Contents
Introduction
Properties of Arsenic
Compounds and Minerals
Uses of Arsenic
References
Introduction
Arsenic, symbol As, is a rather rare element that by chance has become well known. It is notorious as a poison, but there is more to it than that. Its abundance in the crust of the earth is estimated as 5 x 10-4%, not much more than that of a rare earth metal, and only four times more than that of platinum. However, it very frequently accompanies ores of gold, silver, lead and copper and so becomes an unavoidable by-product. The Boliden gold-copper mine in Sweden from a ton of ore produced 1 oz. gold, 2 oz. silver, 20 lb of copper, but 180 lb of arsenic! This one mine tended to glut the supply of arsenic in the first half of the 20th century. All arsenic comes as a by-product, and ores are not mined specifically for arsenic.
Arsenic is worth studying because of its curious uses, and as an example of an element that stands with one foot in the metals camp, and one foot in the nonmetals camp. Actually, it is erroneous to call an element a "metal" or a "nonmetal" as if this were some inbred quality. An atom is just a certain number of electrons neutralizing the electric charge of a nucleus, and all its properties are only those of the structures in which it takes part. There are no qualities in the atom itself. This is strikingly true in the case of arsenic.
Properties of Arsenic
Arsenic has atomic number 33, so it has 33 electrons in the configuration 1s2 2s22p6 3s23p23d10 4s24p3. The same outer shell s2p3 is shared by N, P, Sb and Bi, the two lighter elements above it in the periodic table, and the two heavier elements below. Of these, N and P are evident nonmetals, while Sb and Bi are definitely metals, with metallic lustre and conducting electricity. As falls between, and cannot be classified so simply. The outer shell suggests valences of 3 and 5 for arsenic, and indeed these are observed. The first ionization potential of As is 10.5 V, which is quite high for a metal, but rather low for a nonmetal. The atomic radius of As is 1.25 ?, just a little larger than that of S, 1.06 ?. The single-bond covalent radius is also given as 1.21 ?, and the nonbonding packing radius as 2.0 ?. The only stable isotope has mass number 75. The atomic weight of arsenic is 74.9216 (C12 = 12.0000).
Arsenic by itself likes to form the tetrahedral molecule As4, in which each As is bound to the other three covalently (by sharing electrons). Phosphorus does exactly the same thing. As4 is a yellowish gas that condenses to a yellow, waxy solid with density about 2.0 g/cc. The gas has a characteristic garlic-like odor. This is very much like white phosphorus. However, white phosphorus is reasonably stable, while yellow arsenic is not. In the presence of light, yellow or γ-As rearranges itself to structures of higher density and darker color, such as brown or black β-As of density 4.7. There may be several such structures, but all are nonmetallic, without lustre and not electrically conducting. The most stable form at room temperature, which usually arises spontaneously, is metallic α-As, with grey color and metallic lustre. The lustre soon tarnishes on exposure to air. It is electrically conducting, with resistivity 35 μΩ-cm. Its hardness is 3.5, coefficient of expansion 6.95 x 10-4 per ?C, and specific heat 0.0822 cal/g/?C. The substance is definitely a metal, but is brittle and does not melt, but sublimes directly into As4 at 615?C. It is more or less analogous to red phosphorus (which is not a metal, however). We see that arsenic can dress as a metal or a nonmetal, depending on how it aggregates, and how it aggregates depends on the balance between lowest energy and highest entropy (the minimum of the free energy U - TS).
Compounds and Minerals
The ancient world knew only two arsenic compounds, but was, of course, unaware of the connection. These were the sulphides realgar, As2S2, and orpiment, As2S3. Realgar was then called sandarach, Greek sandarakh, sandaRAke. It was a soft, light, translucent substance of a deep red-orange colour that was ground and used as a pigment. The other was a soft, light, translucent substance of a fine yellow colour, also used as a pigment. In Greek, it was also called sandarach or arsenikon, "arsenikon." The Greek word arsen, "arsen," means "male," or by extension "strong." How this came to be associated with the compound is not known. In Latin, it was called auripigmentum, from which the French or-piment directly comes. Gaius Caesar (Caligula) engaged in a little alchemy to see if he couldn't coax the yellow color onto some heavy metal and get gold, but he was not successful. This was long before mystical alchemy was introduced from the Arabs, and should be classed as a chemical endeavour. The toxic nature of arsenic was unknown to the ancients.
References
F. X. M. Zippe, Geschichte der Metalle (Wien: Wilhelm Braum?ller, 1857). pp. 218-226.
J. L. Bray, Non-Ferrous Production Metallurgy, 2nd ed. (New York: John Wiley & Sons, 1947). pp. 83-88.
C. H. Hurlbut, Jr., Dana's Manual of Mineralogy, 16th ed. (New York: John Wiley & Sons, 1952). Native arsenic is on p. 180. Realgar and orpiment are on pp. 204-205. Arsenopyrite is on pp. 212-213.
W. N. Jones, Jr., Inorganic Chemistry (Philadelphia: Blakiston, 1949). pp. 485-494.
M. J. Sienko and R. A. Plane, Chemical Principles and Properties, 2nd ed. (New York: McGraw-Hill International Student Edition, 1974). pp. 609-629.
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