My textbook "chemistry by Julia" says the following

When it comes to expressing the molar mass of elements such as oxygen and hydrogen, we have to be careful to specify what form of the element we mean. For instance, the element oxygen exists predominantly as diatomic molecules (O2). Thus, if we say 1 mole of oxygen and by oxygen we mean O2, the molecular mass is $\pu{32.00 amu}$ and the molar mass is $\pu{32.00 g}$. If on the other hand we mean a mole of atomic oxygen ($\ce{O}$), then the molar mass is only $\pu{16.00 g}$, which is numerically equal to the atomic mass of $\ce{O}$ ($\pu{16.00 amu}$). You should be able to tell from the context which form of an element is intended.

and then it gives examples like the following: How many moles of oxygen react with 2 moles of hydrogen to produce water? here the book says it is $\ce{O2}$ which is true from the chemical equation.

another example says Air is approximately 21% oxygen. and the meaning of oxygen here is again $\ce{O2}$ which is easy to deduce since oxygen found in nature is mostly $\ce{O2}$.

The last example I could not tell why and here it is: Many organic compounds contain oxygen. the meaning here is $\ce{O}$ a single atom, but why? and how to tell from a certain context what oxygen means, whether $\ce{O}$ or $\ce{O2}$?

  • $\begingroup$ You can find useful Notation basics and How can I format math/chemistry expressions on Chemistry SE. See also upright vs italic and Math SE MathJax tutorial. $\endgroup$
    – Poutnik
    Jun 16, 2021 at 20:57
  • 1
    $\begingroup$ It can be ambiguous unless you use the explicit word molecular or atomic to qualify what type of oxygen you mean. Life can be full of ambiguities and you have to read between the lines sometimes and find what makes more sense within a given context. Such problems with usage crop up often in popular science. Chemists are usually careful with nomenclature to make sure it reflects how oxygen is bonded. Oxygen in molecules is usually present alone, that is, not bonded to another oxygen, except of course in molecular oxygen and peroxides. $\endgroup$
    – Buck Thorn
    Jun 17, 2021 at 5:41
  • $\begingroup$ Please note that the old atomic mass unit (amu), which referred to the relative atomic mass of oxygen, became obsolete in 1961 and is no longer used in chemistry. $\endgroup$
    – Loong
    Jan 22, 2022 at 10:12

1 Answer 1


Look at the formula of the compound containing oxygen. In some cases it's obvious, e.g. a mole of $\ce{O2}$ in the air. In other cases, you'd need to know how oxygen is liberated. For example, one mole of hydrogen peroxide, $\ce{H2O2}$, liberates only one half mole of gaseous oxygen: $\ce{H2O2 -> H2O + 1/2 O2}$ or better, $\ce{2H2O2 -> 2H2O + O2}$.

Er, umm... why write this as $\ce{1/2 O2}$, rather than one $\ce{O}$? The answer is because though the oxygen might be liberated in a very reactive atomic, rather than molecular, form, as in "piranha solution", it rapidly links together as molecular $\ce{O2}$.

Yes, it can be confusing, since there are a number oxygen allotropes. Aside from $\ce{O2}$, or dioxygen, which makes up about 1/5 of the air, there is $\ce{O3}$, ozone, and solid oxygen may contain $\ce{O8}$. And, as Herr Lehrer states, "There may be many others, but they haven't been discavard".


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