Oh, that is a different situation, TS, than what I thought you meant. You are describing breaking down H2O molecules into O2 and H2 and the volume of the resulting gas assuming constant pressure. Even if the H2O molecules started out gaseous, two H2O gas molecules will make two H2 gas molecules and an O2 molecule. The number of moles of gas in the container increases, and by the Ideal Gas Law you can see that the pressure and/or volume will increase.
When you wrote "atomic volume" I thought you mean the volume of the atoms, not the volume of the gas.
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