What is the main similarity among elements in group 2?

             Chemical Properties

Explanation:
                   Elements belonging to same group in a periodic table have same chemical properties. This similarity in chemical properties arises mainly from the same number of valence shell electrons. Group 2 elements also called Alkaline Earth Metals have two valence electrons in their valence shell with following electronic configuration

                                                       ns²

Where "n" represents principle quantum number or number of main shell.

The reactivity of these elements is due to the metallic behavior as these elements looses two electrons to form a cation with +2 oxidation number. i.e.

                                          X   →   X⁺²  +  2 e⁻

Therefore, all elements present on Group II will form a cation with +2 oxidation number and behave in the same manner with different reagents.