radical

Radicals (often referred to as free radicals) are atoms, molecules, or ions with unpaired electrons or an open shell configuration. Free radicals may have positive, negative, or zero charge. With some exceptions, the unpaired electrons cause radicals to be highly chemically reactive.

In living organisms, superoxide and nitric oxide and their reaction products regulate many processes, such as control of vascular tone and thus blood pressure. They also play a key role in the intermediary metabolism of various biological compounds. Such radicals can even be messengers in a phenomenon dubbed redox signaling. A radical may be trapped within a solvent cage or be otherwise bound.