A free radical, often simply called a radical, is a reactive species that contains an unpaired electron. It matters because the unpaired electron makes radicals highly reactive, driving chain reactions such as combustion, polymerisation and the halogenation of alkanes. Examples include a chlorine atom, written Cl• with a dot for the unpaired electron, and the methyl radical, written •CH3. Radicals are usually formed by homolysis, the even splitting of a bond.

自由基,常简称游离基,是含有一个未成对电子的活性物种。它之所以重要,是因为这个未成对电子使自由基高度活泼,推动燃烧、聚合以及烷烃卤代等链式反应。例子包括氯原子(写作 Cl•,圆点代表未成对电子)和甲基自由基(写作 •CH3)。自由基通常由均裂(键的均匀断裂)生成。