| Atomic number (Z) | 8 |
| Group | group 16 (chalcogens) |
| Period | period 2 |
| Block | p-block |
| Element category | reactive nonmetal |
| Electron configuration | [He] 2s2 2p4 |
| Electrons per shell | 2, 6 |
Dioxygen is also used in cellular respiration and several main classes of organic molecules in living organisms have oxygen, such as proteins, carbohydrates, nucleic acids, and fats, as do the main fundamental inorganic compounds of teeth, animal shells, and bone. Maximum of the mass of living organisms is oxygen as a constituent of water, the main constituent of life forms. Oxygen is nonstop replenished in Earth’s atmosphere by the process called photosynthesis, which uses the energy of sunlight to create oxygen from carbon dioxide and water. Oxygen is too chemically reactive to remain a free element in the air without being continuously replaced by the photosynthetic action of living organisms. Another type (allotrope) of oxygen, ozone (O3), strongly absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps in protecting the biosphere from ultraviolet radiation (UV). On the other hand, ozone present at the surface is a byproduct of smog and therefore a pollutant.
Occurrence
The remarkably high concentration of oxygen gas on Earth is the outcome of the oxygen cycle. This biogeochemical cycle defines the movement of oxygen within and between its three key reservoirs on Earth: the biosphere, the atmosphere, and the lithosphere. The key driving factor of the oxygen cycle is photosynthesis, which is responsible for modern Earth’s atmosphere. Photosynthesis releases oxygen as their end product into the atmosphere, while respiration, falling-off, and combustion remove it from the atmosphere. In the present symmetry, production and consumption happen at the same rate
Characteristics
Oxygen gas dissolved in water at sea-level
| 5 °C | 25 °C | |
| Freshwater | 9.0 mL | 6.04 mL |
| Seawater | 7.2 mL | 4.95 mL |
Oxygen shrinks at 90.20 K (−182.95 °C, −297.31 °F), and freezes at 54.36 K (−218.79 °C,−361.82 °F). Both liquid and solid O2 are clear materials with a light sky-blue color produced by absorption in the red (when compared with the blue color of the sky, which is due to Rayleigh scattering of blue light). High-purity liquid O2 is typically found by the fractional distillation of liquefied air. Liquid oxygen may also be reduced from air using liquid nitrogen as a coolant.
Oxygen is an extremely reactive compound and must be separated from combustible materials
The spectroscopy of molecular oxygen is related to the atmospheric processes of Aurora and airflow.
Properties and molecular structure
As dioxygen, two oxygen atoms are chemically bound with each other. The bond can be variously defined based on the level of theory, but is reasonable and just described as a covalent double bond that results from the filling of molecular orbitals made from the atomic orbitals of the single oxygen atoms, the filling of which results in a bond order of two. More precisely, the double bond is the outcome of successive, low-to-high energy, or Aufbau, filling of orbitals, and the subsequent cancellation of contributions from the 2s electrons, after successive filling of the low σ and σ* orbitals; σ overlap of the two atomic 2p orbitals that lie in between the O-O molecular axis and PIE π overlap of two pairs of atomic 2p orbitals at right angles to the O-O molecular axis, and then termination of contributions from the remaining two of the six 2p electrons after their incomplete filling of the lowest π and π* orbitals.
This blend of cancellations and sigma( σ )and pie ( π) overlaps the results in dioxygen’s double bond reactivity, character, and a triplet electronic ground state. An electron arrangement with two unpaired electrons, as is seen in dioxygen orbitals that are of the same energy—i.e., degenerate—is an arrangement called a spin triplet state. Therefore, the ground state of the O2 molecule is mentioned as triplet oxygen. The highest energy, partly filled orbitals are antibonding, and so their filling declines the bond order from 3 to 1. Triplet oxygen reacts slowly with most organic molecules because its unpaired electrons, which have paired electron spins; this avoids spontaneous combustion.
Singlet oxygen is a term given to several higher-energy species of molecular O2 in which all the electron spins are paired. It is much extra reactive with common organic molecules than is molecular oxygen. In nature, singlet oxygen is usually produced from water during photosynthesis, using the energy of sunlight. It is also made in the troposphere by the photolysis of ozone by light of short wavelength, and by the immune system as a source of active oxygen. Carotenoids in photosynthetic creatures play an important role in absorbing energy from singlet oxygen and transforming it to the unexcited ground state before it can harm tissues.
Storage
Biological role
In nature, free oxygen is formed by the light-driven dividing of water during oxygenic photosynthesis. According to various estimates, green algae and cyanobacteria in sea environments offer about 70% of the free oxygen formed on Earth, and the rest is made by terrestrial plants. Other estimates of the oceanic involvement to atmospheric oxygen are greater, while some guesses are lower, suggesting oceans yield about 45% of Earth’s atmospheric oxygen every year.
A basic overall formula for photosynthesis is
or
carbon dioxide + sunlight +water → glucose + dioxygen
living organism