How to calculate formal charges of Sulfite (SO32-) ion with lewis structure?
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.
How to calculate the formal charges on SO32- atoms?
The formal charges can be calculated using the formula given below:
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.
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.
∴ 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.
As a final step, the SO32- Lewis structure is enclosed in square brackets, and a -2 charge is placed at the top right corner.
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-.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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