Oxidation and reduction are complementary chemical processes that involve the transfer of electrons:

• Oxidation is the loss of electrons by an atom, ion, or molecule.
• Reduction is the gain of electrons by an atom, ion, or molecule.

A helpful way to remember this is the acronym OIL RIG:

• Oxidation Is Loss (of electrons)
• Reduction Is Gain (of electrons)

These processes always occur together in a redox (reduction–oxidation) reaction. When one substance loses electrons (oxidized), another gains those electrons (reduced).

Examples:

• In the reaction between sodium and chlorine, sodium is oxidized (loses an electron), and chlorine is reduced (gains an electron), forming NaCl.
• In cellular respiration, glucose is oxidized, and oxygen is reduced to form water and carbon dioxide.

Redox reactions are fundamental to energy production, corrosion, metabolism, electrochemistry, and many industrial processes.