Oxygen (O)
Reactive nonmetal
Oxygen is the element that keeps fires burning and cells respiring. It is the most abundant element in the Earth's crust and, as O₂, makes up about 21% of the atmosphere.
Position on the periodic table
Atomic properties
| Atomic number | 8 |
|---|---|
| Atomic mass | 15.999 amu |
| Electron configuration | 1s2 2s2 2p4 |
| Noble gas shorthand | [He] 2s2 2p4 |
| Electrons per shell | 2, 6 |
| Valence electrons (outer shell) | 6 |
| Common oxidation states | -2 |
| Electronegativity (Pauling) | 3.44 |
| Covalent radius (approx.) | 66 pm |
| First ionization energy | 1314 kJ/mol |
| Electron affinity | 141 kJ/mol |
Physical properties
| State at 25 °C | Gas |
|---|---|
| Density | 1.429 g/L |
| Melting point | -218.79 °C (54.36 K) |
| Boiling point | -182.96 °C (90.19 K) |
| Appearance | Not available |
| Radioactive | No |
| Origin | Occurs naturally |
Electron configuration of oxygen
Oxygen's ground-state electron configuration is 1s2 2s2 2p4, usually shortened to [He] 2s2 2p4. Its electrons occupy 2 shells (2, 6), placing it in period 2 of the p-block. The 6 outer-shell electrons drive its bonding behaviour. Explore it interactively in the electron configuration calculator.
Uses of oxygen
- Medical oxygen for breathing support
- Steelmaking (blowing oxygen through molten iron)
- Rocket oxidiser (liquid oxygen)
- Water treatment and welding
Biological role: Essential for respiration — the final electron acceptor that lets cells release energy from food.
Occurrence: About 46% of the Earth's crust by mass (in silicates and oxides) and 21% of the atmosphere.
Common compounds of oxygen
H2O
Water
CO2
Carbon dioxide
SiO2
Silica (quartz)
Fe2O3
Iron(III) oxide (rust)
History and discovery
Discovered: 1774 — Carl Wilhelm Scheele. Name origin: From Greek oxys (sharp/acid) + genes (forming) — early chemists wrongly believed all acids contained oxygen.
Discovered independently by Carl Wilhelm Scheele (1771–2) and Joseph Priestley (1774); Lavoisier explained its role in combustion.
Safety notes
Pure oxygen dramatically accelerates burning — oxygen-rich environments have strict no-flame rules.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Oxygen
1. What is the chemical symbol of Oxygen?
2. What is the atomic number of Oxygen?
3. Which category does Oxygen belong to?
4. What is the state of Oxygen at room temperature?
5. Which period of the periodic table is Oxygen in?
Oxygen FAQs
How many valence electrons does oxygen have?
Six. Oxygen sits in group 16 with the electron arrangement 2, 6, so it has six electrons in its outer shell — which is why it typically gains two electrons to form the O²⁻ ion.
Is oxygen a gas?
Yes. At room temperature oxygen is a colourless, odourless gas (O₂) that makes up about 21% of the air. It only becomes a pale-blue liquid when cooled below about −183 °C.
What is the atomic number of oxygen?
8. Every oxygen atom has 8 protons in its nucleus, which is what defines it as oxygen.
What is the symbol for oxygen?
O. From Greek oxys (sharp/acid) + genes (forming) — early chemists wrongly believed all acids contained oxygen.
Is oxygen a metal, nonmetal, or metalloid?
Oxygen is classified as a reactive nonmetal.
What state is oxygen at room temperature?
At about 25 °C, oxygen is a gas. It melts at -218.79 °C.
What is oxygen used for?
Key uses include: medical oxygen for breathing support; steelmaking (blowing oxygen through molten iron); rocket oxidiser (liquid oxygen).
Sources & references
- Oxygen — Element information — Royal Society of Chemistry
Atomic masses and element data follow current IUPAC/NIST standard values. Educational content only — for graded or laboratory work, verify against your course materials. Last reviewed July 11, 2026.
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