Chemical transformation (Decomposition)

[chemistry1]

Hi, I was wondering when you need to write the decomposition of a substance, how do you know if the number is going to be a coefficient or subscript ?

Example:

2H2O ===>2H2 + 02 would be the answer

But why not

2H2O ==> 2H2 + 2O

Knowing that we have 2 moles of O in the beginning...

So ?

Thank you!

 

[Bananas40]

Although 2H2O ==> 2H2 + 2O does not violate Lavoisier's law, oxigen is a diatomic molecule and therefore the substance 'O' that appears in that equation doesn't truly exist.