How does an unpaired electron make a free radical highly reactive?

In chemistry, a radical is a molecule that contains at least one unpaired electron. While most stable molecules consist of atoms held together by pairs of shared electrons, radicals possess an "odd" electron that makes them highly reactive.1

This high reactivity stems from the radical's inherent drive to achieve a stable electronic configuration. To complete their electron pairs, radicals typically interact in one of the following ways:

• Combination: They may combine with another radical or an atom that also carries a single unpaired electron to form an ordinary molecule where all electrons are paired.1
• Abstraction: They may react with intact molecules by abstracting parts of those molecules to complete their own electron pairs, a process that simultaneously generates new free radicals.1

Because of this tendency to seek stability, many simple radicals have only a fleeting existence and act as transient intermediates in various chemical processes, including combustion, polymerization, and chain reactions. In biological contexts, this reactivity can also lead to the damage of cells, proteins, and DNA.23