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How do chlorine radicals keep the methane chlorination chain reaction going?

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Underwater view of a glowing chain of tunicates with a scuba diver and boat visible near the water's surface in the background. — photochemical reaction Chain of fluorescent tunicates.

In the photochemical reaction between methane and chlorine, the chain reaction is sustained through a cycle of propagation steps where chlorine radicals are continuously regenerated.12

The process functions as follows:

Initiation: The reaction begins when ultraviolet light causes a chlorine molecule ( $C l_{2}$ ) to dissociate into two chlorine atoms ( $\cdot C l$ ).12
Propagation:
- A chlorine radical collides with a methane molecule, producing a methyl radical ( $\cdot C H_{3}$ ) and hydrogen chloride ( $H C l$ ).12
- This methyl radical then reacts with another chlorine molecule ( $C l_{2}$ ) to form the product, chloromethane ( $C H_{3} C l$ ), and releases a new chlorine radical ( $\cdot C l$ ).12

Because the chlorine radical consumed in the first propagation step is regenerated in the second, a single chlorine atom can facilitate the production of many molecules of chloromethane. This cycle continues until the chain is broken by termination steps, such as the combination of two radicals.21