Question

Which alkali metal is radioactive in nature.

Answer 1

The radioactive elements are present at the bottom of the periodic table i.e. in the last period. The radioactive element of each group is located there only.
The last element in the alkali metals series is radioactive in nature. It belongs to seventh and group I of the periodic table.

Complete answer:
Alkali metals belong to the first group of the periodic table . The alkali metals are so called because on reaction with water, they form alkalies . They are highly reactive in nature . They are situated at the extreme left of the periodic table. The alkali metals are so reactive that they are generally found in combined states in nature . They have one valence electron that they readily lose to form ionic bonds.
The alkali metals include $Li,Na,K,Rb,Cs,Fr$ . Out of this , the last element of the series is radioactive in nature. Francium is radioactive in nature. In the Francium atom, the number of neutrons is more than the number of protons and this is why its nucleus is unstable. In order to gain stability , it shows radioactivity and disintegrates its atom to become stable. Its nuclei is unstable and it gives off atomic radiation as a part of a process of attaining stability . It is the heaviest element of the group I.
The half- life of a francium atom is very less making it highly unstable in nature. It exists for a very short span of time.

Note: Francium is the second rarest element on earth. It has a half life of about 22 minutes only. It is used in cancer diagnostics , spectroscopic experiments etc.