Question

What is the most reactive alkali metal?

Answer 1

Most people will say francium (Fr) because of its position in the alkali metals group, and it is in theory. It is also probably the answer your teacher is looking for. However, experimentally, cesium (Cs) is the most reactive.

The reactivity of the alkali metals increases down the group. In theory, francium (Fr) should be the most reactive, but it is radioactive with a maximum half-life of 22 minutes. Less than 30 grams exist in the earth's crust at any given time. It is the least electronegative element, so it should be the most reactive. Because of its extreme heat of decay and its short half-life, at this time it would be impossible to get a laboratory-produced sample large enough to experiment with because it would vaporize immediately.
http://www.chemicool.com/elements/francium.html
http://en.m.wikipedia.org/wiki/Francium

So, in terms of experimental evidence, cesium is the most reactive alkali metal. In terms of theory, francium is the most reactive.