Francium is element No. 87 on the periodic table. The element can be prepared by bombarding thorium with protons. An extremely small amount occurs naturally in uranium minerals, but it is so rare and radioactive that there has never been enough of it to actually see what would happen if a piece was dropped into water. However, scientists know for sure that the reaction would be energetic, possibly even explosive. ​

The piece of francium would blow apart, while the reaction with water would produce hydrogen gas, francium hydroxide, and a lot of heat. The entire area would be contaminated with radioactive material.

Why Francium Reacts so Strongly

The reason for the strong exothermic reaction is because francium is an alkali metal. As you move down the first column of the periodic table, the reaction between the alkali metals and water becomes increasingly violent, as follows:

A small amount of lithium will float on water and burn.

Sodium burns more readily.

Potassium breaks apart, burning with a violet flame.

Rubidium ignites with a red flame.

Cesium releases enough energy that even a small piece blows up in water.

Francium is below cesium on the table and would react more readily and violently.

This occurs because each of the alkali metals is characterized by having a single valence electron. This electron easily reacts with other atoms, such as those in water. As you move down the periodic table, the atoms become larger and the lone valence electron is easier to remove, making the element more reactive.

In addition, francium is so radioactive that it is expected to release heat. Many chemical reactions are accelerated or enhanced by temperature. Francium would input the energy of its radioactive decay, which is expected to magnify the reaction with water.