A free radical is a single, unpaired electron that is found in the valence or outer shell of an atom. An atom that has a single electron in its valence shell will not follow the octet rule. A free radical is chemically very reactive since it seeks an electron to complete its outer shell.
The cleavage of the chlorine molecule by ultraviolet (UV) light is an example of how free radicals may be generated. The single covalent bond between the two chlorine atoms is disrupted by the high energy UV light. The resulting products are two chlorine free radicals. Note that each chlorine free radical has only 7 valence electrons. The high energy chlorine free radical is extremely reactive and will seek to obtain another electron to fulfill the Octet Rule and subsequently have lower energy.
Another example of a free radical is shown in the following structural formula of nitric oxide (NO). Note that there are a total of 11 electrons in this structure. The more electronegative atom oxygen has 8 electrons in its outer shell while nitrogen has only 7 electrons in its outer shell. This extremely reactive free radical seeks to obtain another electron to fulfill the octet rule and become a lower energy species.
Structural formula of nitric oxide - NO
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