How many pi bonds are in SO_2?

1 Answer
Owen Bell
Mar 27, 2016

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

Explanation:

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

Source: Wikipedia

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:

Source: Wikipedia

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

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