Free Radicals

The human body is composed of about 7x10²⁷ atoms, of which 4.2x10²⁷ are hydrogen. A hydrogen atom consists of two particles, a proton and an electron. Hydrogen is not found without its electron in ordinary chemistry, as ionized hydrogen is highly chemically reactive. Since the nucleus of an atom is always surrounded by a cloud of electrons in pairs for the atom to be stable, pure hydrogen atom usually exists in pair so that each proton will share a pair of electrons. In the human body, hydrogen is usually combined with oxygen, carbon and nitrogen to form various molecules, including water. In a living cell, energy-producing reactions must take place, involving the transfer of hydrogen atoms or electrons from one molecule to another, known as biological oxidation, in fact initiated by dehydrogenation of the foodstuffs, such as the carbohydrates.

In biological oxidation, the electrons are brought to the electron transport chain by NADH. 
The electron transport chain consists of carriers, including flavoproteins, cytochromes, and ubiquinones. The electrons are passed from one carrier to the next while energy is being produced. The final electron acceptor is irreversibly reduced; it may be oxygen (aerobic oxidation) or another inorganic molecule (anaerobic oxidation). The normal electron transport chain from NADH to oxygen can be illustrated schematically as follows (Midlandtech.Edu.)

During the process of biological oxidation, many atoms will lose an electron temporarily, leaving the atom with an unpaied electron, so that they will take up an electron from an upstream atom, called an electron donor in this normal flow chain of electrons. However, the atom with an unpaired sometimes can rob an electron from a nearby atom which is not in the normal flow chain of electron transportation. Then the molecule containing an atom with an unpaired electron, which is not confined in the normal electron transport chain, is called a “free radical” because it is very reactive and tends to rob another electron from any of its neighboring atoms to make up its electron number in pair. This type of uncontrolled oxidative process (losing an electron) will initiate a free radical chain reaction, which is damaging to the cellular structures, including the DNA in our genes, and is part of the aging process, including cancer formation. Heavy metals, even iron, catalyze the free radical chain reactions several thousands times.