228 | The History and Use of Our Earth’s Chemical Elements The lanthanides, sometimes referred to as “rare-earths,” are another important group of

228 | The History and Use of Our Earth’s Chemical Elements The lanthanides, sometimes referred to as “rare-earths,” are another important group of

oxides. Most exist in nature as their oxides. (See the section titled “Lanthanide Series.”) There are general formulas for other oxygen compounds. For example, alcohols can be generalized as R-OH, aldehydes as R-CHO, and carboxylic acids as R-COOH.

Hazards Although oxygen itself is not flammable or explosive, as is sometimes believed, its main

hazard is that, in high concentrations, oxygen can cause other materials to burn much more rapidly.

Oxygen is toxic and deadly to breathe when in a pure state at elevated pressures. In addi- tion, such pure oxygen promotes rapid combustion and can produce devastating fires, such as the fire that killed the Apollo 1 crew on a test launch pad in 1967. It spread rapidly because the pure oxygen was at normal pressure rather than the one-third pressure used during flight.

Oxygen used for therapeutic purposes in adults can cause convulsions if the concentration is too high. At one time, high levels of oxygen were given to premature infants to assist their breathing. It was soon discovered that a high concentration of O 2 caused blindness in some of the infants. This practice has been abandoned, or the oxygen levels have since been reduced, and this is no longer a medical problem.

Oxygen involved in metabolic processes are prone to form “free radicals,” which are thought to cause damage to cells and possibly be associated with cancer and aging.

Ozone (Also Group 16)

This section on ozone is included under oxygen in group 16 because of its importance today in the lives of citizens and its effect on the environment. It is treated as another element with its own properties and characteristics, uses, and hazards.

Properties Ozone is an allotropic molecular form of oxygen containing three atoms of oxygen (O 3 ).

It is a much more powerful oxidizing agent than diatomic oxygen (O 2 ) or monatomic oxygen (O). It is the second most powerful oxidizer of all the elements. Only fluorine is a stronger oxidizer. It is not colorless as is oxygen gas. Rather, ozone is bluish in the gaseous state, but blackish-blue in the liquid and solid states (similar to the color of ink).

Ozone’s boiling point is –112°C, and its freezing point is –192°C. Characteristics

Ozone has a very distinctive pungent odor. It exists in our lower atmosphere in very small trace amounts. In higher concentrations it is irritating and even poisonous. Ozone is in rela- tively low concentrations at sea level. In the upper atmosphere, where it is more concentrated, it absorbs ultraviolet radiation, which protects the Earth and us from excessive exposure to ultraviolet radiation.

Electrical discharges in the atmosphere produce small amounts of ozone. You can recog- nize the odor when running electrical equipment that gives off sparks. Even toy electric trains can produce ozone as they spark along the track. Ozone can be produced by passing dry air between two electrodes that are connected to alternating electric current with high voltage. Such a system is sometimes used to purify the air in buildings or provide ozone for commer- cial uses. Ozone is produced during the electrical discharges of lightning during storms. This is what makes the air seem so fresh after a thunderstorm or electrical storm. Besides being produced by electrical discharges, ozone is produced in the upper atmosphere or stratosphere

by ultraviolet (UV) radiation from the sun striking O 2 molecules, breaking them down and reforming them as O 3 molecules. The vast majority of ozone is produced in the atmosphere

230 | The History and Use of Our Earth’s Chemical Elements sunlight. Normal wind currents carry the ozone to the polar regions of the Earth where it is

thickest. Following are the chemical reactions for the production of ozone at ground level. Ozone can be formed when a mixture of O 2 and NO 2 is exposed to sunlight. Given that this reaction is very slow at normal temperatures, it is not a problem until hot gases in the cylinders of automobiles’ internal combustion engines cause the following more rapid reactions.

N 2 (gas) + O 2 (gas) → + heat 2NO (gas). The NO formed inside the automobile engine reacts spontaneously with O 2 to form NO 2 : for example, 2NO (gas) + O 2 (gas) → 2NO 2 (gas). This nitrogen dioxide is a reddish-brown color seen in smog that dissociates when exposed to strong sunlight: NO 2 (gas) + sunlight → NO (gas) + O (gas). In the final step, the monoat- omic oxygen form is very reactive and combines with O 2 to form ozone: O (gas) + O 2 (gas) →O 3 (ozone gas). On sunny days in high-traffic areas, the concentration of ozone can reach levels that are harmful to plants and animals.

