The elements $Be$,$Mg$,and $Ca$,each having two electrons in their outermost shells,are in periods $2$,$3$,and $4$ respectively of the modern periodic table. Answer the following questions,giving justification in each case:
$(i)$ Write the group to which these elements belong.
$(ii)$ Name the least reactive element.
$(iii)$ Name the element having the largest atomic radius.

(A-D) $(i)$ These elements belong to Group $2$ because they all have the same number of valence electrons ($2$ valence electrons).
$(ii)$ $Be$ (Beryllium) is the least reactive element. In a group,the chemical reactivity of metals increases from top to bottom as the tendency to lose electrons increases due to the increase in atomic size.
$(iii)$ $Ca$ (Calcium) has the largest atomic radius. Atomic radius increases down a group because the number of electron shells increases ($Be$ has $2$ shells,$Mg$ has $3$ shells,and $Ca$ has $4$ shells).