What Does Reactivity Mean in Chemistry?

Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.

In chemistry, reactivity is a measure of how readily a substance undergoes a chemical reaction. The reaction can involve the substance on its own or with other atoms or compounds, generally accompanied by a release of energy. The most reactive elements and compounds may ignite spontaneously or explosively. They generally burn in water as well as the oxygen in the air. Reactivity is dependent upon temperature. Increasing temperature increases the energy available for a chemical reaction, usually making it more likely.

Another definition of reactivity is that it is the scientific study of chemical reactions and their kinetics.

Reactivity Trend in the Periodic Table

The organization of elements on the periodic table allows for predictions concerning reactivity. Both highly electropositive and highly electronegative elements have a strong tendency to react. These elements are located in the upper right and lower left corners of the periodic table and in certain element groups. The halogens, alkali metals, and alkaline earth metals are highly reactive.

• The most reactive element is fluorine, the first element in the halogen group.
• The most reactive metal is francium, the last alkali metal (and most expensive element). However, francium is an unstable radioactive element, only found in trace amounts. The most reactive metal that has a stable isotope is cesium, which is located directly above francium on the periodic table.
• The least reactive elements are the noble gases. Within this group, helium is the least reactive element, forming no stable compounds.
• Metal can have multiple oxidation states and tend to have intermediate reactivity. Metals with low reactivity are called noble metals. The least reactive metal is platinum, followed by gold. Because of their low reactivity, these metals don't readily dissolve in strong acids. Aqua regia, a mixture of nitric acid and hydrochloric acid, is used to dissolve platinum and gold.

How Reactivity Works

A substance reacts when the products formed from a chemical reaction have lower energy (higher stability) than the reactants. The energy difference can be predicted using valence bond theory, atomic orbital theory, and molecular orbital theory. Basically, it boils down to the stability of electrons in their orbitals. Unpaired electrons with no electrons in comparable orbitals are the most likely to interact with orbitals from other atoms, forming chemical bonds. Unpaired electrons with degenerate orbitals that are half-filled are more stable but still reactive. The least reactive atoms are those with a filled set of orbitals (octet).

The stability of the electrons in atoms determines not only the reactivity of an atom but its valence and the type of chemical bonds it can form. For example, carbon usually has a valence of 4 and forms 4 bonds because its ground state valence electron configuration is half-filled at 2s 2 2p 2 . A simple explanation of reactivity is that it increases with the ease of accepting or donating an electron. In the case of carbon, an atom can either accept 4 electrons to fill its orbital or (less often) donate the four outer electrons. While the model is based on atomic behavior, the same principle applies to ions and compounds.

Reactivity is affected by the physical properties of a sample, its chemical purity, and the presence of other substances. In other words, reactivity depends on the context in which a substance is viewed. For example, baking soda and water are not particularly reactive, while baking soda and vinegar readily react to form carbon dioxide gas and sodium acetate.

Particle size affects reactivity. For example, a pile of corn starch is relatively inert. If one applies a direct flame to the starch, it's difficult to initiate a combustion reaction. However, if the corn starch is vaporized to make a cloud of particles, it readily ignites.

Sometimes the term reactivity is also used to describe how quickly a material will react or the rate of the chemical reaction. Under this definition the chance of reacting and the speed of the reaction are related to each other by the rate law:

Rate = k[A]

Where rate is the change in molar concentration per second in the rate-determining step of the reaction, k is the reaction constant (independent of concentration), and [A] is the product of the molar concentration of the reactants raised to the reaction order (which is one, in the basic equation). According to the equation, the higher the reactivity of the compound, the higher its value for k and rate.

Stability Versus Reactivity

Sometimes a species with low reactivity is called "stable", but care should be taken to make the context clear. Stability can also refer to slow radioactive decay or to the transition of electrons from the excited state to less energetic levels (as in luminescence). A nonreactive species may be called "inert". However, most inert species actually do react under the right conditions to form complexes and compounds (e.g., higher atomic number noble gases).