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=Technology Tool Belt=



Oxygen is a member of the [|chalcogen] [|group] on the [|periodic table], and is a highly [|reactive] [|nonmetallic] element that readily forms [|compounds] (notably [|oxides]) with almost all other elements. By mass, oxygen is the [|third most abundant] element in the universe after [|hydrogen] and [|helium][|[][|1][|]] and the [|most abundant] element by mass in the [|Earth's crust], making up almost half of the crust's mass.[|[][|2][|]] Free oxygen is too chemically reactive to appear on Earth without the [|photosynthetic] action of living organisms, which use the energy of sunlight to produce elemental oxygen from water. Elemental O2 only began to accumulate in the atmosphere after the evolutionary appearance of these organisms, roughly 2.5 billion years ago.[|[][|3][|]] Diatomic oxygen gas constitutes 20.8% of the volume of [|air].[|[][|4][|]] Because it comprises most of the mass in water, oxygen comprises most of the mass of living organisms (for example, about two-thirds of the human body's mass). All major classes of structural molecules in living organisms, such as [|proteins], [|carbohydrates], and [|fats], contain oxygen, as do the major [|inorganic compounds] that comprise animal shells, teeth, and bone. Elemental oxygen is produced by [|cyanobacteria], [|algae] and plants, and is used in [|cellular respiration] for all complex life. Oxygen is toxic to [|obligately anaerobic organisms], which were the dominant form of [|early life] on Earth until O2 began to accumulate in the atmosphere. Another form ([|allotrope]) of oxygen, [|ozone] ( O3 ), helps protect the biosphere from [|ultraviolet radiation] with the high-altitude [|ozone layer], but is a pollutant near the surface where it is a by-product of [|smog]. At even higher [|low earth orbit] altitudes atomic oxygen is a significant presence and a cause of [|erosion for spacecraft].[|[][|5][|]] media type="youtube" key="b4wveY2-lCo" height="200" width="400"