Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements and other compounds. After hydrogen and helium, oxygen is the third-most abundant element in the universe by mass. At standard temperature and pressure, two atoms bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas currently constitutes 20.95% of the Earth's atmosphere, though this has changed considerably over long periods of time. Oxygen makes up almost half of the Earth's crust in the form of oxides.

Pnictogens and Chalcogens

A pnictogen is one of the chemical elements in group 15 of the periodic table. This group is also known as the nitrogen family. The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family.

Pnictogens consist of the elements nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi),  and perhaps the chemically uncharacterized synthetic element moscovium (Mc).