An oxygen atom has 8 electrons, with 2 in the K shell and 6 in the L shell.
When it gains 2 electrons, it has the same arrangement of electrons as neon, so oxygen readily forms the doubly charged negative ion \mathrm{O}^{2-}. This ion is called the oxide ion.
Oxygen is an element of group 16 of the periodic table, and it is also the most abundant element in the Earth's crust.
By volume, dry air contains about 78 % nitrogen, about 21 % oxygen and about 0.9 % argon. Carbon dioxide makes up only about 0.04 %.
Oxygen as an element, \mathrm{O_2}, is a colourless, odourless gas that dissolves slightly in water. In the laboratory, it is made by adding manganese(IV) oxide to hydrogen peroxide solution, and it is collected over water. The manganese(IV) oxide acts as a catalyst and is unchanged at the end of the reaction.
2\mathrm{H_2O_2} \longrightarrow 2\mathrm{H_2O} + \mathrm{O_2}
A reaction in which a substance combines vigorously with oxygen, giving out heat and light, is called combustion. Oxygen itself does not burn, but it helps other substances to burn.
Steel wool, a ball of fine iron fibres, burns slowly in air, glowing red. In oxygen, it burns fiercely, throwing out sparks.
After burning, black iron oxide is left.
When an electric discharge is passed through oxygen, or strong ultraviolet light shines on it, ozone, \mathrm{O_3}, is formed.
3\mathrm{O_2} \longrightarrow 2\mathrm{O_3}
Ozone is a pale blue gas with a distinctive smell, and it is a strong oxidizing agent. It turns moist potassium iodide-starch paper blue-black, so this paper can be used to test for ozone.
Different forms of the same element with different properties, like oxygen and ozone, are called allotropes of each other.
At heights of about 10 to 50 km above the ground, there is a layer rich in ozone, called the ozone layer. The ozone layer absorbs much of the harmful ultraviolet light coming from the Sun and protects living things on the ground.
Chlorofluorocarbons, which used to be used in refrigerators and sprays, were found to destroy the ozone layer. A large thinning of the ozone over Antarctica, known as the ozone hole, was discovered, and the production and use of chlorofluorocarbons are now regulated internationally.