**8. Free radicals**

Most of the chemical compounds of biological relevance are made by atoms joined together by covalent bonds, where two different atoms share a pair of electrons in the same orbital, and each electron rotates in the opposite direction to its pair. In the cells, chemical reactions that break these bonds heterolytically take place continuously, making one of the parts take two electros and generating unstable nucleophilic or electrophilic compounds, known as anions and cations. However, some chemical reactions, electromagnetic radiation, and other factors may break bonds homolytically, resulting in two parts that have one electron each; these are known as free radicals [47].

Generally speaking, a free radical is an atom or molecule that has one or more unpaired electrons in the external orbitals and is capable of existing independently. It is very reactive and tends to reduce in order to stabilize, which means that subtracts an electron from stable atoms or molecules that are in turn oxidized. Once the free radical has obtained its missing electron, the stable molecule is oxidized and is left with an unpaired electron, which makes it a new free radical that initiates a chain reaction [6].
