Hydrogen, since in H2O there are two hydrogen atoms per one oxygen atom. So the hydrogen atoms would run out first so that there are 12 H2O molecules, where all the hydrogen is used and there are 12 oxygen atoms left over
Total electrons in Bromine(Br) = 35
Plus one -ve ion = 35+1 = 36
Well the reason why the water valve was because of the force of gravity. Without the force of gravity the water would have flown so if you had to try to move the water up the waterfall you would have to fight the force of gravity