O · Element 8 · Nonmetal
The element that rusts iron, burns fuel and keeps you alive — and the most abundant element in the Earth's crust.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where oxygen sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All oxygen atoms have the same number of protons, 8, and differ only in their number of neutrons: those variants are its isotopes. The most common, O-16, has 8 neutrons and makes up 99.757% of natural oxygen.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| O-15 | 7 | — | 122.24 sec | ε/β⁺ |
| O-16 most common | 8 | 99.757% | Stable | — |
| O-17 | 9 | 0.03835% | Stable | — |
| O-18 | 10 | 0.2045% | Stable | — |
Here the classroom shortcut, rounded atomic mass (15.999) minus atomic number, gives the same answer: 8.
Decay modes
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 60 pm — that is 0.06 nm, so about 8.33 million of them side by side would span a millimetre.
Thermal range
Oxygen is liquid over a 36 K window, from 54 K to 90 K.
The story
Oxygen is the third most abundant element in the universe and the most abundant element in Earth's crust, comprising about 21% of our atmosphere and about 30% of the planet's mass (and about 46% of its crust). This highly reactive, colorless, and odorless gas is essential for most life forms on Earth through the process of cellular respiration.
Discovered independently by Carl Wilhelm Scheele and Joseph Priestley in the 1770s, oxygen transformed our understanding of combustion and respiration. Its name, derived from the Greek words "oxys" (acid) and "genes" (producer), reflects Antoine Lavoisier's mistaken belief that all acids contained oxygen.
Fun Fact: The oxygen we breathe today was largely produced by ancient cyanobacteria through photosynthesis over billions of years, fundamentally changing Earth's atmosphere and enabling complex life to evolve.Oxygen exists in several allotropes, with dioxygen (O₂) being the form we breathe, and ozone (O₃) forming the protective layer in our stratosphere. In industry, oxygen supports combustion processes, steel production, and serves as an oxidizing agent in countless chemical reactions.
From the water we drink (H₂O) to the minerals in Earth's crust, oxygen forms compounds with virtually every other element, making it fundamental to chemistry, biology, and geology. Its unique electronic configuration gives oxygen its characteristic reactivity and ability to form double bonds, crucial for organic chemistry and life processes.
Applications
The largest industrial use of oxygen is in steel manufacturing, where it dramatically improves efficiency and quality:
Oxygen serves as a critical oxidizing agent in numerous industrial processes:
Oxygen transformed paper manufacturing through environmentally friendly bleaching:
Oxygen-based technologies clean water and reduce pollution:
Liquid oxygen (LOX) is the most widely used rocket oxidizer:
Oxygen combustion improves glass production efficiency and quality:
Where it comes from
461000 mg/kg of Earth's crust · more abundant than 100% of elements
Oxygen is the third most abundant element in the universe, comprising approximately 1% of ordinary matter by mass. It is primarily formed through stellar nucleosynthesis in massive stars:
The oxygen in our solar system was made in earlier generations of stars and was inherited when the solar system formed from a molecular cloud about 4.6 billion years ago.
Earth's atmosphere contains 20.946% oxygen by volume, representing the second most abundant atmospheric gas after nitrogen. This massive reservoir contains approximately 1.2 × 10¹⁵ tons of oxygen gas.
Fact: Virtually all atmospheric oxygen is biogenic - produced by photosynthetic organisms over billions of years. The Great Oxidation Event 2.4 billion years ago fundamentally changed Earth's atmosphere.The atmospheric oxygen cycle involves constant exchange between:
Oxygen dominates Earth's crust, comprising approximately 46.1% by weight - making it the most abundant element in Earth's crust and mantle. Major oxygen-bearing minerals include:
Water (H₂O) contains 88.8% oxygen by mass, making Earth's hydrosphere a massive oxygen reservoir:
Aquatic ecosystems depend on dissolved oxygen (DO) levels:
Photosynthetic organisms are the original source of Earth's oxygen:
Oxygen occurs throughout the solar system and universe:
Handling
Oxygen's role in combustion creates severe safety risks:
Compressed oxygen safety measures:
Healthcare oxygen administration requires careful monitoring:
OSHA and regulatory standards for workplace oxygen:
Liquid oxygen (LOX) at -183°C presents additional dangers:
Emergency procedures for oxygen incidents:
Environmental impact and waste management:
DOT regulations for oxygen transport:
Quick answers
Oxygen (symbol O) is element 8 on the periodic table, a nonmetal in period 2, group 16. The element that rusts iron, burns fuel and keeps you alive — and the most abundant element in the Earth's crust. At room temperature it is a gas.
[He] 2s² 2p⁴, giving 2 occupied shells with the distribution 2, 6. Its outer shell holds 6 electrons, which is what sets its bonding behaviour.Oxygen melts at 54.4 K (-218.8 °C) and boils at 90.2 K (-182.9 °C).
The standard atomic weight of oxygen is 15.999 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
The most common oxygen atom, O-16, has 8 neutrons: its mass number, 16, minus its atomic number, 8. It makes up 99.757% of natural oxygen. The classroom shortcut, rounded atomic mass (15.999) minus atomic number, gives the same 8.
Oxygen is a gas at room temperature; at 0 °C and 1 atm its density is about 1.429 g/L, so it is heavier than air.
Oxygen has a Pauling electronegativity of 3.44. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). A value this high means it tends to pull electrons away from almost anything it bonds with.
Oxygen was discovered in 1774 by Joseph Priestley and Carl Wilhelm Scheele. Origin of the name: Greek oxys + genes, "acid-former".
Oxygen makes up about 46.1% of the Earth's crust — one of the most abundant elements there is.