How do chain reactions end during the chlorination of methane?

In the chain reaction between methane and chlorine, termination occurs when free radicals collide and combine, effectively removing the reactive species necessary to sustain the propagation steps.12

Because these radicals possess unpaired electrons, they are highly reactive and typically combine to form stable molecules with paired electrons. In the specific context of chlorination, the primary termination steps include:3

• Dimerization of methyl radicals: Two methyl radicals ($·{\text{CH}}_3$) combine to form ethane (${\text{CH}}_3{\text{CH}}_3$).12
• Combination of methyl and chlorine radicals: A methyl radical ($·{\text{CH}}_3$) reacts with a chlorine radical ($·\text{Cl}$) to produce chloromethane (${\text{CH}}_3\text{Cl}$).1

These processes end the chain by consuming the intermediates that would otherwise continue to react with methane or chlorine molecules.12