OXYGEN Photo by: Molodec

Overview Oxygen is the first element in Group 16 (VIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. The elements in Group 16 are said to belong to the chalcogen family. Other elements in this group include sulfur, selenium, tellurium, and polonium. The name chalcogen comes from the Greek word chalkos, meaning "ore." The first two members of the family, oxygen and sulfur, are found in most ores. Oxygen is by far the most abundant element in the Earth's crust. Nearly half of all the atoms in the earth are oxygen atoms. Oxygen also makes up about one-fifth of the Earth's atmosphere. Nearly 90 percent of the weight of the oceans is due to oxygen. In addition, oxygen is thought to be the third most abundant element in the universe and in the solar system. The discovery of oxygen is usually credited to Swedish chemist Carl Wilhelm Scheele (1742-86) and English chemist Joseph Priestley (1733-1804). The two discovered oxygen at nearly the same time in 1774, working independently of each other. SYMBOL

0 ATOMIC NUMBER

8 ATOMIC MASS

15.9994 FAMILY

Group 16 (VIA)

Chalcogen PRONUNCIATION

OK-si-jun Oxygen is necessary for the survival of all animal life on Earth. Animals breathe in oxygen and breathe out carbon dioxide. One important use of oxygen is in medicine. People who have trouble breathing are given extra doses of oxygen. In many cases, this "extra oxygen" keeps people alive after they would otherwise have died. But oxygen has many commercial uses also. The most important use is in the manufacture of metals. More than half of the oxygen produced in the United States is used for this purpose. Oxygen usually ranks third in the list of chemicals produced in the United States each year. In 1996, about 668 billion cubic feet of oxygen was manufactured in the United States. The gas is prepared almost entirely from liquid air.

Discovery and naming What is air? Ancient peoples thought deeply about that question. And that should not be surprising. It is easy to see how essential air is to many processes. Objects cannot burn without air. Human life cannot survive without air. In fact, ancient peoples thought air must be an "element." But they used the word "element" differently than do modern scientists. To ancient people, an element was something that was very important and basic. Air fit that description, along with fire, water, and earth. They often thought of air as an element in the modern sense—that it was as simple a material as could be found. Yet, some early scholars believed otherwise. For example, some Chinese scholars, as early as the eighth century A.D., thought of air as having two parts. They called these parts the yin and yang of air. The properties of the Chinese yin and yang can be compared to the properties of oxygen and nitrogen. The first person in Western Europe to describe the "parts" of air was Italian artist and scientist Leonardo da Vinci (1452-1519). Leonardo pointed out that air is not entirely used up when something is burned in it. He said that air must consist, therefore, of two parts: one part that is consumed in burning and one part that is not. For many years, Leonardo's ideas were not very popular among scholars. One problem was that early chemists did not have very good equipment. It was difficult for them to collect samples of air and then to study it. In the early 1700s, chemists began to find out more about air, but in a somewhat roundabout way. For example, in 1771 and 1772, Scheele studied the effect of heat on a number of different compounds. In one experiment, he used silver carbonate (Ag 2 CO 3 ), mercury carbonate (HgCO 3 ), and magnesium nitrate (Mg(NO 3 ) 2 ). When he heated these compounds, he found that a gas was produced. He then studied the properties of that gas. He found that flames burned brightly in the gas. He also found that animals could live when placed in the gas. Without knowing it, Scheele had discovered oxygen. (See sidebar on Scheele in the chlorine entry in Volume 1.) About two years later, Priestley conducted similar experiments by heating mercury oxide (HgO) in a flame. The compound broke down, producing liquid mercury metal and a gas:

When Priestley tested the new gas, he found the same properties that Scheele had described. Priestley even tried breathing the new gas he had produced. His description of that experience has now become famous: Antoine-Laurent Lavoisier | French chemist A ntoine-Laurent Lavoisier (1743-94) is often called the father of modern chemistry. He has been given that title for a number of reasons. The most important reason is the explanation he discovered for the process of combustion (burning). Prior to Lavoisier's research, chemists thought that a burning object gave off a substance to the air. They called that substance phlogiston. When wood burned, for example, chemists said that phlogiston escaped from the wood to the air. Lavoisier showed that this idea was incorrect. When something burns, it actually combines with oxygen in the air. Combustion, Lavoisier said, is really just oxidation (the process by which something combines with oxygen). This discovery gave chemists a whole new way to look at chemical changes. The phlogiston theory gradually began to die out. Many of the ideas used in modern chemistry began to develop. No wonder Lavoisier is called the father of this revolution. Lavoisier led an unusually interesting life. He was an avid chemist who carried out many experiments. But he also had a regular job as a tax collector. His job was to visit homes and businesses and collect taxes. This did not make him a very popular man! Lavoisier also made some important enemies early in his life. One of these enemies was Jean-Paul Marat (1743-93). Marat thought of himself as a scientist and applied for membership in the French Academy of Scientists. Lavoisier voted against Marat's application. He said that Marat's research was not very good. Less than a decade later, Lavoisier had reason to regret that decision. Marat had become a leader in the French Revolution (1774-1815). He accused Lavoisier of plotting against the revolution. He also said that Lavoisier was carrying out dangerous secret experiments. These accusations were not true. But Marat was now a very powerful man. He was able to have Lavoisier convicted of the charges against him. On May 8, 1794, Lavoisier was beheaded and buried in an unmarked grave. Some people have said that Lavoisier's death was the worst single consequence of the French Revolution. The feeling of it [the new gas, oxygen] to my lungs 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. Some people think Scheele should get credit for discovering oxygen. He completed his experiments earlier than did Priestley. But his publisher was very slow in printing Scheele's reports. They actually came out after Priestley's reports. So most historians agree that Scheele and Priestly should share credit for discovering oxygen. Neither Scheele nor Priestley fully understood the importance of their discovery. That step was taken by French chemist Antoine-Laurent Lavoisier (1743-94). Lavoisier was the first person to declare that the new gas was an element. He was also the first person to explain how oxygen is involved in burning. In addition, he suggested a name for the gas. That name, oxygen, comes from Greek words that mean "acidic" (oxy-) and "forming" (-gen). Lavoisier chose the name because he thought that all acids contain oxygen. Therefore, the new element was responsible for "forming acids." In this one respect, however, Lavoisier was wrong. All acids do not contain oxygen, although some do.

