Polonium conductivity

There are two procedures for doing this. The first makes use of a metal probe coated with an emitter such as polonium or Am (around 1 mCi) and placed above the surface. The resulting air ionization makes the gap between the probe and the liquid sufficiently conducting that the potential difference can be measured by means of a high-impedance dc voltmeter that serves as a null indicator in a standard potentiometer circuit. A submerged reference electrode may be a silver-silver chloride electrode. One generally compares the potential of the film-covered surface with that of the film-free one [83, 84]. 

Polonium is unique in being the only element known to crystallize in the simple cubic form (6 nearest neighbours at 335 pm). This a-form distorts at about 36° to a simple rhombohedral modification in which each Po also has 6 nearest neighbours at 335 pm. The precise temperature of the phase change is difficult to determine because of the self-heating of crystalline Po (p. 751) and it appears that both modifications can coexist from about 18° to 54°. Both are silvery-white metallic crystals with substantially higher electrical conductivity than Te. 

Polonium is more metallic in its properties than the elements above it in group 16. It is the only element in group 16 that is naturally radioactive. It is in a position on the periodic table of elements where it can be a metal, metalloid, or nonmetal. It is more often considered a metal because of its electrical conductivity decreases with an increase in temperature. 

In the fourth group, carbon and silicon are both non-metallic, while germanium has a very small electrical conductivity. It is only with white tin and lead that the electrical conductivity approaches the normal values for true metals. In the fifth group, arsenic and antimony are just on the limit between metallic and non-metallic properties, while of the elements of the sixth group, only polonium might be considered to have real metallic properties. The halogens, in the seventh group, show no trace of metallic properties. 

Curie, Pierre (1859 1906). French chemist, educated at the Sorbonne, conducted researches on piezoelectricity magnetism, and became professor of physics at the Sorbonne. Pierre is known expecially for his work with his wife, Maria, on radioactivity leading to their discovery of polonium radium for which they were awarded with A.H.Becquerel the 1003 Nobel... 

The air electrode consists of an insulated metal wire with its tip 1-2 mm above the liquid surface and with polonium deposited on the tip to make the air gap conducting. 

For measurement of the surface potential at an oil-water interface, the polonium electrode can be used, if of very large size, but must be placed in the air just above the thinnest possible layer of the oil. If the polonium electrode is placed in the oil, close to the water surface, no conductivity is obtained, and potential measurements have not been found possible. 

Elements along a rough diagonal from boron to polonium are intermediate in behavior, in some cases having both metallic and nonmetallic allotropes (elemental forms) these elements are designated as metalloids or semimetals. As described in Chapter 7, some elements, such as silicon and germanium, are capable of having their conductivity finely tuned by the addition of small amounts of impurities and are consequently of enormous importance in the manufacture of semiconductors in the computer industry. 

This method has been extensively used by Schulman and Rided and is indicated diagrammatically in Fig. 47. A small amount of radioactive material, commonly polonium, is mounted on or near the electrode. The a-particle radiation ionises the air between the electrode and the liquid, making it conducting. An electrometer can then be used to measure the potential difference across the gap. Normally, the potential difference is balanced against a standard cell with a potentiometer to obtain a null reading. The electrode E is usually silver-silver chloride, although others have been tried. The main requirement is freedom from appreciable drift during the duration of the experiment. There are references in the literature to the preparation of radioactive sources, but they are now readily available commercially. 

These elements are "halfway" between metals and nonmetals. Depending on the way they are treated, they can act as insulators like nonmetals or conduct electricity like metals. This makes several metalloids extremely important as semiconductors in computers and other electronic devices. The eight metalloid elements are boron, silicon, germanium, arsenic, antimony, tellurium, polonium, and astatine. 

The trend from nonmetalfic to metallic character upon going down the group is exemphfied by the conductivities of these elements. Sulfur is an insulator, selenium and tellurium are semiconductors, while the conductivity of polonium is typical of a true metal. In addition, the conductivities of sulfur, selenium, and tellurium increase with increasing temperature, behavior typical of nonmetals, whereas that of polonium increases at lower temperatures, typical of metals, see also Arsenic Boron Carbon Fullerenes Oxygen Ozone Phosphorus Selenium Sulfur Tin. 

The trend towards greater metallic character in the elements is complete at polonium. Whereas sulfur is a true insulator (specific resistivity in fiQ-cm = 2 x 1023), selenium (2X1011) and tellurium (2 x10s) are intermediate in their electrical conductivities, and the temperature coefficient of resistivity in all three cases is negative, which is usually considered characteristic of non-metals. Polonium in each of its two allotropes has a resistivity typical of true metals ( 43 juQ-cm) and a positive temperature coefficient. The low-temperature allotrope, which is stable up to about 100°, has a cubic structure, and the high temperature form is rhombohedral. In both forms the coordination number is six. 

Indications have been reported for metallic conduction in sulfur at high temperature at ultra high pressures ranging from 87 to 230 kb (9,10,11), The metallic conduction is believed to arise from a transition to a metallic structure, perhaps similar to that of polonium. However, there is some question as to whether this transition took place experimentally in the solid state (9, 12) as a result of the uncertainty in the location of the liquidus at ultra high pressures. The metallic sulfur state is characterized by a conductivity which decreases with increasing pressure and has electrical properties different from typical metals (II). 

Metalloid is a term for elements that are sort-of metals, and sort-of not metals. Sometimes this group of elements is referred to as semimetals. To be more precise, these elements exhibit some of the physical and chemical properties of metals. Generally metalloids have some electrical conductivity, but not nearly as much as true metals. Because of these ambiguous definitions, even which elements are called metalloids can vary. Usually boron, silicon, germanium, arsenic, antimony, and tellurium are included as metalloids sometimes polonium and astatine rarely selenium. 

The third place is held by France where fifteen elements were discovered chromium (1797), beryllium (1798), boron (1808), iodine (1811), bromine (1826), gallium (1875), samarium (1879), gadolinium (1886), dysprosium (1886), radium (1898), polonium (1898), actinium (1899), europium (1901), lutecium (1907), francium (1939). It is not surprising that the radioactive elements polonium, radium, and actinium were discovered by French scientists. These discoveries proceeded from the pioneering studies of radioactivity conducted in France. A brilliant spectral analyst P. Lecoq de Boisbaudran discovered by means of spectral analysis four new elements—gallium and three rare-earth elements (samarium, gadolinium, and dysprosium). Chromium and beryllium were discovered by L. Vauquelin who was such a skillful analytical chemist that it would be unjust if he had not given the world at least one new element. 

The elements on the periodic table can be classified as metals, nonmetals, metalloids, or noble gases. Metals tend to be shiny and have atoms that give up electrons. Metals are malleable and tend to be excellent conductors of heat and electricity. By nature, nonmetals do not conduct electricity and have atoms that do not naturally give up electrons however, they do tend to accept electrons. The metalloid elements are located along the heavy black stair-step line on the right-hand side of the periodic table. Boron, silicon, germanium, arsenic, antimony, tellurium, polonium, and astatine are classified as metalloids. 

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