The three basic types of chemical reactions:

• Spontaneous reaction, where a substance by itself reacts to form something else; radioactive decay is a common form of this.
• Collisional reaction, where two or more substances collide with each other and a reaction takes place.
• Photodissociation, or photolysis, where a substance absorbs a photon of electromagnetic radiation (light) and splits into products.

Click on the picture of each reaction to see a QuickTime movie of that reaction.

Radioactive decay is a typical example of a spontaneous reaction. Spontaneous reactions are not common in photochemical smog.

Some collisions do not result in reactions, because an activation energy barrier must be crossed by supplying energy (such as heat) to the reaction. Others give off energy (usually two or more molecules simply sticking together to form a single molecule). Some reactions, such as combustion, combine these effects: there is an activation energy barrier, but after the reaction takes place, more energy is released than was taken in to promote the reaction (this is then a net exothermic reaction).

Three-body reactions are much less common than two-body reactions because in a population of randomly moving molecules, the chances of having three molecules in one place at one time is less likely than for two.

A photon of light provides energy for splitting apart a molecule. For molecules that are very strongly bonded, it will require a particularly energetic photon of light, such as ultraviolet.

This type of reaction is what gives L.A. photochemical smog its name.