English: Oxygen (O) is # 8 on the Periodic Table of Elements. It forms by late-stage nuclear fusion in certain types of stars. At Earth surface conditions, it occurs as an invisible gas (O2 - molecular oxygen). Earth is the only world with abundant free oxygen gas in the atmosphere - it makes up 21% of modern air. The earliest Earth had no free oxygen. O2 gas is principally produced by photosynthesizers - organisms that make their own food using sunlight, with molecular oxygen as a waste product. Examples include plants, algae, many protists, and many bacteria.

Oxygen is the # 1 most common element in Earth's crust. It is in all common minerals (e.g., silicates and oxides) and all common rocks (e.g., granite and basalt). The oxides are a large group of minerals - all have one or more O-2 anions (one oxygen atom with a double-negative electric charge). All but the most inert elements form oxides. Water ice (H2O) is the most common oxide mineral on Earth.

Eighteen isotopes of oxygen are known - four occur naturally. Oxygen-16 is the most common. Oxygen-17 and oxygen-18 occur in very small amounts. Oxygen-15 is an unstable, radioactive variety occuring in trace amounts.

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Info. at: en.wikipedia.org/wiki/Oxygen and

en.wikipedia.org/wiki/Isotopes_of_oxygen