Physical properties Oxygen is a colorless, odorless, tasteless gas. It changes from a gas to a liquid at a temperature of -182.96°C (-297.33°F). The liquid formed has a slightly bluish color to it. Liquid oxygen can then be solidified or frozen at a temperature of -218.4°C (-361.2°F). The density of oxygen is 1.429 grams per liter. By comparison, the density of air is about 1.29 grams per liter. Oxygen exists in three allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The three allotropes of oxygen are normal oxygen, or diatomic oxygen, or dioxygen; nascent, atomic, or monatomic oxygen; and ozone, or triatomic oxygen. The three allotropes differ from each other in a number of ways. First, they differ on the simplest level of atoms and molecules. The oxygen that we are most familiar with in the atmosphere has two atoms in every molecule. Chemists show this by writing the formula as O 2 . The small "2" means "two atoms per molecule." By comparison, nascent oxygen has only one atom per molecule. The formula is simply O, or sometimes (O). The parentheses indicate that nascent oxygen does not exist very long under normal conditions. It has a tendency to form dioxygen:

That is, dioxygen is the normal condition of oxygen at room temperature. The third allotrope of oxygen, ozone, has three atoms in each molecule. The chemical formula is O 3 . Like nascent oxygen, ozone does not exist for very long under normal conditions. It tends to break down and form dioxygen:

Ozone does occur in fairly large amounts under special conditions. For example, there is an unusually large amount of ozone in the Earth's upper atmosphere. That ozone layer is important to life on Earth. It shields out harmful radiation that comes from the Sun. Ozone is also sometimes found closer to the Earth's surface. I t is produced when gasoline is burned in cars and trucks. It is part of the condition known as air pollution. Ozone at ground level is not helpful to life, and may cause health problems for plants, humans, and other animals. The physical properties of ozone are somewhat different from those of dioxygen. It has a slightly bluish color as both a gas and a liquid. It changes to a liquid at a temperature of -111.9°C (-169.4°F) and from a liquid to a solid at -193°C (-135°F) . The density is 2.144 grams per liter.

Chemical properties Oxygen's most important chemical property is that it supports combustion. That is, it helps other objects to burn. The combustion (burning) of charcoal is an example. Charcoal is nearly pure carbon (C):

Oxygen also combines with elements at room temperature. Rusting is an example. Rusting is a process by which a metal combines with oxygen. When iron rusts, it combines with oxygen:

Oxygen also reacts with many compounds. Decay is an example. Decay is the process by which once-living material combines with oxygen. The products of decay are mainly carbon dioxide (CO 2 ) and water (H 2 O):



(The chemical formula for "dead matter" is too complicated to use here.) Oxygen itself does not burn. A lighted match in a container of pure oxygen burns much brighter, but the oxygen does not catch fire.

Occurrence in nature Oxygen occurs mainly as an element in the atmosphere. It makes up 20.948 percent of the atmosphere. It also occurs in oceans, lakes, rivers, and ice caps in the form of water. Nearly 89 percent of the weight of water is oxygen. Oxygen is also the most abundant element in the Earth's crust. Its abundance is estimated at about 45 percent in the earth. That makes it almost twice as abundant as the next most common element, silicon. Oxygen occurs in all kinds of minerals. Some common examples include the oxides, carbonates, nitrates, sulfates, and phosphates. Oxides are chemical compounds that contain oxygen and one other element. Calcium oxide, or lime or quicklime (CaO), is an example. Carbonates are compounds that contain oxygen, carbon, and at least one other element. Sodium carbonate, or soda, soda ash, or sal soda (Na 2 CO 3 ), is an example. It is often found in detergents and cleaning products. Nitrates, sulfates, and phosphates also contain oxygen and other elements. The other elements in these compounds are nitrogen, sulfur, or phosphorus plus one other element. Examples of these compounds are potassium nitrate, or saltpeter (KNO 3 ); magnesium sulfate, or Epsom salts (MgSO 4 ); and calcium phosphate (Ca 3 (PO 4 ) 2 ).

