The element of group 1 in the periodic table are highly reactive in nature and reacts with water to form an alkaline solution with the evolution of hydrogen gas.
So, due to the formation of alkaline solution, group 1 elements except hydrogen are known as alkali metals.
The Lightest Alkali metals:
The lightest alkali metal is Lithium .
Lithium has the lowest atomic weight in group 1 and hence is the lighter element.