O2
Oxygen gas (O2)
Also known as: dioxygen, molecular oxygen
O₂ is the form of oxygen you breathe — two oxygen atoms sharing a double bond. It makes up about 21% of the atmosphere, all of it produced by photosynthesis. O₂ drives respiration and combustion alike: both are, chemically, controlled and uncontrolled oxidation.
Molar mass breakdown
31.998 g/mol| Element | Atoms | Mass (g/mol) | % by mass |
|---|---|---|---|
| Oxygen (O) | 2 | 31.998 | 100% |
What oxygen gas is used for
- Respiration — the final electron acceptor in cells
- Medical oxygen therapy
- Steelmaking and welding (oxy-fuel torches)
- Rocket oxidiser (liquid O₂)
Key facts
- Molar mass ≈ 32.00 g/mol — exactly double atomic oxygen's 16.
- An element in molecular form, not a compound.
- Ozone (O₃) is a different molecular form (allotrope) of the same element.
Frequently asked questions
What is the molar mass of O2?
About 32.00 g/mol — two oxygen atoms at 15.999 each. Watch for this in gas problems: use 32, not 16.
Is O2 a compound?
No — a compound needs at least two different elements. O₂ is an element in its molecular form.
Why is oxygen written as O2 in equations?
Because oxygen gas exists as diatomic molecules. Seven common elements do: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.