Oxidation & Reduction

Oxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants entails changes in energy to form new products and will be either endothermic or exothermic.

Oxidation is a chemical reaction in which electrons are lost/donated during a reaction by a chemical species. Specifically, it means the substance that donates electrons is oxidized. For example, within the protoplanetary disc, OH molecules and free oxygen atoms created oxidizing conditions that enabled carbon dust to form simple molecules like CO and more complex molecules such as CH4 and C2Hdepending on their distance from the sun1. A much simpler example is when iron rusts, in this process the free iron interacts with oxygen (the oxidizer/oxidant/oxidizing agent) to produce iron oxide (FeO). Though the term used to imply that oxygen was the only oxidizer in the reaction, in modern chemistry, oxygen does not have to be present in a reaction for it to be an oxidizing reaction.

The opposite process is called reduction in which electrons are gained (accepted) during a reaction by a chemical species. When we describe a metal like Fe (iron) as being reduced, we mean that the reactions used to extract that iron from, for example, its oxides (FeO) is produced by adding another molecule under various system conditions so that the added molecule has greater affinity for the Fe-bound oxygen than the iron itself.

Note, there are many other and more complex ways to oxidize/reduce iron, and the FeO example was simply chosen for simplicity.