Oxygen
Oxygen is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element that readily forms compounds (notably oxides) with most elements except the noble gases Helium and Neon. Oxygen is a strong oxidizing agent and only fluorine has greater electronegativity. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium and the most abundant element by mass in the Earth's crust, making up almost half of the crust's mass. Oxygen is too chemically reactive to remain a free element in Earth's atmosphere without being continuously replenished by the photosynthetic action of living organisms, which use the energy of sunlight to produce elemental oxygen from water. Free elemental O2 only began to accumulate in the atmosphere about 2.5 billion years ago (see Great oxygenation event) about a billion years after the first appearance of these organisms. Diatomic oxygen gas constitutes 20.8% of the volume of air.
Oxygen constitutes most of the mass of living organisms, because water is their major constituent (for example, about two-thirds of human body mass). Many major classes of organic molecules in living organisms, such as proteins, nucleic acids, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that are constituents of animal shells, teeth, and bone. Elemental oxygen is produced by cyanobacteria, algae and plants, and is used in cellular respiration for all complex life. Oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until O2 began to accumulate in the atmosphere. Another form (allotrope) of oxygen, ozone (O3), strongly absorbs UVB radiation and consequently the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation, but is a pollutant near the surface where it is a by-product of smog. At even higher low earth orbit altitudes atomic oxygen is a significant presence and a cause of erosion for spacecraft.
Oxygen was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774, but Priestley is often given priority because his work was published first. The name oxygen was coined in 1777 by Antoine Lavoisier, whose experiments with oxygen helped to discredit the then-popular phlogiston theory of combustion and corrosion. Oxygen is produced industrially by fractional distillation of liquefied air, use of zeolites with pressure-cycling to concentrate oxygen from air, electrolysis of water and other means. Uses of elemental oxygen include the production of steel, plastics and textiles, brazing, welding and cutting of steels and other metals, rocket propellant, oxygen therapy and life support systems in aircraft, submarines, spaceflight and diving.
Occurrence
Oxygen is the most abundant chemical element by mass in the Earth's biosphere, air, sea and land. Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium. About 0.9% of the Sun's mass is oxygen. Oxygen constitutes 49.2% of the Earth's crust by mass and is the major component of the world's oceans (88.8% by mass).[5] Oxygen gas is the second most common component of the Earth's atmosphere, taking up 20.8% of its volume and 23.1% of its mass (some 1015 tonnes). Earth is unusual among the planets of the Solar System in having such a high concentration of oxygen gas in its atmosphere: Mars (with 0.1% O2 by volume) and Venus have far lower concentrations. However, the O2 surrounding these other planets is produced solely by ultraviolet radiation impacting oxygen-containing molecules such as carbon dioxide.
The unusually high concentration of oxygen gas on Earth is the result of the oxygen cycle. This biogeochemical cycle describes the movement of oxygen within and between its three main reservoirs on Earth: the atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for modern Earth's atmosphere. Photosynthesis releases oxygen into the atmosphere, while respiration and decay remove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate of roughly 1/2000th of the entire atmospheric oxygen per year.
Cold water holds more dissolved O2.
Free oxygen also occurs in solution in the world's water bodies. The increased solubility of O2 at lower temperatures (see Physical properties) has important implications for ocean life, as polar oceans support a much higher density of life due to their higher oxygen content. Water polluted with plant nutrients such as nitrates or phosphates may stimulate growth of algae by a process called eutrophication and the decay of these organisms and other biomaterials may reduce amounts of O2 in eutrophic water bodies. Scientists assess this aspect of water quality by measuring the water's biochemical oxygen demand, or the amount of O2 needed to restore it to a normal concentration.
| Symbol | O | |
| Atomic Number | 8 | |
| Atomic Weight | 15.9994 | |
| Oxidation States | -2 | |
| Electronegativity, Pauling | 3.44 | |
| State at RT | Gas, Nonmetal | |
| Melting Point, K | 54.8 | |
| Boiling Point, K | 90.19 |
Interesting Facts about Oxygen
- Air is 78 percent nitrogen and 21 percent oxygen. Oxygen is about twice as soluble in water as nitrogen. If it had the same solubility as nitrogen, much less oxygen would be present in seas, lakes and rivers, and life would be very different.
- Almost two-thirds of the weight of living things comes from oxygen, mainly because living things contain a lot of water and 88.9 percent of water’s weight comes from oxygen.
- Oxygen (O2) is unstable in our planet’s atmosphere and must be constantly replenished by photosynthesis in green plants. Without life, our atmosphere would contain almost no O2.
- If we discover any other planets with atmospheres rich in oxygen, we will know that life is almost certainly present on these planets; significant quantities of O2 will only exist on planets when it is released by living things.
- Just five elements make up over 90 percent of the weight in the Earth’s crust. Almost half of the weight of the crust comes from oxygen. (Silicon, aluminum, iron and calcium are the other four main elements in the crust.)
- Oxygen is made in stars which have a mass of five or more Earth suns when they burn helium and carbon or just carbon in nuclear fusion reactions. Oxygen is part of the ‘ash’ formed by these nuclear fires.
- Oxygen is the third most abundant element in the universe.
- Green and red colors in the aurora borealis (and australis) are caused by oxygen atoms.
- Highly energetic electrons from the solar wind split oxygen molecules high in earth’s atmosphere into excited (high energy) atoms.
- These atoms lose energy by emitting photons, producing awe-inspiring light shows.
- These are usually polar displays, because solar electrons accelerate along our planet’s magnetic field lines until they hit the atmosphere in the polar regions.
Appearance and Characteristics
Harmful effects:
- O2 is non-toxic under normal conditions.
- However, exposure to oxygen at higher than normal pressures, e.g. scuba divers, can lead to convulsions.
- Ozone (O3) is toxic and if inhaled can damage the lungs.
Characteristics:
- Oxygen in its common form (O2) is a colorless, odorless and tasteless diatomic gas.
- Oxygen is extremely reactive and forms oxides with nearly all other elements except noble gases.
- Oxygen dissolves more readily in cold water than warm water. As a result of this, our planet’s cool, polar oceans are more dense with life than the warmer, tropical oceans.
- Liquid and solid oxygen are pale blue and are strongly paramagnetic.
- Ozone (O3), another form (allotrope) of oxygen, occurs naturally in the Earth’s upper atmosphere. It is made by the action of ultraviolet light on O2. Ozone shields us from much of the harmful ultraviolet radiation coming from the sun. In Earth’s early atmosphere, before oxygen and hence ozone levels were sufficiently high, the ultraviolet radiation reaching our planet’s surface would have been lethal to many organisms.
- The reaction with oxygen is one of the criteria we use to distinguish between metals (these form basic oxides) and non-metals (these form acidic oxides).
Uses of Oxygen
- The major commercial use of oxygen is in steel production. Carbon impurities are removed from steel by reaction with oxygen to form carbon dioxide gas.
- Oxygen is also used in oxyacetylene welding, as an oxidant for rocket fuel, and in methanol and ethylene oxide production.
- Plants and animals rely on oxygen for respiration.
- Pure oxygen is frequently used to help breathing in patients with respiratory ailments.
Liquid oxygen is pale blue and paramagnetic.
Laboratory electrolysis of water. Electrical energy is used to split water. Watch out for the different ways the two gases are collected.
