- the range of circumstances (conditions that include temperature, pressure, or presence of catalysts) in which the substance reacts,
- the range of substances that will react with it,
- the equilibrium point of the reaction (that is, the extent to which all of it reacts), or
- the speed of the reaction.
Reactivity involves both the thermodynamics and kinetics of the particular chemical reaction. A reaction will be more reactive if the total energy of the products is less than the total energy of the reactants. A reaction will be more reactive if the energy of its transition is low.
Lithium reacts with water and air. Gold does not react with water or air. Gold is unreactive, thus low on the reactivity series. Lithium is reactive. There are several ways to measure reactivity. Some are reducing agents, and others can be oxidizing agents. Some absorb water easily. Some break down easily.
Reactivity (chemistry) Facts for Kids. Kiddle Encyclopedia.