# How many pi bonds are there in "CO"_2?

Jan 3, 2015

$C {O}_{2}$ has 2 pi bonds.

First, start with the molecule's Lewis structure, which allows you to determine the hybridization of each atom.

We can see that $C$ has two regions of electron density around it, which means it has a steric number equal to 2. This implies that it is $s p$ hybridized, therefore has 2 unhybridized p-orbitals with which it can form pi bonds with the oxygen atoms.

On the other hand, each $O$ atom has three regions of electron density around it, which means it is $s {p}^{2}$ hybridized. This allows each $O$ atoms to have 1 unhybridized p-orbital with which to form a pi bond.

The bonding in the $C {O}_{2}$ molecule looks like this:

$C$'s sp hybridized orbitals are shown in yellow and its two unhybridized p-orbitals are shown in blue. $O$'s $s {p}^{2}$ hybridized orbitals are shown in green, while its remaining unhybridized p-orbital is shown in blue.

So, $C$ uses its two sp hybridized orbitals to form sigma bonds with the two $O$ atoms (each $O$ atom uses an $s {p}^{2}$ orbital for this).

Let's say $C$ has its ${p}_{z}$ and ${p}_{x}$ orbitals left unhybridized. Its ${p}_{x}$ orbital will form a pi bond with the $O$ atom that has its ${p}_{x}$ orbital unhybridized, while its ${p}_{z}$ orbital will for a pi bond with the other $O$ atom's ${p}_{z}$ orbital.

Therefore, $C {O}_{2}$ has 2 pi bonds and 2 sigma bonds.

A faster way to determine how many pi bonds the molecule has is to know that a double bond is comprised of 1 sigma and 1 pi bond. Since $C {O}_{2}$ has 2 double bonds, it will have 2 pi bonds.