Oxygen is a chemical element with symbol O and atomic number 8. Oxygen is a highly reactive non-metal and oxidizing agent that easily forms oxides with most elements as well as with other compounds. Oxygen is the most abundant element on Earth, and after hydrogen and helium, it is the third most abundant element in the universe.
Diatomic oxygen now makes up 20.95% of the Earth's atmosphere. All plants, animals and fungi need oxygen for cellular respiration, which takes energy by reacting oxygen with food particles and produces carbon dioxide as a waste product. In quadrupeds, breathing brings oxygen to the lungs, where gas exchange takes place, carbon dioxide dissipates from the blood, and oxygen dissipates into the blood. The blood system transports oxygen to the cells, where cellular respiration takes place.
One of the first known experiments on the relationship between combustion and air was carried out in the 2nd century BC by the Greek mechanic writer Philo of Byzantium. In his work Pneumatica, Philo noted that turning a vessel over a burning candle and surrounding the vessel's neck with water caused some of the water to float up to the neck.
Polish alchemist, philosopher and doctor Michał Sędzivoj described the substance contained in the air. During his experiments between 1598 and 1604, Sędziwo correctly concluded that the substance was equivalent to the gaseous by-product released by the thermal decomposition of potassium nitrate. However, Sędziwoj's discovery was often denied by successive generations of scientists and chemists.
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Oxygen was isolated by Michael Sędziwoj before 1604, but it is widely believed to have been discovered independently by Carl Wilhelm Scheele of Uppsala in 1773 or earlier and Joseph Priestley of Wiltshire in 1774. Priestley is often treated as a priority because his work was published first. Priestley, however, called oxygen "dephlogistic air" and did not recognize it as a chemical element. The name oxygen was coined in 1777 by Antoine Lavoiser, who was the first to recognize oxygen as a chemical element and correctly characterize its role in combustion.
