Oxygen (O), nonmetallic chemical element of Group 16 (VIa, or the oxygen group)
of the periodic table. Oxygen is a colourless, odourless, tasteless gas essential
to living organisms, being taken up by animals, which convert it to carbon dioxide;
plants, in turn, utilize carbon dioxide as a source of carbon and return the oxygen
to the atmosphere. Oxygen forms compounds by reaction with practically any other
element, as well as by reactions that displace elements from their combinations with
each other; in many cases, these processes are accompanied by the evolution of heat
and light and in such cases are called combustions. Its most important compound is water.