When oxygen (O2) is passed through an electrical spark (lightning is a good natural source of an electrical spark), ozone (O3) is formed. When the equation is balanced, the coefficient of oxygen is

the coefficient of oxygen is 3

Explanation:

When oxygen molecule is pass through an electrical spark ozone O3 molecules are formed. This same reaction takes place in the Earth's atmosphere during the formation of the ozone layer.

The balanced reaction looks like

3O2 > 2O3 [ in presence of electric spark]

From the balance equation we can say that the coefficient of oxygen is 3 and that of ozone is 2

However, lab preparation of ozone, uses a Laser spark plug ignition system.