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( Article Type: Explanation )

Oxygen is a colourless, odourless, tasteless gas that is essential to life on earth. Its most important compound is water (H2O). The word ‘oxygen’ comes from the Greek oxys ‘sharp acid’ and -genes ‘born’ meaning ‘acid former’. The occurrence, by weight, of oxygen in the atmosphere is 23%, in seawater 85.5%, and in the earth’s crust 46.6%. It is the most abundant of all elements in the earth’s crust, occurring in rock, water and air. In the lower atmosphere, oxygen is found as O2 but in the upper atmosphere it appears as ozone (O3). During respiration, animals and lower plants take oxygen from the atmosphere and return carbon dioxide (CO2) whereas, using photosynthesis, green plants assimilate carbon dioxide in the presence of sunlight and produce oxygen. Almost all free oxygen present in the atmosphere is the result of photosynthesis.