ElementsWhat is oxygen?
Oxygen is the gas that people, animals and fires need. It makes up about 21 percent of the air and, by weight, nearly half of the Earth's crust, where it is bound in rocks and water.
What is oxygen used for?
Steel making, hospital oxygen for patients, welding and metal cutting, water treatment, and rocket engines.
Where the name comes from
From the Greek words oxys and genes, meaning 'acid former', from an early and mistaken idea that all acids contain it.
Key facts
- Symbol
- O
- Atomic number
- 8
- Atomic mass
- 15.999 u
- Family
- Nonmetal
- Period
- 2
- Group
- 16
- Block
- p
- State at room temperature
- Gas
- Melting point
- -218.8 °C (54.36 K, -362 °F)
- Boiling point
- -182.9 °C (90.2 K, -297 °F)
- Density
- 0.001429 g/cm³ (as a gas at 0 °C)
- Electrons per shell
- 2, 6
- Electron configuration
- [He]2s2 2p4
- Common oxidation states
- -2
- Electronegativity (Pauling)
- 3.44
- First ionisation energy
- 13.618 eV
- Electron affinity
- 1.461 eV
- Atomic radius (van der Waals)
- 152 pm
- Discovered
- 1774
Place in the periodic table
Oxygen is element number 8. It sits in period 2 (row 2) of the periodic table, in group 16 (column 16), and belongs to the p-block. Just before it comes nitrogen (7), and just after it comes fluorine (9). In the same column, sulfur is below it. Elements in one column tend to behave alike.
Family: Nonmetals.
Physical properties
At room temperature oxygen is a gas. It melts at -218.8 °C and boils at -182.9 °C, so it stays liquid over a range of 36 degrees. One litre of the gas weighs about 1.429 grams at 0 °C and normal pressure. That is heavier than air, which weighs about 1.29 grams a litre. It has the 5th lowest melting point of all elements.
Inside the atom
An atom of oxygen has 8 protons in its nucleus and, when neutral, 8 electrons around it. The electrons are arranged in 2 shells: 2, 6. The outermost shell holds 6 electrons. Written in full, the electron configuration is [He]2s2 2p4.
Its electronegativity on the Pauling scale is 3.44. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 13.618 electronvolts (about 1,314 kilojoules per mole). One mole of oxygen weighs 15.999 grams, so a single gram contains about 3.76 × 1022 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains oxygen with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.