Which alkali metal is radioactive in nature.

None of the naturally occurring alkali metals are radioactive. The alkali metals are a group of elements in the periodic table that includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). Among these, francium (Fr) is the most radioactive and unstable due to its large atomic size and low stability.

However, it's important to note that francium is extremely rare and highly radioactive, with a very short half-life. Because of its scarcity and limited availability, it is challenging to study and work with francium in the laboratory, and most of its properties are inferred from theoretical calculations.

In general, naturally occurring isotopes of the alkali metals (lithium, sodium, potassium, rubidium, and cesium) are stable and not radioactive.