Oxygen (O)

Oxygen is the most abundant element in the Earth's crust at about 46% by mass, mostly as silicates rather than as anything you can breathe. Free O₂ in the atmosphere is a biological product: cyanobacteria began releasing it around 2.4 billion years ago in what is now called the Great Oxidation Event, which poisoned most of the life then on Earth. Liquid oxygen is pale blue and is attracted to a magnet, because the O₂ molecule has two unpaired electrons — a fact that ordinary Lewis-dot bonding gets flatly wrong.

Properties

Atomic number

8

Symbol

O

Standard atomic weight

15.999 u

Category

Nonmetal

Group

16

Period

2

Block

p-block

Electron configuration

[He] 2s² 2p⁴

Electrons per shell

2, 6

Melting point

54.36 K — -218.8 °C — -361.8 °F

Boiling point

90.2 K — -182.9 °C — -297.3 °F

Density

0.001429 g/cm³

Electronegativity

3.44 (Pauling)

Oxidation states

-2, -1

Discovered

1774 by Carl Wilhelm Scheele and Joseph Priestley

Electron configuration

Full

1s² 2s² 2p⁴

Noble gas shorthand

[He] 2s² 2p⁴

Shells

K: 2 · L: 6

What it is used for

• Steelmaking — blowing oxygen through molten iron burns off the carbon
• Medical oxygen in hospitals, ambulances and home concentrators
• Oxy-fuel cutting and welding torches
• Rocket oxidiser, usually as liquid oxygen
• Aeration in sewage treatment and fish farming

Hazards

• Not itself flammable, but makes almost everything else burn faster and hotter
• Oil or grease on an oxygen fitting can ignite spontaneously
• Breathing pure oxygen at pressure causes lung damage and, above about 1.6 bar, seizures

IUPAC publishes an interval, [15.99903, 15.99977]; 15.999 is the conventional value. Density is the gas at 0 °C and 101.325 kPa.

Neighbours in the table

Group 16

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Oxygen sits in group 16 with sulfur, selenium, tellurium, polonium, livermorium.