Explain the trend in reactivity of group 2 elements with water as you go down the group.

As you go down group 2, atom size increases. Larger atoms experience more shielding. There is an increase in nuclear charge going down group 2 elements so you would expect the outer electrons to be held more tightly by the atom due to electrostatic interactions. However, shielding of larger atoms mean that outer electrons are lost more easily as you go down group 2. Therefore, group 2 elements become more reactive with water going down the group.