What Metal Is Most Reactive?

Francium

• Francium was first discovered in 1939 by chemist Marguerite Perey in Paris. Scientists can produce the element artificially, but the amount found on Earth is incredibly small -- less than a single ounce exists at any one given time. Even if artificially produced, it's very difficult to study because only small quantities can be produced; moreover, the element is very radioactive and decays rapidly. The half-life of the longest-lived isotope is just 22 minutes. Consequently, although francium is without a doubt an extremely reactive metal, very little is known about its chemistry.

Cesium

• According to Los Alamos National Laboratory (LANL), the chemistry of francium is very similar to the chemistry of cesium. Cesium is also much easier to study than francium because it is found in the mineral pollucite and other ores. Isolating it, however, is very difficult because it is extremely reactive. Not only does it react violently with water, it's also pyrophoric, meaning it has the tendency to spontaneously combust in air. Typically it's stored in mineral oil or under an argon atmosphere.

Francium vs. Cesium

• Ionization energy measures the amount of energy needed to remove an electron from an atom. One reason why cesium and rubidium are so reactive is their extremely low ionization energies, which imply it's very easy for them to donate electrons to other molecules like the water molecules in your beaker. At 376 kilojoules per mole, cesium actually has a slightly lower ionization energy than francium. Consequently, cesium is actually slightly more reactive than francium, although it's difficult to make comparisons since francium is so difficult to study.

Relativistic Effects

• In general, ionization energies fall as you travel down a column in the periodic table. From period 5 onward, however, the theory of relativity as it applies to quantum chemistry becomes increasingly important, and these relativistic effects play a complicated but significant role. Owing to relativistic effects, francium actually has a slightly smaller atomic radius than cesium -- hence the slight though counterintuitive increase in ionization energy going from cesium down to francium. Ultimately, however, the two are highly reactive and very similar in their chemistry.