Cyanide Lewis Structure showing one single bond between hydrogen, carbon atom and a triple bond between carbon and nitrogen atom. Also showing 2 electrons around nitrogen atom as 2 dots.

Step 1
Number of valence electrons in free Nitrogen atom = 5
(Electronic configuration of Nitrogen is 1s22s22p3 so the number of electrons in valence orbit are 5)

Step 2
Number of valence electrons in bonded Nitrogen atom in Hydrogen Cyanide = 2
(Two little dots around the Nitrogen atom denotes 2 electrons)

Step 3
Number of bonded electrons around Nitrogen atom = 6
(As Nitrogen atom is bonded with 3 single bonds to carbon atom)

Step 4
Putting all the numbers together in the mathematical formula for figuring out Formal Charge.
Formal Charge of Nitrogen atom in Hydrogen Cyanide = 5 – 2 – 1/2 X 6 = 5 – 2 – 3 = 0
So Nitrogen atom doesn’t have any Formal Charge in the Hydrogen Cyanide molecule.

Example 3 – Calculating Formal Charge of Manganese Atom in Manganese Oxide MnO4

Lewis Structure of Manganese Oxide MnO4- showing one manganese atom bonded with three oxygen atoms by double bonds and with one oxygen atom by single bond.
In the Lewis Structure, 3 oxygen atoms have 4 lone electrons each while one oxygen electron have 6 lone electrons

Step 1
Number of valence electrons in free Manganese atom = 7
(Electronic configuration of Manganese is 1s22s22p63s23p53d54s2 or [Ar] 3d5 4s2 so number of valence electrons are 7)

Step 2
Number of valence electrons in bonded Manganese atom in Manganese Oxide = 0
(As it clear from above structure of MnO4 there is no electron around Manganese atom)

Step 3
Number of bonded electrons around Manganese atom = 8
(As Manganese atom have 3 double bonds, 1 single bond with other Oxygen atoms)

Step 4
Putting all the numbers together in the mathematical formula for figuring out Formal Charge.
Formal charge of Manganese atom in Manganese Oxide = 7 – 0 – 1/2 X 8 = 7 – 4 = 3
So Manganese atom have Formal Charge of +3 in Managenese Oxide.

Final Thoughts

If you have any further question regarding “Formal Charge” let me know in the comments below, I would try my best to help you out.

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