Oxygen
Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table and is a highly reactive nonmetal and oxidizing agent that readily forms compounds (notably oxides) with most elements. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium.
At a glance
- Atomic number
- 8
- Atomic mass
- 15.999 u
- Phase
- Gas
- Density
- 1.429 g/L
- Melting point
- 54.36 K
- Boiling point
- 90.188 K
- First ionization energy
- 1,313.9 kJ/mol
- Electronegativity
- 3.44
- Electron affinity
- 140.976 kJ/mol
- Molar heat capacity
- Unavailable
- Period
- 2
- Appearance
- Unavailable
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Oxygen’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
Preparing the orbital view…
8 protons8 neutrons
Oxygen-16: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FGas
At this temperature, oxygen is a gas.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
ONeutral atom · no charge
A neutral oxygen atom. Removing its first electron takes 1,314 kJ/mol.
Oxygen’s outer shell holds 6 electrons. Electron 7 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 5.4× as much energy as the one before.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 6, then 2 bars. Read backwards, that’s 2, 6: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 6 electrons before the first big jump are oxygen’s valence electrons, the ones it uses in chemical reactions. That’s why oxygen sits in group 16: every element in the group has the same number of outer electrons, so they react in similar ways.
- Removing electrons from oxygen is expensive from the start (1,314 kJ/mol for the first). So in reactions oxygen tends to gain or share electrons rather than lose them.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 2 | 1,314 | – | Measured |
| 2 | 2 | 3,389 | 2.6 | Measured |
| 3 | 2 | 5,301 | 1.6 | Estimated |
| 4 | 2 | 7,469 | 1.4 | Measured |
| 5 | 2 | 10,990 | 1.5 | Measured |
| 6 | 2 | 13,326 | 1.2 | Estimated |
| 7 | 1 | 71,334 | 5.4 | Calculated |
| 8 | 1 | 84,078 | 1.2 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
The oxygen you breathe, and its other forms
Oxygen is so familiar we forget it’s there: colourless, odourless, and about a fifth of every breath. But oxygen atoms also team up in threes as ozone, which shields us from the Sun’s ultraviolet. And cooled far enough, oxygen turns into a pale blue liquid that sticks to a magnet.
O₂ Oxygen gas (dioxygen)
- Shape
- Two atoms, joined by a double bond
- In the air
- About 21% of every breath
- Colour and smell
- None as a gas; pale blue as a liquid
- Boils at
- −183 °C
- Magnetic?
- Yes, weakly: two unpaired electrons
The yellow arrows are oxygen’s two unpaired electrons. They make each molecule a tiny magnet: try the demo below.
Cool it to −183 °C: a blue liquid that sticks to a magnet
Pour a liquid past the magnet’s tips.
Each oxygen molecule has two unpaired electrons, the yellow arrows above, which make it a tiny magnet. A magnet pulls them in. Nitrogen’s electrons are all paired, so it doesn’t notice the magnet at all.
The ozone layer: sunscreen for the planet
300 Dobson units3 mm
All the ozone above you, squeezed down to the pressure at sea level, would make a layer only this thick.
- UV-C 100–280 nm
- Stopped completely, high up, by oxygen and ozone.
- UV-B 280–315 nm
- Reaching the ground: A small amount. Mostly stopped by ozone. It causes sunburn, so where ozone is thin, more reaches the ground.
- UV-A 315–400 nm
- Ozone barely touches it: most reaches the ground either way.
In the 1980s scientists found the ozone over Antarctica thinning every spring, eaten away by CFCs from fridges and spray cans. The 1987 Montreal Protocol banned them, and the ozone layer is slowly healing: over Antarctica it should be back to 1980 levels around 2066.
Atoms are drawn smaller than their true size so the shapes are clear. Ozone’s bonds are drawn as one and a half each: its electrons are shared across both bonds. The widths of the ultraviolet arrows are a rough guide based on how ozone absorbs light, not measurements.
How does it compare?
Density
0.00143 g/cm³5th lowest of 99
Melting point
54.4 K4th lowest of 96
Boiling point
90.2 K7th lowest of 97
Electronegativity
3.442nd highest of 96
First ionization energy
1,314 kJ/mol7th highest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- OxygenRoyal Society of Chemistry
- OxygenWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database