You could say that hydrogen and Oxygen molecules are similar in that their chemical formula is H2 and O2. That means there are 2 "atoms" per molecule. I don't know how to put water into this because water molecules are quite different from H2 and O2. Even the intramolecular forces are quite different.
H2 and O2 are different in their masses per molecule.One is quite a bit heavier than the other. They can be quite explosive if they are put in the same container. I once heard a NASA engineer say that Hydrogen is a "real snake" to handle. It is used for rocket fuel. Both are compressed a great deal.
