Oxygen (O)
Element 8 · Nonmetal · atomic mass 15.999 u · group 16, period 2
Electron shells: 2, 6
Oxygen is the most abundant element in the Earth's crust and the oxidiser that powers aerobic life. Its reactivity is what makes both fire and metabolism possible.
Earth's oxygen atmosphere is a waste product — cyanobacteria have been exhaling it for 2.4 billion years.
- Atomic number
- 8
- Atomic mass
- 15.999 u
- Electron configuration
- [He] 2s² 2p⁴
- Group · period · block
- 16 · 2 · p
- Oxidation states
- −2
- Electronegativity
- 3.44 (Pauling)
- Melting point
- -219 °C
- Boiling point
- -183 °C
- Density
- 0.001429 g/cm³
- State at 20 °C
- Gas
- Appearance
- Colourless gas; pale blue as a liquid
- Discovered
- Carl Wilhelm Scheele and Joseph Priestley, 1774
8 protons, and 8 electrons when neutral
IUPAC interval [15.99903, 15.99977]
Chalcogens
What Oxygen is used for
- Steelmaking
- Medical oxygen therapy
- Water treatment
- Rocket oxidiser
Atomic structure of Oxygen
A neutral oxygen atom has 8 protons and 8 electrons. Its most common isotope carries about 8 neutrons, which is where the mass number 16 comes from. Change the neutron count and you have a different isotope of oxygen; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 6. In orbital notation that is [He] 2s² 2p⁴, leaving 6 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Oxygen's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Oxygen sits in the periodic table
Discovery and naming
Oxygen was discovered by Carl Wilhelm Scheele and Joseph Priestley in 1774. Its name comes from Greek oxys + genes, "acid-forming".
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Oxygen?
The standard atomic weight of Oxygen is 15.999 u. Because the natural isotope mix varies between sources, IUPAC publishes it as the interval [15.99903, 15.99977], and 15.999 is the conventional single value to use in calculations.
How many protons, neutrons and electrons does Oxygen have?
Oxygen has 8 protons, because the atomic number is the proton count and that is what makes an atom Oxygen at all. A neutral Oxygen atom has the same number of electrons, 8. The neutron count varies by isotope; the most common one has about 8, since 16 − 8 = 8.
What is the electron configuration of Oxygen?
The electron configuration of Oxygen is [He] 2s² 2p⁴. Its electrons are arranged 2, 6 across 2 shells.
Is Oxygen a metal?
No. Oxygen is classified as a nonmetal, on the right-hand side of the table where elements gain or share electrons rather than losing them.
What group and period is Oxygen in?
Oxygen is in group 16 (the chalcogens), period 2, in the p-block. The group is its column and the period is its row.
What is the charge of Oxygen?
Oxygen most commonly forms a 2− ion. It is 2 electrons short of a full outer shell, so it gains them rather than losing 6.
What is the melting point of Oxygen?
Oxygen melts at -219 °C and boils at -183 °C. That puts it in the gases at everyday temperatures.
What is Oxygen used for?
Steelmaking, Medical oxygen therapy, Water treatment, Rocket oxidiser. Oxygen is the most abundant element in the Earth's crust and the oxidiser that powers aerobic life.
Who discovered Oxygen, and where does its name come from?
Oxygen (O) was discovered by Carl Wilhelm Scheele and Joseph Priestley in 1774. The name comes from Greek oxys + genes, "acid-forming".