The molecules consist of Hydrogen and Oxygen that have equal atomic masses, either side of the equation, due to the identical numbers of protons, neutrons and e

The molecules consist of Hydrogen and Oxygen that have equal atomic masses, either side of the equation, due to the identical numbers of protons, neutrons and electrons. Any mass gained or lost will be due to changes in bond energies, and the corresponding emission or absorption of photons that change the energy states of the electrons in the bonds.