[kevsim]

I am confused with diatomic modules, for example Oxygen.
When it is standing alone it is O2, when included in a formula, for example, "water", H2O
the 2 is dropped.

When balancing a formula is Oxygen treated as O or O2?

For some reason I can not get my head around it.

I would appreciate some advise.

kevsim

[Arkcon]

It's hard to understand where your point of confusion is.   Oxygen gas exists as a diatomic molecule, that is, two oxygen atoms bond to each other, we call that diatomic oxygen.  The oxygen atom in water is alone, bonded to two hydrogen atoms.  We haven't "lost" the number two, it just doesn't belong because there's only one.

Suggestion: look up a reaction that has diatomic oxygen as a reactant, and water as a product.  There are many examples: the production of water from hydrogen and oxygen, or combustion of fuels in oxygen.  If they're properly balanced, you'll be able to see where things go.

[Irlanur]

O₂ : One Oxygen Molecule, composed out of two Oxygen atoms
2O: Two separate oxygen atoms.
2O₂: Two oxygen molecules, each composed out of two oxygen atoms
H₂O: One water molecule, composed out of two hydrogen atoms and one oxygen atom.

"Oxygen" can mean the atom or the molecule. If you see it in reaction (e.g. out of the air), oxygen is usually present in diatomic molecules, O₂. so if you balance an equation, e.g. the oxidation of a fuel, you have to use O₂ as a unit.