Hydrogen is the most abundant element in the universe. On Earth, it is usually found as a diatomic gas (H2 ). The reaction of hydrogen gas with oxygen gas to form water releases a tremendous amount of energy:
2H2 (g) + O2 (g) –> 2 H2 O (l) + Energy
As a result of the large amount of energy released in this reaction, hydrogen gas has been used as a fuel for rockets.