Is SO3 2- Polar or Nonpolar? (And Why?)

SO3 2- (Sulfite ion) is a POLAR ion.

But why? 

And how can you say that SO3 2- ion is a polar ion?

Want to know the reason?
Let’s dive into it!

SO3 2- is a POLAR ion because the Oxygen (O) atom is more electronegative and it also has lone pair, which results in an asymmetric shape of the ion.
Because of this, the partial positive (ẟ+) and partial negative (ẟ-) charge appears on the ion. These ẟ+ and ẟ- charges are responsible to make the entire SO3 2- ion polar.

Let me explain this in detail with the help of SO3 2- lewis structure and its 3D geometry.

Why is SO3 2- a Polar ion? (Explained in 3 Steps)

SO3 2- ion is a polar ion because it has poles of partial positive charge (ẟ+) and partial negative charge (ẟ-) on it.

Let me explain this to you in 3 steps!

Step #1: Draw the lewis structure

Here is a skeleton of SO3 2- lewis structure and it contains three Sulfur-Oxygen bonds.

(Note: If you want to know the steps of drawing the SO3 2- lewis dot structure, then visit this short 2 minute video).

So from the above diagram we have come to know that the SO3 2- ion has three Sulfur-Oxygen bonds.

Now in the next step we have to check whether these bonds are polar or nonpolar.

And we also have to check the molecular geometry of SO3 2- ion.

Step #2: Check whether individual bonds are polar or nonpolar

The chemical bonds can be either nonpolar, polar or ionic depending on the difference of the electronegativity values (ΔEN) between the two atoms.

Have a look at the above image.

• If the electronegativity difference (ΔEN) is less than 0.4, then the bond is nonpolar covalent bond.
• If the electronegativity difference (ΔEN) is between 0.4 to 1.7, then the bond is polar covalent bond. 
• If the electronegativity difference (ΔEN) is greater than 1.7, then the bond is an ionic bond. ^{[1] [2] [3] [4] [5]}

Now let’s come to the example of SO3 2- ion. It has three Sulfur-Oxygen bonds.

You can see the electronegativity values of Sulfur (S) and Oxygen (O) atoms from the periodic table given below.

From the above image;

• Electronegativity of Sulfur (S) = 2.58 ^[6]
• Electronegativity of Oxygen (O) = 3.44 ^[7]

Now let’s see the polarity of each bond.

For Sulfur-Oxygen bond;
The electronegativity difference (ΔEN) = 3.44 – 2.58 = 0.86
This value lies between 0.4 to 1.7, which indicates that the bond between Sulfur (S) and Oxygen (O) is polar.
Hence, the Sulfur-Oxygen bond is a polar covalent bond.

You can see in the above image that because of higher electronegativity of Oxygen atom, the partial positive charge (ẟ+) appears on the Sulfur atom (S) and partial negative charge (ẟ-) appears on the Oxygen atoms (O).

But wait, we also have to look at the molecular geometry of SO3 2- ion to know whether it has a symmetric shape or not.

Step #3: Check whether the ion is symmetric or not

Have a look at this 3D structure of SO3 2- ion. The Sulfur atom (S) is at the center and it is surrounded by 3 Oxygen atoms (O).

It also has one lone pair on the Sulfur atom (S).

Due to the lone pair on the sulfur atom (S), its molecular geometry becomes asymmetric.

Because of this, there are positive and negative poles of charges on the overall SO3 2- ion.

Hence, the SO3 2- ion is a polar ion.

I hope you have understood the reason behind the polar nature of SO3 2- ion (sulfite ion).

See the polarity of other molecules to make your concepts clear:
Is AsF5 Polar or Nonpolar?
Is ClF Polar or Nonpolar?
Is CH3OCH3 Polar or Nonpolar?
Is BrCl3 Polar or Nonpolar?
Is HI Polar or Nonpolar?