Reactive nonmetal
Oxygen is a chemical element; it has the symbol O and its atomic number is 8. It is a member of the chalcogen group in the periodic table. It is highly reactive, a nonmetal, and a potent oxidizing agent that readily forms oxides with most elements as well as with other compounds. Oxygen is the most abundant element in Earth's crust, making up almost half of the Earth's crust in the form of various oxides such as water, carbon dioxide, iron oxides, and silicates. It is also the third-most abundant element in the universe after hydrogen and helium.
At standard temperature and pressure, two oxygen atoms will bind covalently to form dioxygen, a colorless and odorless diatomic gas with the chemical formula O2. Dioxygen gas currently constitutes approximately 20.95% molar fraction of the Earth's atmosphere, though this has changed considerably over long periods of time in Earth's history. The much rarer allotrope of oxygen, ozone (O3), strongly absorbs the UVB and UVC wavelengths and forms a protective ozone layer at the lower stratosphere, which shields the biosphere from ionizing ultraviolet radiation. However, ozone present at the surface is a corrosive byproduct of smog and thus an air pollutant.
All eukaryotic organisms, including plants, animals, fungi, algae, and most protists, need oxygen for cellular respiration, a process that extracts chemical energy by the reaction of oxygen with organic molecules derived from food and releases carbon dioxide as a waste product. …
The modern concept of the element oxygen developed over five centuries and included many related discoveries and unsuccessful theories. Multiple people made different contributions to the concept. No one person discovered oxygen.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd-century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Ibn al-Nafis, writing in 1250 CE, correctly described oxygenation of blood in the circulatory system; Michael Servetus rediscovered this concept in 1553 but his books were systematically destroyed. A scientifically based and influential description was published by William Harvey in 1628.
Leonardo da Vinci observed that a portion of air is consumed during combustion and respiration.
Polish alchemist, philosopher, and physician Michael Sendivogius (Michał Sędziwój), writing in 1604, described a substance contained in air, referring to it as cibus vitae ('food of life'); this substance is identical with oxygen. …
At standard temperature and pressure, oxygen is a colorless, odorless, and tasteless gas with the molecular formula O2, referred to as dioxygen.
As dioxygen, two oxygen atoms are chemically bound to each other. The bond can be variously described based on level of theory, but is reasonably and simply described as a covalent double bond that results from the filling of molecular orbitals formed from the atomic orbitals of the individual oxygen atoms, the filling of which results in a bond order of two. More specifically, the double bond is the result of sequential, low-to-high energy, or Aufbau, filling of orbitals, and the resulting cancellation of contributions from the 2s electrons, after sequential filling of the low σ and σ* orbitals; σ overlap of the two atomic 2p orbitals that lie along the O–O molecular axis and π overlap of two pairs of atomic 2p orbitals perpendicular to the O–O molecular axis, and then cancellation of contributions from the remaining two 2p electrons after their partial filling of the π* orbitals.
This combination of cancellations and σ and π overlaps results in dioxygen's double-bond character and reactivity, as well as the presence of a triplet electronic ground state. An electron configuration with two unpaired electrons, as is found in dioxygen orbitals (see the filled π* orbitals in the diagram), that are of equal energy—i.e., degenerate—is a configuration termed a spin triplet state. …
Naturally occurring oxygen is composed of three stable isotopes, 16O, 17O, and 18O, with 16O being the most abundant (99.762% natural abundance).
16O is one of the dominant fusion products in massive stars. It is synthesized at the end of the triple-alpha process with some synthesis in the neon burning process. Both 17O and 18O require seed nuclei. 17O is primarily made by the burning of hydrogen into helium during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars. Most 18O is produced when 14N (made abundant from CNO burning) captures a 4He nucleus, making 18O common in the helium-rich zones of evolved, massive stars.
Fifteen radioisotopes have been characterized, ranging from 11O to 28O. The most stable are 15O with a half-life of 122.24 seconds and 14O with a half-life of 70.606 seconds. All of the remaining radioactive isotopes have half-lives that are less than 27 seconds, and the majority of them have half-lives that are less than 83 milliseconds. The most common decay mode of the isotopes lighter than 16O is β+ decay to yield nitrogen, and the most common mode for the isotopes heavier than 18O is beta decay to yield fluorine.
Uptake of O2 from the air is the essential purpose of respiration, so oxygen supplementation is used in medicine. Treatment not only increases oxygen levels in the patient's blood but has the secondary effect of decreasing resistance to blood flow in many types of diseased lungs, easing work load on the heart. Oxygen therapy is used to treat emphysema, pneumonia, some heart disorders (congestive heart failure), some disorders that cause increased pulmonary artery pressure, and any disease that impairs the body's ability to take up and use gaseous oxygen.
Treatments are flexible enough to be used in hospitals, the patient's home, or increasingly by portable devices. Oxygen tents were once commonly used in oxygen supplementation, but have since been replaced mostly by the use of oxygen masks or nasal cannulas.
Hyperbaric (high-pressure) medicine uses special oxygen chambers to increase the partial pressure of O2 around the patient and, when needed, the medical staff. Carbon monoxide poisoning, gas gangrene, and decompression sickness (the 'bends') are sometimes addressed with this therapy. Increased O2 concentration in the lungs helps to displace carbon monoxide from the heme group of hemoglobin. Oxygen gas is poisonous to the anaerobic bacteria that cause gas gangrene, so increasing its partial pressure helps kill the bacteria and alleviates gas gangrene. …
Naturally occurring oxygen is composed of three stable isotopes, 16O, 17O, and 18O, with 16O being the most abundant (99.762% natural abundance).
16O is one of the dominant fusion products in massive stars. It is synthesized at the end of the triple-alpha process with some synthesis in the neon burning process. Both 17O and 18O require seed nuclei. 17O is primarily made by the burning of hydrogen into helium during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars. Most 18O is produced when 14N (made abundant from CNO burning) captures a 4He nucleus, making 18O common in the helium-rich zones of evolved, massive stars.
Fifteen radioisotopes have been characterized, ranging from 11O to 28O. The most stable are 15O with a half-life of 122.24 seconds and 14O with a half-life of 70.606 seconds. All of the remaining radioactive isotopes have half-lives that are less than 27 seconds, and the majority of them have half-lives that are less than 83 milliseconds. The most common decay mode of the isotopes lighter than 16O is β+ decay to yield nitrogen, and the most common mode for the isotopes heavier than 18O is beta decay to yield fluorine.
In nature, free oxygen is produced as a byproduct of light-driven splitting of water during chlorophyllic photosynthesis. According to some estimates, marine photoautotrophs such as red/green algae and cyanobacteria provide about 70% of the free oxygen produced on Earth, and the rest is produced in terrestrial environments by plants. Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year.
A simplified overall formula for photosynthesis is
6 CO2 + 6 H2O + photons → C6H12O6 + 6 O2
or simply
carbon dioxide + water + sunlight → glucose + dioxygen
Photolytic oxygen evolution occurs in the thylakoid membranes of photosynthetic organisms and requires the energy of four photons. Many steps are involved, but the result is the formation of a proton gradient across the thylakoid membrane, which is used to synthesize adenosine triphosphate (ATP) via photophosphorylation. The O2 remaining (after production of the water molecule) is released into the atmosphere.
Oxygen is used in mitochondria of eukaryotes to generate ATP during oxidative phosphorylation. …