Oxygen
Oxygen is the first element in Group 16 of the periodic table, a group of elements sometimes known as the oxygen family. Oxygen has an atomic number of 8, an atomic mass of 15.9994, and a chemical symbol of O.
Properties
Oxygen is a colorless, odorless, tasteless gas with a density of 1.429 grams per liter, slightly greater than that of air (1.29 grams per liter). Its boiling point is -297.33°F (-182.96°C) and its freezing point is -361.2°F (-218.4°C). Liquid oxygen has a slightly bluish color to it and is slightly magnetic.
Oxygen exists in three allotropic forms: diatomic oxygen, or dioxygen (O2), atomic or nascent oxygen (O), and ozone (O3). Atomic oxygen is a highly reactive species that forms diatomic oxygen readily and reacts with other elements and compounds easily. Ozone also tends to be unstable and break down into dioxygen and nascent oxygen and to react readily with other substances.
Ozone has a slightly bluish color as both a gas and a liquid. It has a boiling point of -169.4°F (-111.9°C), a freezing point of -315°F (-193°C), and a density of 2.144 grams per liter.
Oxygen is a relatively inactive substance at room temperature, as evidenced by its abundance in the atmosphere. It does react slowly with a number of elements and compounds, however. For example, it reacts slowly with many metals to form the metal oxide in a process sometimes known as rusting, and it reacts with many organic compounds in the process known as decay.
At elevated temperatures, however, oxygen becomes much more active. It reacts with a variety of substances in the process known as combustion. The combustion of wood and fossil fuels (coal, oil, and natural gas) is arguably one of the most important chemical processes in human society. The oxidation of any organic material, whether by decay or combustion, results in the formation of carbon dioxide (CO2) and water (H2O), as well as carbon monoxide (CO) and other products in many instances.
Occurrence and Extraction
Oxygen occurs in the Earth's atmosphere primarily as an element. It makes up 20.948% of the atmosphere, the second most abundant gas after nitrogen. Oxygen also occurs in the hydrosphere in the form of water, of which it makes up nearly 89% by weight, and in the Earth's crust. In the earth, it has an abundance of about 45%, making it by far the most abundant element in the crust. It occurs in all kinds of minerals, such as oxides, carbonates, nitrates, sulfates, and phosphates. Oxygen is produced commercially by the fractional distillation of liquid air.
Discovery and Naming
Oxygen was discovered almost simultaneously in about 1774 by the Swedish chemist Carl Wilhelm Scheele and the English chemist Joseph Priestley. Both chemists followed a similar approach in their research, heating compounds of oxygen until they broke down. In his classic experiment, for example, Priestley heated red oxide of mercury (mercuric oxide; HgO) and found that he obtained liquid mercury and a new gas: 2HgO --heat 2Hg + O2. Priestley carried out a number of tests on the new gas, including breathing it himself. He described the sensation by saying that the new gas, was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards. Who can tell but that, in time, this pure air may become a fashionable article in luxury? Hitherto, only two mice and myself have had the privilege of breathing it. Priestley's prediction came to pass at the end of the twentieth century with the introduction of "oxygen bars" at which patrons could, for a price, spend a few minutes breathing pure oxygen.
Oxygen was named by the French chemist Antoine-Laurent Lavoisier a few years after its discovery. Lavoisier thought that oxygen was present in all acids, so he suggested the name from two Greek words, oxy-, for "acidic," and -gen, for "forming." Lavoisier was wrong about the presence of oxygen in all acids, but the name was retained for the element.
Uses
The most important single application of oxygen is in metallurgy where it is used to extract metals from their ores. For example, oxygen is used to burn off carbon, silicon, and other impurities present in iron ore during the process of making steel. The carbon dioxide thus formed escapes into the air, while the silicon dioxide formed becomes part of a "slag" that is scraped off the molten steel produced in the reaction.
Oxygen is also used in the production of other metals, such as copper, lead, and zinc. These metals occurs in the earth in the form of sulfides, such as copper sulfide (CuS), lead sulfide (PbS), and zinc sulfide (ZnS). The first step in recovering these metals is to convert them to oxides:
2CuS + 3O2 2CuO + 2SO2
2PbS + 3O2 2PbO + 2SO2
2ZnS + 3O2 2ZnO + 2SO2
The oxides thus formed are then heated with carbon to make the pure metals:
2CuO + C 2Cu + CO2
2PbO + C 2Pb + CO2
2ZnO + C 2Zn + CO2
Another use of oxygen is in high-temperature torches. The oxyacetylene torch, for example, produces heat by burning acetylene gas (C2H2) in pure oxygen. The torch can produce temperatures of 5,400°F (3,000°C), hot enough to cut through steel and other tough alloys.
One of the best known applications of oxygen is in the health care field. Individuals sometimes develop respiratory conditions, such as bronchitis or emphysema, which make it very difficult for them to breathe. In such conditions, they may be provided with oxygen masks that make it easier for their lungs to obtain the oxygen needed by the body.
Oxygen is also used extensively in the chemical industry as the starting point in making a variety of compounds. Sometimes, the steps in getting from oxygen to the final compound are lengthy and complex. For example, ethylene gas (C2H4) can be treated with oxygen to form ethylene oxide (CH2H4O). Ethylene oxide, in turn, is used to produce ethylene glycol [CH2CH2(OH)2], which is an antifreeze as well as the starting point in the manufacture of polyester fibers, film, plastic containers, bags, packaging materials, and many other consumer products. Thousands of oxygen-containing compounds have important commercial uses. Many of these compounds are discussed under other elements in this book.
Health Issues
Nearly all organisms require oxygen for their survival. Most animals, as an example, can survive for weeks and even months without food and for many days without water. But they can not survive more than a few minutes without oxygen.
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