Oxygen makes up one fifth of the atmosphere, with the remainder being mostly nitrogen. About 89% of the mass of the oceans is oxygen, and oxygen is the most abundant element in the Earth's crust accounting for almost half its total mass. In fact, most of the rocks on Earth are composed of oxygen-rich silicates, such as forsterite (Mg2SiO4), pyroxene (Fe2SiO4) and olivine (MgFeSiO4). Although nowadays, oxygen is abundant in the atmosphere, this was not always true. The atmosphere of the newly-formed earth contained mainly carbon dioxide and sulphur compounds. Oxygen only made an appearance when the first single-celled lifeforms evolved. These blue-green bacteria (prokaryotes) split water into hydrogen and oxygen (discarding the oxygen), and also liberate oxygen from CO2 in the process of making carbohydrates. Thus the oxygen in the atmosphere was originally a waste product - a pollutant - of the primeval planet. The vast quantities of O2 that were released by these photosynthetic processes oxidised the iron in the seas and, in effect, the Earth rusted! We can date this event by the great deposits of red iron ore that are found deep underground in certain regions of the world.
Oxygen is a colourless, odourless, tasteless gas composed of the simple diatomic molecule, O2. It condenses at -183°C to a pale blue liquid, that is unusual in that it is magnetic. Indeed, a magnet has the ability to pick up liquid oxygen. The gas is also magnetic, and this property can be used as a means to detect how much O2 is present in gas samples - for example in incubators for premature babies.
Oxygen is highly reactive, and O2 reacts with all the other elements (except the halogens, a few noble metals and the noble gases) either at room temperature or on heating. Passing an electric discharge through oxygen or the action of ultraviolet light produces ozone, O3, which is responsible for the bracing effect of seaside air.
Oxygen is vital for respiration in most living organisms. Biological processes within a body, such as protein construction or muscle contraction, require energy, and this is obtained by oxidizing food products, using the molecular oxygen that has been breathed in (animals) or absorbed (plants), and converting them into adenosine triphosphate (ATP). The by-product of this process is carbon dioxide CO2, which is expelled (exhaled). In animals, the oxygen is carried around the body using hemoglobin, a protein found in red blood cells. The vital part of hemoglobin is the heme group. This is a porphyrin ring with an iron atom at the centre, which has the property of loosely binding O2. The O2 attaches to the hemoglobin, and travels in the blood from the lungs to where it is required, such as muscles and tissues. The loosely bound O2 is then released to the waiting cells, and replaced by the waste-product, CO2, which is then transported back to the lungs and exhaled. The fact that the O2-heme bond is weak allows these reversible reactions to take place easily. It also explains why a similar gas, carbon monoxide CO, is toxic when inhaled, because it forms a very strong bond with heme and once attached, cannot be dislodged. In effect, CO 'uses up' all the available hemoglobin, leaving none left to carry oxygen, so the animal dies from asphyxiation.
Oxygen is produced during photosynthesis, the process whereby plants turn carbon dioxide and water into carbohydrates. This involves the use of a receptor molecule called chlorophyll, which traps the energy in sunlight and uses it to split water molecules to produce O2, which is then excreted from the plant as a waste product.
Oxygen is important for all combustion process, such as the burning of the hydrocarbon fuels (oil, coal, petrol, natural gas) which heat our homes and power our cars. Fires need O2 to burn, and removal of O2, by for example smothering or spraying with CO2, is one way to extinguish fires. Welding, using oxy-acetylene torches, is another important industrial application, whereby acetylene gas (the fuel) and oxygen are mixed in the correct proportions and ignited to provide an intensely hot flame. In the steelmaking industry huge quantities of O2 are blown through the impure molten ore, where it burns off any impurities that are present (particularly carbon). About 1 tonne of O2 is required for every tonne of finished steel!