Question

How many pi bonds are in `SO_2`?

Answer 1

There are two `pi` bonds in a single `"SO"_2` molecule.

Explanation:

First, let us consider the structure of the `"SO"_2` molecule:

As you can see, the molecule is bent / v-shaped / angular, and there are three regions of electron density: two `"O = S"` double bonds and a lone pair of electrons.

Now, recall that the composition of a double bond is as follows:

• 1x `sigma` bond
• 1x `pi` bond

Therefore, with two double bonds we have two `pi` bonds.

The `sigma` bond results from the head-on overlap of orbitals, and a `pi` bond is the side-on overlap of unhybridised p-orbitals that may occur parallel to a `sigma` bond. The sulfur atom is `"sp"^2` hybridised with one unhybridised p-orbital: this results in resonance structures of sulfur dioxide that constantly alternate back-and-forth to result in the above structure, as follows:

In molecular orbital theory, this is more commonly thought of as a delocalised `pi` system that stretches across the molecule.