Actinium is the most reactive element or metal in group 3.
Explanation:
Reactivity of an element or metal is defined as its tendency to loose or gain atoms. Group 3 elements consist of metal. We know that reactivity of metal increases as we go down to the bottom of the column.
group 3 metals have 3 electrons in valence shell. They react by loosing 3 electrons. it has oxidation state of +3 and are electropositive.
Group 3 consists of scandium, yttrium, lanthanum and actinium.
Actinium is called actinide.
