(a) On the left side of the periodic table in the first group as the elements belonging to first group when reacted with air/water react vigorously to give large amount of heat which can be fatal (Sodium Metal)

(b) The element with variable valency that must be stored under water is phosphorous. It shows variable valency of 3 and 5 and is stored in water as it is highly reactive in air. It is placed in group 15 and period 3 of the periodic table.