What are free radicals?

Free radicals are highly reactive chemical species characterized by having unpaired electrons in their outer shell. This unpaired electron makes them seek out other electrons to achieve stability, often leading to various chemical reactions with other molecules. In biological systems, free radicals can play a role in processes such as aging and inflammation, and they are also involved in some chemical reactions in the atmosphere.

The other options reflect concepts that do not accurately describe free radicals. Stable chemical compounds, for instance, have all their electrons paired, which contrasts with the essential nature of free radicals. Elements in a gaseous state does not capture the unique reactivity of free radicals, nor does it speak to the importance of unpaired electrons. Lastly, the definition of free radicals is not limited to molecules containing hydrogen; they can consist of a wide range of atoms and are not restricted to any specific elemental composition. Thus, option B correctly captures the defining feature of free radicals.