Characteristics and Molecular Structure of Oxygen

Oxygen is a chemical compound with symbol O and atomic number 8. In the periodic table, it is the member of the chalcogen group, an extremely reactive nonmetal, and an oxidizing agent that readily produces oxides with most elements as well as with other elements. By weight, oxygen is the third-most abundant element in the world, after hydrogen and helium. At constant pressure and temperature, two atoms of the element attached to form dioxygen, an odorless and colorless diatomic gas with the formula O2. Diatomic oxygen gas creates 20.8% of the Earth's atmosphere. As compounds containing oxides, the compound makes up almost half of the Earth's crust.

Atomic number (Z)8
Groupgroup 16 (chalcogens)
Periodperiod 2
Element category reactive nonmetal
Electron configuration[He] 2s2 2p4
Electrons per shell2, 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.


Oxygen is the most copious chemical element by mass in the Earth's environment, air, sea, and land. Oxygen is the 3rdmostcopious chemical element in the world, after hydrogen and helium. About 0.8% of the Sun's mass is oxygen. Oxygen constitutes 49.3% of the Earth's crust by mass as part of oxide compounds like silicon dioxide and is the most copious element by mass in the Earth's crust. It is also the main component of the oceans (88.9% by mass). Oxygen gas is the 2ndsimplest component of the Earth's atmosphere, taking up 20.9% of its volume and 23.1% of its mass (some 10 tons). Earth is rare among the planets of the Solar System in having such a great concentration of oxygen gas in its atmosphere: Mars (with 0.1% O2 by volume) and Venus have much less. The O2 surrounding those planets is made solely by the action of ultraviolet radiation (UV) on oxygen-containing molecules such as carbon dioxide CO2.

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


Physical properties

Oxygen dissolves more freely in water than nitrogen, and in freshwater more freely than seawater. Water in symmetry with air has approximately 1 molecule of dissolved O2 for every 2 molecules of N2 in a ratio of (1:2), compared with an atmospheric ratio of about 1:4. The solubility of oxygen in water is depending upon temperature, and about double as much (14.6 mg•L−1) dissolves at 0 °C than at 20 °C (7.6 mg•L−1). At 26 °C and 1 standard atmosphere (101.3 kPa) of air, freshwater has about 6.04 milliliters (mL) of oxygen per liter, and seawater has about 4.95 mL per liter. At 5 °C, the solubility rises to 9.0 mL (more than at 25 °C)/ liter for water and 7.2 mL per liter for sea water(45% more).

Oxygen gas dissolved in water at sea-level
5 °C25 °C
Freshwater9.0 mL6.04 mL
Seawater7.2 mL4.95 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

At customary pressure and temperature, oxygen is an odorless, colorless and tasteless gas with the molecular formula O2, mentioned to as dioxygen.

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.


Oxygen storage technique includes cryogenics, high-pressure oxygen tanks, and chemical compounds. For reasons of economy, oxygen is often conveyed in bulk as a liquid in particularly insulated tankers, since one liter of liquefied oxygen is equal to 840 liters of gaseous oxygen at atmospheric pressure and 22 °C (70 °F). Such tankers are used to replenish bulk liquid oxygen storage bottles, which stand outside hospitals and other institutes that need great volumes of pure oxygen gas. Liquid oxygen is passed through heat exchangers, which transform the cryogenic liquid into gas before it arrives in the building. Oxygen is also stored and transported in smaller cylinders having the compressed gas; a form that is useful in some portable medical applications and oxy-fuel welding and cutting.

Biological role

Photosynthesis and respiration

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

6 CO2 + 6 H2O + photons → C6H12O6 + 6 O2
carbon dioxide + sunlight +water → glucose + dioxygen

living organism

The independent or free oxygen incomplete pressure in the body of a living vertebrate organism is peak in the respiratory system, and drops along any arterial system, peripheral tissues, and venous system, respectively. Partial pressure or incomplete pressure is the pressure that oxygen would have if and only if it occupies the volume.