Oxygen

Oxygen is so chemically active that it is seldom found in the atomic form, except where temperatures are so high or ultraviolet radiation so intense that any kind of molecule is regularly disrupted. At earth-normal temperatures, it is normally found in chemical combination with some other elements, but if it is found free, it is usually in molecular form, O2, as a gas.
One of my interests is in science-fictional world building, as a way to apply a few accumulated bits of knowleds, and to give some variety to the way things are.
For astronomical applications, hydrogen is by far the most abundant of the chemical elements, followed by helium. But helium is chemically inert and forms no compounds, at least not at ordinary temperatures. Third on the list in abundance is oxygen.
On the earth, Oxygen is the single most abundant element. Most of it is found in chemical combination with other elements. It is one of the few places where it can be found free in the atmosphere. It poisonous to some forms of bacteria, because of its presence in the atmosphere, environments without free oxygen are rather uncommon on the earth, so the anaerobic bacteria are likewise uncommon. It is continually produced by plants, which are thought to be the ultimate source of it in the atmosphere, and is essential to the respiration of most animals. It is thought that in the earliest stages of the earth's history, there was no oxygen in the atmosphere, which would dramatically alter the conditions of life in ways that are difficult to predict.
There are some applications to industrial processes, as oxygen can be separated from the mixtures and compounds that naturally contain it, although purifying it and liquefying it is inconvenient and expensive enough that it's not done unless there is a significant benefit to be gained.