To remember what the reactant is, it always comes before the yielding sign (arrow). The elements after the arrow are known as the 'products'. So, with this information, let's look for our reactants. Remember, the reactants are the elements BEFORE the yielding sign (arrow). We have Hydrogen and Oxygen before the arrow.
Calcium is more reactive than magnesium because calcium atom is larger than magnesium atom and it has one more energy level. ... Thus Ca is more reactive than Mg.
