Final answer:
NO (nitric oxide) is the free radical among the given options as it contains an odd number of electrons and is highly reactive with an unpaired electron (2).
Step-by-step explanation:
Identification of a Free Radical
The definition of a free radical is a molecule that contains an odd number of electrons. Among the options given, NO (nitric oxide) is an example of a free radical because it contains an unpaired electron, making it highly reactive. Nitric oxide is produced under high-temperature conditions like in internal combustion engines, where oxygen and nitrogen gases react. It is distinct from nitrogen dioxide (NO2), which has an even number of electrons.
Compounds such as CO (carbon monoxide), NO2 (nitrogen dioxide), SO3 (sulfur trioxide), and O3 (ozone) are not free radicals because they have an even number of electrons. Oxygen species like superoxide and peroxide, along with the hydroxyl radical, are reactive oxygen species (ROS) that contain an unpaired electron and are harmful to biological systems.