How to calculate formal charges of Sulfur Trioxide (SO3) with lewis structure?

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In covalently bonded molecules, formal charge is the charge assigned to an atom based on the assumption that the bonded electrons are equally shared between concerning atoms, regardless of their electronegativity.

The overall formal charge present on a molecule is a measure of its stability.

The fewer the formal charges present on the bonded atoms in a molecule (close to zero), the greater the stability of its Lewis structure.

Sulfur Trioxide is a colorless liquid and, upon air exposure, can “magically” produce white fumes (by reacting to moisture).

In this article, we will calculate the formal charges present on the bonded atoms in SO3 and also the overall charge present on the molecule.

 So, continue reading!

Name of the moleculeSulfur Trioxide
Chemical formulaSO3
The formal charge on the S atom0
The formal charge on double-bonded O-atoms0
The overall formal charge on SO30

How to calculate the formal charges on SO3 atoms?

The formal charges can be calculated using the formula given below:

formal charge formula

The formal charge of an atom = [valence electrons of an atom – non-bonding electrons – ½ (bonding electrons)]

  • The valence electrons (V.E) of an atom are the total number of electrons present in its valence shell. Valence electrons can be determined by locating the position of the elemental atom in the Periodic Table.
  • Non-bonding electrons (N.E) are the number of lone pairs present on the atom. (1 lone pair means 2 nonbonding electrons).
  • Bonding electrons (B.E) are the total electrons shared with the atom via covalent chemical bonds. (1 single bond means 2 bonding electrons).

Now let us use this formula to calculate the formal charges in the most preferred Lewis structure of SO3.

The most preferred Lewis representation of the sulfur trioxide (SO3) molecule is as shown below.

best stable lewis structure of so3

It consists of a total of 24 valence electrons. A Sulfur (S) atom is present at the center, which is bonded to three atoms of oxygen (O) via three double covalent bonds.

No lone pair of electrons is present at the central S-atom. Each of the three double-bonded O-atoms contains 2 lone pairs.

It is the best possible Lewis structure of SO3 because the formal charges are minimized in it, and thus, it is also the most stable.

Let’s find out how we can determine the formal charges present on each atom in the SO3 Lewis structure.

For the central Sulfur atom

calculate the sulfur atom formal charge in so3

  • Valence electrons of Sulfur = It is present in Group VI A = 6 valence electrons
  • Bonding electrons around Sulfur = 3 double bonds = 3(4) = 12 electrons
  • Non-bonding electrons on Sulfur = no lone pair = 0 electrons
  • Formal charge on the Sulfur atom = 6 – 0 – 12/2 = 6 – 0 – 6 = 6 – 6 = 0

The formal charge on the Sulfur (S) atom in SO3 is 0.

 For each double-bonded oxygen atom

calculate the oxygen atom formal charge in so3

  • Valence electrons of oxygen = It is present in Group VI A = 6 valence electrons
  • Bonding electrons around Oxygen = 1 double bond = 4 electrons
  • Non-bonding electrons on Oxygen = 2 lone pairs = 2(2) = 4 electrons
  • Formal charge on the Oxygen atom = 6 – 4 – 4/2 = 6 – 4 – 2 = 6 – 6 = 0

The formal charge on each double-bonded oxygen (O) atom in SO3 is 0.

This calculation shows that zero formal charges are present on the central S atom as well as on each of the three double-bonded O-atoms in the SO3 Lewis structure.

so3 formal charge

As a result, there is no overall charge on the SO3 molecule, and thus it is a neutral molecule.

so3 lewis structure formal charge

Also, check –

FAQ

How can you calculate SO3 formal charges?

The formal charges present on the bonded atoms in SO3  can be calculated using the formula given below:

V.E – N.E – B.E/2

Where –

⇒ V.E = valence electrons of an atom

⇒ N.E = non-bonding electrons, i.e., lone pairs

⇒ B.E = bonding electrons

What is the formal charge on S-atom in SO3?

The central Sulfur (S) atom carries a zero formal charge in SO3.

What is the formal charge on O-atoms in SO3?

Zero or no formal charges are present on S=O double-bonded oxygen atoms in SO3.

What is the overall formal charge on SO3?

The overall formal charge on SO3 is 0.

Do all three O-atoms carry the same formal charge in SO3

Yes, zero formal charges is present on all the O-atoms in SO3.

Summary

  • The best possible Lewis structure of a molecule is the one in which the bonded atoms carry formal charges as close to zero as possible.
  • The formal charge formula is [ V.E – N.E – B.E/2].
  • In SO3, zero formal charges is present on the central S-atom.
  • The double-bonded O-atoms also have zero formal charges in SO3.
  • The overall formal charge on SO3 is 0.
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