Oxygen (O)
Oxygen (O) is a reactive nonmetal with atomic number 8 and a standard atomic weight of 15.999. At room temperature it is a gas that melts at -219 °C and boils at -183 °C.
Oxygen key facts
Oxygen at a glance
Identity
| Symbol | O |
|---|---|
| Atomic number (Z) | 8 |
| Atomic weight | 15.999 ± 0.001 (CIAAW 2024) |
| Category | Reactive nonmetal |
| Group | 16 — chalcogens (oxygen group) |
| Period | 2 |
| Block | p-block |
| CAS number | 7782-44-7 |
Atomic properties
| Electron configuration | [He] 2s2 2p4 Full: 1s2 2s2 2p4 |
|---|---|
| Electrons per shell | 2, 6 |
| Oxidation states | -2 Less common: -1, 0, +1, +2 |
| Electronegativity (Pauling) | 3.44 |
| First ionisation energy | 13.618 eV (1,313.9 kJ/mol) 2nd: 35.121 eV · 3rd: 54.935 eV |
| Electron affinity | 1.461 eV |
| Atomic radius (empirical) | 60 pm |
| Covalent radius | 63 pm |
| Van der Waals radius | 152 pm |
Physical properties
| State at 25 °C | Gas |
|---|---|
| Melting point | -218.8 °C · -361.8 °F · 54.36 K |
| Boiling point | -183 °C · -297.3 °F · 90.19 K |
| Density | 1.429 g/L (gas at 0 °C, 1 atm) |
| Specific heat capacity | 0.918 J/(g·K) |
| Thermal conductivity | 0.027 W/(m·K) |
| Crystal structure | Cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 16O 99.757%, 18O 0.2045%, 17O 0.0384% |
| Stable isotopes | 5 |
| Known isotopes | 18 |
| Abundance in Earth's crust | 461,000 mg/kg |
| Abundance in seawater | 857,000 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Oxygen electron configuration
Oxygen has 8 electrons. Written in short form, its ground-state configuration is [He] 2s2 2p4 — the electrons of the noble gas helium, plus 2s2 2p4.Written out in full: 1s2 2s2 2p4.
As a member of group 16 it has 6 valence electrons in its outer shell (shell 2), which largely decide its chemistry.
Discovery and name
1771–1774 — Carl Wilhelm Scheele and Joseph Priestley (independently) · Sweden / England.
Greek oxys (sharp, acid) + genes — Lavoisier thought all acids contained it.
Known for
About 21% of air and the most abundant element in Earth's crust by mass. Essential for respiration and combustion; ozone (O₃) in the stratosphere filters ultraviolet light.
Isotopes of oxygen
Naturally occurring oxygen is a mix of 3 isotopes. Each has 8 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 16O | 8 | 8 | 99.757% | stable |
| 18O | 8 | 10 | 0.2045% | stable |
| 17O | 8 | 9 | 0.0384% | stable |
18 isotopes of oxygen are known in our data set (half-lives as published in nuclear data tables).
Oxygen compared with group 16
The other elements in group 16 (chalcogens (oxygen group)), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Oxygen | 8 | 15.999 | -219 °C | -183 °C | 1.429 g/L | 3.44 |
| Sulfur | 16 | 32.06 | 115 °C | 445 °C | 2.07 g/cm³ | 2.58 |
| Selenium | 34 | 78.971 | 221 °C | 685 °C | 4.809 g/cm³ | 2.55 |
| Tellurium | 52 | 127.60 | 450 °C | 988 °C | 6.232 g/cm³ | 2.1 |
| Polonium | 84 | [209] | 254 °C | 962 °C | 9.2 g/cm³ | 2 |
| Livermorium | 116 | [293] | — | — | 12.9 g/cm³ (pred.) | — |