Isotopes There are three naturally occurring isotope of oxygen: oxygen-16, oxygen-17, and oxygen-18. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope. Five radioactive isotopes of oxygen are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive. None of the radioactive isotopes of oxygen has any commercial use.

Extraction Oxygen is made from liquid air. Liquid air is made by cooling normal atmospheric air to very low temperatures. As the temperature drops, the gases contained in air turn into liquids. At -182.96°C (-297.33°F), oxygen changes from a gas into a liquid. At -195.79°C (-320.42°F), nitrogen changes from a gas into a liquid. And so on. Eventually, all the gases in air can be made to liquefy (change into a liquid). But the reverse process also takes place. Suppose liquid air in a container warms up slowly. When its temperature reaches -195.79°C, liquid nitrogen changes back to a gas. A container can be put into place to catch the nitrogen as it boils off the liquid air. When the temperature reaches -182.96°C, oxygen changes from a liquid back to a gas. Another container can be put into place. The escaping oxygen can be collected. Oxygen with a purity of 99.995 percent can be made by this method. It is the only method by which oxygen is made for commercial purposes.

Uses Many people are familiar with oxygen to help preserve lives. In some cases, people are not able to breathe on their own. Conditions such as emphysema damage the lungs. Oxygen cannot pass through the lungs into the blood stream. One way to treat this condition is to force oxygen into the lungs with a pump. The same method is used to treat other medical conditions. For example, carbon monoxide poisoning occurs when carbon monoxide gas gets into the blood stream. Auto exhaust, poorly maintained oil furnaces, and wood fires produce carbon monoxide. The carbon monoxide replaces oxygen in the blood. Cells get carbon monoxide instead of oxygen. But they cannot use carbon monoxide, so they begin to die. Forcing oxygen into the blood can reverse some of the damage. In high enough amounts, it can force the carbon monoxide out of the blood and cells can recover. Oxygen has other interesting uses. For example, it is used in rocket fuels. It is combined with hydrogen in the rocket engines. When hydrogen and oxygen combine, they give off very large amounts of energy. The energy is used to lift the rocket into space. Metal production accounts for the greatest percentage of oxygen use. For example, oxygen is used to burn off carbon and other impurities that are in iron to make steel. A small amount of these impurities may be desirable in steel, but too much makes it brittle and unusable. The carbon and other impurities are burned off in steel-making by blasting oxygen through molten iron. Two chemical changes that take place during steel-making are shown below:



The carbon dioxide escapes from the steel-making furnace as a gas. The silicon dioxide (SiO 2 ) forms slag. Slag is a crusty, metallic material that is scraped off after the steel is produced. Other impurities removed by a blast of oxygen are sulfur, phosphorus, manganese, and other metals. Oxygen is also used in the production of such metals as copper, lead, and zinc. These metals occur 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:





The oxides are then heated with carbon to make the pure metals:





Another use of oxygen is in high-temperature torches. The oxy-acetylene torch, for example, produces heat by burning acetylene gas (C 2 H 2 ) in pure oxygen. The torch can produce temperatures of 3,000°C (5,400°F) and cut through steel and other tough alloys. Oxygen is also used in the chemical industry as a beginning material in making some very important compounds. Sometimes, the steps to get from oxygen to the final compound are lengthy. As an example, ethylene gas (C 2 H 4 ) can be treated with oxygen to form ethylene oxide (CH 2 CH 2 O):

About 60 percent of ethylene oxide produced is made into ethylene glycol (CH 2 CH 2 (OH) 2 ). Ethylene glycol is used in antifreeze and as a starting point in making polyester fibers, film, plastic containers, bags, and packaging materials.

Compounds Thousands of oxygen compounds have important commercial uses. Many of these compounds are discussed under other elements. Coming soon: An oxygen bar near you! Joseph Priestley was correct! He predicted that "taking a whiff" of oxygen might someday be a luxury for people. For professional athletes, it already is a common practice. Football players who have run long yardage may inhale some oxygen. It helps them get their energy back. But now, "taking a whiff" has become even more popular. Oxygen bars are open in Japan, the United States, and other parts of the world. People come to breathe pure oxygen for a few minutes. Of course, they pay a fee for the privilege. Patrons say that pure oxygen makes them feel better and think more clearly. Others think it improves their looks. Owners of oxygen bars say that city air is often too polluted. They encourage people to "take a whiff" of pure oxygen for their health. For more information about oxygen bars, contact the National Oxygen Bar Association, 104 North Willow Plaza, Broomfield, CO, 80020; telephone (303) 464-1744; or visit the following website: noba@o2bars.com.

Health effects Nearly all organisms require oxygen—bacteria, plants, and animals. Humans, for example, can go weeks and even months without food. They can survive for many days without water. But they cannot survive more than a few minutes without oxygen. Oxygen is used by the cells of animal bodies. It is used to "burn" chemicals and produce energy that cells need to stay alive. Without oxygen, cells begin to die in minutes.