(a) All three elements have valency 2. Thus, they should be present in group 2 of the periodic table. (b) On going down a group in the periodic table, the reactivity of an element increases because of decrease in the effective nuclear charge and increase in the atomic size; this makes the removal of valence electrons easy. Thus, among the given elements, 4Be is the least reactive element. (c) On going down a group in the periodic table, the atomic size of elements increases because of the addition of new shells. Thus, among the given elements, 20Ca has the largest atomic size.