The main group metals are elements of group 1 and 2. The reactivity of group 1 elements (alkali metals) increases down the group. Lithium is a bit vigorous and then disappears. The reaction generates little heat because of Lithium's melting point. While the last element down the group, Cesium, explodes on contact possibly shattering the container; forming Cesium hydroxide and hydrogen. The group 2 elements are not as vigorous with water like group 1 but their reactivity also increases as we go down the group. Beryllium reacts with steam at high temperatures while Calcium, for instance, reacts fairly vigorously with cold water in an exothermic reaction
