The most reactive metal is cesium, while the most reactive nonmetal is fluorine. So, the most reactive element on the periodic table is either one of these elements. But, reactivity means different things to different chemists, plus it depends on a few factors.
The Most Reactive Metal
The reason cesium top is the most reactive metal is because it tops the metal activity series. This is a list of metals (and hydrogen gas, for comparison) where a metal displaces others below it in chemical reactions. For example, if you react cesium with zinc oxide, the oxygen is more attracted to cesium than to zinc and you get cesium oxide. Additionally, metals higher on the activity series more readily react with acids and water.
Other Contenders for the Title of Most Reactive Metal
It is possible francium is more reactive than cesium. Francium is directly below cesium on the periodic table in the alkali metals group. Metal reactivity is a trend on the periodic table, with the most reactive and most electropositive elements on the bottom lefthand side of the table. But, francium is extraordinarily rare and also radioactive, so its rapid decay deters research into its properties. There is insufficient empirical data to say for sure whether or not francium is more reactive than cesium.
Textbooks sometimes cite potassium as the most reactive metal because it is near the top of the metal activity series and is also readily available for chemists to use in the lab. Francium (presumably), cesium, and rubidium are actually more reactive, but less commonly encountered.
The Most Reactive Element on the Periodic Table
While cesium or francium is the most reactive metal, what does it react with most readily? Just as the alkali metals are the most reactive metals, the halogens are their counterparts on the righthand side of the periodic table that are the most reactive nonmetals. The most reactive nonmetal is fluorine, which is the element with the highest electronegativity value.
So, the most reactive elements on the periodic table are cesium and fluorine.
Factors That Affect Reactivity
Reactivity is a measure of how easily an element participates in a chemical reaction and forms new chemical bonds. Highly electropositive or electronegative elements are extremely reactive because their valence electron shells are only one electron away from a stable configuration. The alkali metals easily donate their single valence electron, while the halogens readily accept a single valence electron.
But, other factors determine whether or not one element is more reactive than another, including particle size and temperature. For example, hydrogen (H2) very readily reacts with oxygen (O2) and forms water. Even though the equilibrium constant for this reaction is very high and hydrogen is above many metals on the reactivity series, hydrogen and oxygen gas don’t react until you introduce a flame.
Grinding elements into smaller particles increases their reactivity because of the increase in surface area. So, a solid lump of a metal higher on the activity series might be less reactive than the powdered form of an element beneath it on the list.
Impurities also affect reactivity, but the nature of the effect depends on the impurity. The form or allotrope also matters. For example, carbon as graphite has different reactivity than carbon as diamond. Also, some elements react more readily with certain substances than others. In this case, comparing reactivity really depends on the nature of the reaction and not just which element is more electropositive or electronegative.
References
- Bickelhaupt, F. M. (1999). “Understanding reactivity with Kohn–Sham molecular orbital theory: E2–SN2 mechanistic spectrum and other concepts”. Journal of Computational Chemistry. 20 (1): 114–128. doi:10.1002/(sici)1096-987x(19990115)20:1<114::aid-jcc12>3.0.co;2-l
- Pauling, L. (1932). “The Nature of the Chemical Bond. IV. The Energy of Single Bonds and the Relative Electronegativity of Atoms”. Journal of the American Chemical Society. 54 (9): 3570–3582. doi:10.1021/ja01348a011
- Wolters, L. P.; Bickelhaupt, F. M. (2015). “The activation strain model and molecular orbital theory”. Wiley Interdisciplinary Reviews: Computational Molecular Science. 5 (4): 324–343. doi:10.1002/wcms.1221