Yes because a theory is based on results and the results are part of the experiment and it being tested. You have to test the experiment and get results so yes a theory is testable.
Alkali Metals (Group 1) elements experience an increase in the vigour of their reaction in water as they go down the group (as the atomic number increase). As such the most reactive Alkali Metal would be
FRANCIUM, which is at the base of Group One.
Quite frankly, you do not want Francium to react with water- that's a huge explosion on your hand.
Each one is a living organism.