History It was once believed that air was a single element, but by the fifteenth century ce, scientists

began to question whether it was possibly at least two separate gases. Leonardo da Vinci was one of the first to suggest the air consisted of at least two gases. He even determined that one of them would support life and fire.

In 1839 Christian Friedrich Schonbein (1799–1868) discovered a gas with an unusual odor coming from some electrical equipment. He did not know what it was, but because it had an odd smell, he called it “ozone,” after the Greek word for “I smell.” Although he knew that it was a chemical substance, he mistakenly associated ozone with the halogens (group 17). Others before Schonbein had smelled the gas but had not recognized its importance. Thomas Andrews (1813–1885) and several other scientists, through different experiments, identified ozone as a form of oxygen (an allotrope). It was not until 1868 that J. Louis Soret established the formula to be O 3.

Common฀Uses Ozone is much more reactive than O 2 , which makes it a very powerful oxidizing agent.

Only fluorine is more reactive. It has many commercial uses. It is a strong oxidizer, particularly of organic compounds, it is a strong bleaching agent for textiles, oils, and waxes, and it is a powerful germicide. It is also used in the manufacture of paper, steroid hormones, waxes, and cyanide and in the processing of acids.

Ozone produced by electrical discharge is used to purify drinking water and to treat industrial wastes and sewage. It is also use to deodorize air and kill bacteria by passing dry air through special ozone-producing electronic devices.

Hazards High concentrations of ozone are a fire and explosion hazard when in contact with any

organic substance that can be oxidized. In moderately high concentrations ozone is very toxic when inhaled, and in lesser concen- trations, it is irritating to the nose and eyes. Ozone in the lower atmosphere contributes to air pollution and smog. It can cause damage to rubber, plastics, and paints. These low concentra- tions can cause headaches, burning eyes, and respiratory irritation. It is particular harmful to

231 The U.S. EPA defines as “unhealthy” ambient air containing 125 parts per billion (ppb)

Guide to the Elements |

of ozone. (This means the ordinary air we breathe.) In different regions of the United States, ozone alerts are issued when the ozone level is both higher and lower than this standard. Most regions have their own standards that range from 100 ppb to 350 ppb.

Ozone฀Controversy The effects of natural and man-made chemicals on the ozone layer of the upper atmosphere

have been suspected and controversial for over 40 years. In 1974 F. Sherwood Rowland, from the University of California at Irving, and Mario Molina, a professor of environmental stud- ies at MIT suggested that the problem chemicals were chlorofluorocarbons (CFCs) used in refrigeration and as a propellant in spray cans. As early as the 1960s, several chemists suspected that the effects of CFCs, which are extremely stable on Earth, were negatively impacting the atmospheric ozone layer. They had experimental laboratory evidence of the chemical reactions involved, but no actual data that related to the atmospheric ozone layer. Since the mid-1970s, both the scientific facts and the arguments about potential problems have become ensnared in political and environmental ideologies.

The ozone issue, as well as the possibly related issue of global warming and other environ- mental concerns, is often replete with invectives due to a lack of understanding of both the science and the political arguments involved.

In 1913 Charles Fabry (1867–1945) identified large quantities of ozone in the upper atmosphere found at a range of about 10 to 30 miles above the Earth’s surface. This layer is call the ozonosphere, or more commonly, the ozone layer. The ozone gas layer of the ozono- sphere, although thin, is massive and covers most of the Earth at high altitudes. It is essential because it partially blocks and absorbs the UV rays of the sun, preventing them from reaching the Earth, particularly in the tropical and subtropical areas. Not all the UV is absorbed by the ozone, but a large portion of the stronger, shorter-wavelength rays, which are the most harmful to living organisms, are absorbed and do not reach the Earth. Some UV radiation is needed to activate vitamin D, but excessive ultraviolet light can cause skin cancer, cataracts, mutations, and death in plants and animal cells. One theory is that early in formation of life and in the following ages, the lack of an ozone layer permitted short UV rays to reach Earth, and this many have been partially responsible for the proliferation of the species, because of genetic mutations.