Reactivity is the degree to which a particular atom, radical, or molecule is active. It has to do with how able a molecule is to combine chemically with another molecule, resulting in a chemical or physical change. Reactivity varies according to the substance under scrutiny and the materials with which that material comes into contact. It is a variable in a chemical reaction, which brings about the chemical mingling of two or more substances, and is a measurement of how fast one chemical reacts compared with another (i.e., its kinetic properties).

Reactivity is a relative term, and as such only has meaning when used in reference to a certain set of conditions. Thus, it is not a measurement that chemists use when dealing with reactions or general compound reactions. Instead, a chemical species (a group of substances with identical molecular entities) might be called reactive if it...