As the atom gets larger there is now a smaller charge/volume ratio (they are becoming much larger (increase volume) but their charges remain the same (+2)) and the delocalised electrons are further away from the positive nucleus which means it takes much less energy to break this metallic bonding attraction. Melting points generally decrease down the group this is because they are all metals and hence have metallic bonding which consists positive metal ions surrounded by a sea of delocalised electrons. This is because, as explained previously, it is much easier to remove an outer shell electron as you go further down the group (lower ionisation energies). The reactivity of the group 2 elements increase as you go down the group.
