How to calculate formal charges of Sulfite (SO32-) ion 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.

In this article, we will calculate the formal charges present on the bonded atoms in [SO3]2- and also the overall charge present on the molecular ion.

So, continue reading!

Name of the molecular ion Sulfite/ Sulphite
Chemical formula[SO3]2-
The formal charge on the S-atom0
The formal charges on double-bonded O-atom0
The formal charge on each single-bonded O-atoms-1
The overall formal charge on [SO3]2--2

How to calculate the formal charges on SO32- 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 the Sulfite ion [SO3]2-.

The most preferred Lewis representation of [SO3]2- is as shown below.

best stable lewis structure of SO32-

It consists of a total of 26 valence electrons. A Sulfur (S) atom is present at the center, which is bonded to three atoms of oxygen (O) via one double covalent bond and two single covalent bonds.

One lone pair of electrons is present at the central S-atom. The double-bonded O-atom contains 2 lone pairs of electrons while each of the two single-bonded O-atoms contains 3 lone pairs of electrons.

It is the best possible Lewis structure of [SO3]2- because the formal charges present on bonded atoms are minimized in it, and thus, it is the most stable.

Let’s find out how we can determine the formal charges present on each atom in the sulfite [SO3]2- ion Lewis structure.

For the central Sulfur atom

calculating formal charge on sulfur in so32-

  • Valence electrons of Sulfur = It is present in Group VI A = 6 valence electrons
  • Bonding electrons around Sulfur = 1 double bond + 2 single bonds = 1(4) + 2(2) = 8 electrons
  • Non-bonding electrons on Sulfur = One lone pair = 2 electrons
  • Formal charge on the Sulfur atom = 6 – 2 – 8 / 2 = 6 – 2 – 4 = 6 – 6 = 0

The formal charge on the Sulfur (S) atom in [SO3]2- is 0.

For single-bonded oxygen atoms

calculating formal charge on single bonded oxygen in so32-

  • Valence electrons of oxygen = It is present in Group VI A = 6 valence electrons
  • Bonding electrons around Oxygen = 1 single bond = 2 electrons
  • Non-bonding electrons on Oxygen = 3 lone pairs = 3(2) = 6 electrons
  • Formal charge on the single bonded Oxygen atom = 6 – 6 – 2/2 = 6 – 6 – 1 = 6 – 7 = -1

The formal charge on each single-bonded oxygen (O) atom in [SO3]2- is -1.

For double-bonded oxygen atoms

calculating formal charge on double bonded oxygen in so32-

  • 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 double bonded Oxygen = 6 – 4 – 4/2 = 6 – 4 – 2 = 6 – 6 = 0

The formal charge on the double-bonded oxygen (O) atom in [SO3]2- is 0.

This calculation shows zero formal charges are present on the central S atom and one double-bonded O-atom while each of the two single-bonded oxygen atoms has a -1 formal charge that when added up (-1 + (-1) = -2) results in -2, which is also the charge present on the Sulfite ion [SO3]2- overall, as shown below.  

so32- lewis structure with formal charge

As a final step, the SO32- Lewis structure is enclosed in square brackets, and a -2 charge is placed at the top right corner.

so32- formal charge

Also, check –

FAQ

How can you calculate [SO3]2- formal charges?

The formal charges present on the bonded atoms in [SO3]2- 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]2-?

The central Sulfur (S) atom carries zero formal charges in [SO3]2-.

What is the formal charge on O-atoms in [SO3]2-?

Zero or no formal charge is present on one double-bonded (S=O) bonded oxygen atom in [SO3]2-. In contrast, a -1 formal charge is present on each of the two single-bonded (S-O) oxygen atoms in [SO3]2-.

What is the overall formal charge on [SO3]2-?

The overall formal charge on [SO3]2- is –2.

Do all three O-atoms carry the same formal charge in [SO3]2-

No, the double bonded (S=O) oxygen atom in [SO3]2- carries zero formal charges while a -1 formal charge is present on each single bonded (S-O) oxygen atom in [SO3]2-.

Summary

  • The best possible Lewis structure of a molecule or molecular ion 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]2-, zero formal charges are present on the central S-atom.
  • The double-bonded O-atom also has zero formal charges in [SO3]2-.
  • The single-bonded O-atoms have -1 formal charges in [SO3]2-.
  • The overall formal charge on [SO3]2- is -2.
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