In a chain reaction during radical substitution, what occurs in each step of propagation?

In a chain reaction during radical substitution, the propagation step is characterized by the formation of new products, while simultaneously maintaining the concentration of radicals. This occurs because the radical species generated in the reaction initiates further reactions with stable molecules, producing additional radicals as products.

During the first part of the propagation, a radical reacts with a stable molecule, typically resulting in the formation of a new radical and a substituted product. For instance, a chlorine radical might react with methane to produce methyl radical and hydrogen chloride. This newly formed methyl radical can then react further, continuing the chain reaction. The ongoing generation of radicals at each step is essential for sustaining the reaction process, allowing it to propagate efficiently.

This is why the correct answer emphasizes that new products form while maintaining the radical concentration, which is crucial for the continuation of the chain reaction. The presence of radicals ensures that the chain process can occur multiple times until the reaction reaches termination, where the radicals combine and are no longer available to propagate further.