Oxygen at a glance
- Atomic number
- 8
- Symbol
- O
- Atomic mass
- 15.999 u
- Category
- Reactive nonmetal
- Group / period
- 16 / 2
- Electron configuration
- [He] 2s 2 2p 4
- State at 25 °C
- Gas
- Melting point
- -218.8 °C
- Boiling point
- -182.9 °C
- Density
- 1.43 g/L
- Electronegativity
- 3.44
- 1st ionization energy
- 1314 kJ/mol
- Oxidation states
- −2
- Discovered
- 1774
Atomic structure
A neutral oxygen atom has 8 protons and 8 electrons, and its most common isotope carries about 8 neutrons. One atom weighs about 2.66 × 10⁻²⁶ kg, so a mole of oxygen (6.022 × 10²³ atoms) weighs 15.999 g. That nucleus holds 1 neutrons for every proton, close to the one-to-one balance that light nuclei prefer. The electrons fill 2 shells (K 2, L 6), which gives the configuration [He] 2s 2 2p 4. With 6 electrons in its outer shell, it tends to gain or share electrons to complete its outer shell. It has 3 stable isotopes: O-16, O-17 and O-18. No other element has a stable isotope with mass number 16, so O-16 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) oxygen is a gas. It melts at -218.8 °C and boils at -182.9 °C, so it stays liquid across a 35.84-degree range. To turn liquid it has to be cooled below -182.9 °C, colder than dry ice (−78 °C). Only 4 elements have a lower melting point. As a gas it weighs 1.43 g per litre, 1.2 times heavier than air. Its atoms have a radius of about 60 pm (Nitrogen next door is 65 pm). In the solid, its atoms pack into a monoclinic lattice.
Chemical behaviour
Its electronegativity is 3.44 on the Pauling scale, the 2nd highest of all elements, so it pulls hard on shared electrons. Pulling off its outermost electron takes 1314 kJ/mol. A bond between oxygen and chlorine would be almost non-polar covalent, with an electronegativity difference of 0.28. It releases 141 kJ/mol when it gains an extra electron. Its common oxidation states are −2, so in compounds you will usually meet it as O²⁻. Group 16 also holds Sulfur, Selenium, Tellurium, Polonium and Livermorium, which share its outer-shell pattern. Its Lewis symbol is O surrounded by 6 dots. Handle with care: as a pure element it is oxidiser.
Where it comes from
It makes up 46.1 % of Earth's crust, the 1st most abundant element there. In the human body it accounts for about 65 % of body mass, making it one of the main building blocks of life. It was first identified in 1774, making it the 21st element to be discovered (counting the 11 known since antiquity together). When excited it glows at its own set of colours; its brightest visible lines are at 777.2 nm, 615.8 nm and 645.4 nm.