Cesium dosent actually give off much energy as many the lighter alkali metals, the energy gained from ripping off the outer electron is suprisingly low. Actually, lithium is the one which gives of the most energy during the reaction, but cesium requiers far less energy to start the reaction. Also, cesium is quite heavy, so it will sink to the bottom of the water, which will contain the hydrogen that will sometimes fail to ignite. Francium actually wont do much more than cesium, because sheer weight of the atoms slow down the reaction, the energy given off during the reaction might be even lower than of cesium. Potassium gives the most bang for the buck, because it floats, gives of much more heat, and therefore will instantly ignite the hydrogen making a big bang. Compare these two videos, both are 1g [yt]pPdevJTGAYY[/yt] [yt]zCARhVfeX5U[/yt]