The elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po) make up Group 6A of the periodic table. From sulfur to polonium, they are chemically very similar. Oxygen, the only gas in the group, is exceptional in how readily it combines with other elements. Sulfur, selenium, and tellurium are brittle solids whose compounds tend to occur together in nature, whereas polonium is an unstable product of radioactive decay of uranium minerals. Sulfur is a yellow nonmetal; selenium and tellurium have some metallic properties—Te is classified as a metalloid. Polonium is a metal similar to lead—soft, dense, and with a low melting point.
This colorless, odorless gas is the most abundant element in the earth’s crust, making up almost half of it by weight. It also makes up 23 per cent by weight of the atmosphere and 89 per cent by weight of the earth s water.
Oxygen was discovered in 1774 independently by two chemists, Karl Scheele (1742-1786) in Sweden and Joseph Priestly (1733-1804) in England. They obtained the gas by heating mercuric oxide. But it was the French chemist Antoine Lavoisier (1743-1794) who correctly explained the role of oxygen in combustion. He also coined the name oxygen from Greek words meaning acid-maker. He wrongly believed that all acids contain oxygen. Its atomic number is 8, and its atomic mass is 15.9994. Its melting point is -218.4° C, and its boiling point, -182.962° C
Compounds of other elements and oxygen are known as oxides. Atoms of oxygen form oxides by taking up two electrons from atoms of other elements, thereby filling their outer shell and acquiring chemical stability.
Oxides can be classified into three main types; acidic, basic, and neutral. Acidic oxides react with water to form acids. Examples include carbon dioxide, which forms carbonic acid, and sulfur trioxide, which forms sulfuric acid. Basic oxides form bases with water. An example is sodium oxide, which forms sodium hydroxide. Neutral oxides, such as nitrous oxide and water, are neither acidic nor basic.
This solid, yellow nonmetallic element is tasteless and odorless and found in nature in pure form as two allotropes that differ mainly in crystallinity. In plant and animal tissue it occurs in the chemical structure of three amino acids.
Sulfur has been known since ancient times.
It was first classified as an element by the French chemist Antoine Lavoisier (1743-1794) in 1777. The name comes from the Latin sul-phurium, meaning brimstone. Its atomic number is 16, and its atomic mass is 32.06. Its melting point is 113-120° C, and its boiling point, 444.6° C
Two methods are used to produce sulfur commercially. The Frasch process is illustrated in the accompanying diagram. The Claus conversion process extracts sulfur from underground deposits of petroleum and natural gas. Most sulfur is used in making sulfuric acid (H2S04), a dense, oily, colorless, corrosive substance-one of the strongest acids known, and once the world’s most important commercial chemical. About 65 per cent of the sulfuric acid produced now in the United States is used in making fertilizers from phosphates and ammonia. About 5 per cent is used in refining petroleum, about 5 per cent in manufacturing metals, and about 5 per cent in producing chemicals. The acid is used in making dyes, paints, medicines, explosives, paper pulp, and textiles, and as the electrolyte in automobile batteries.
Sulfur dioxide (S02), produced by combustion of sulfur, is a colorless, toxic gas. Its sharp odor is common in the air of densely populated areas where oil refineries or factories burn sulfurous coal or oil. Dissolved in water droplets, it can form acid precipitation, which is harmful to vegetation and wildlife. Emission of sulfur dioxide into the atmosphere is now subject to U.S. government regulations.
Selenium, tellurium, and polonium
Selenium, although a nonmetal, conducts electricity when light shines on it This “photoelectric” property makes it useful in devices that use light to regulate the flow of electricity, such as TV cameras, copying equipment, and electric eyes. Selenium was discovered by the Swedish chemist Jons Berzelius (1779-1848).
He named it after Selene, the Greek word for the moon. Its atomic number is 34, and its atomic mass is 78.96. In its gray form its melting point is 217° C, and its boiling point, 684.9±1° C.
Tellurium, a metalloid obtained as a byproduct of copper refining, is used in refining petroleum, curing rubber, making certain alloys, and in the screens of TV picture tubes. Tellurium was extracted from Transylvanian ores in 1782 by the Austrian chemist Franz Muller von Reichenstein (1740-1825). He named it after the Latin tellus, meaning earth. Its atomic number is 52, and its atomic mass is 127.6. Its melting point is 449.8° C, and its boiling point, 989.9° C.
Polonium, a dangerously radioactive metal, occurs naturally as a decay product in uranium ore. Polonium itself decays to an isotope of lead. An isotope, 2,0Po, is used as a power source in space satellites. Polonium was discovered in pitchblende in 1898 by the French scientists Pierre Curie (1859-1906) and Marie Curie (1867-1934). It was named after Poland, Marie Curie’s homeland. Its atomic number is 84, and the atomic mass of its most stable isotope is 209.