Physical Properties of Arsenic

Crystal Structure of Metallic Arsenic

Arsenic in the stable metallic form consists of light-greyish lustrous crystals possessing the symmetry of the ditrigonal scalenohedral class (dihexagonal alternating) of the rhombohedral system. This describes both the native and artificial crystals, although the former are usually indistinct. The axial ratio referred to hexagonal axes a:c is, according to Rose, 1:1.4025, and to von Zepharovich 1:1.4013; if the three edges of the unit rhombohedron which meet in the trigonal axis be taken, the angle θ (100):(010) is 94°56'; this corresponds to an angle γ 84° 36' between the three axes. The crystals, when obtained by sublimation, often twin parallel to the (110) face. Cleavage occurs most readily parallel to the (111) face, but it may occur parallel to the (110) face. Examination of an X-ray spectrograph of powdered metallic arsenic shows that the crystal structure consists of two interpenetrating rhombohedral space lattices of axial ratio 2.805, the length of edge of the rhomb being 4.145 A. This is represented in fig., the black circles indicating atoms lying in a single lattice. Regarded as a face-centred lattice the rhombohedron ABCDEF is the unit, the distance AB being 5.60 A. In each unit rhomb there are four atoms of the first lattice and four of the second lattice, the latter being shown as plain circles. The smaller and more acute rhombohedron AGHDKL is the unit cell of the structure, regarding it as being composed of two simple rhombohedral lattices. The points MNOS of the second lattice correspond to the points AGLR of the first, and A and M constitute a point pair. The general structure is similar to that of antimony and bismuth, but the edge of the unit rhomb, corresponding to AB above, in the case of antimony is 6.20 A. and of bismuth 6.56 A. The shorter interatomic distance of the atoms of the arsenic crystal is stated to be 2.51 A. Various values for the atomic radius of arsenic have been derived, namely: 1.26, 1.36, 1.37 and 1.16 A.

As already described, other crystalline forms of arsenic besides the rhombohedral are known or suspected to exist. The crystals of the yellow allotrope belong to the cubic system, while native arsenolamprite contains crystals belonging to the rhombic, or possibly to the monoclinic, system. Yellow arsenic is soluble in carbon disulphide.

Metallic arsenic is brittle and of hardness 3.5 (Mohs' scale), the Brinell number being147.0. The fracture is uneven and finely granular. The mean compressibility at 20° C. between 100 and 500 megabars is 4.5×10^-6. The specific heat has been determined over various ranges of temperature and also at specific temperatures. The results are given in the following table, together with the atomic heats at the specific temperatures; unless otherwise stated, the data refer to the metallic form.

The linear coefficient of thermal expansion at 40° C. is 5.59×10^-6, and the increase in unit length over the range 0° to 100° C. is 6.02×10^-4.

Temperature Range, ° C.	Mean Specific Heat.
21-66	0.0830
0-100	0.0822
-188-20	0.0705
21-65	0.0758amOrph
0-100	0.0840 grey

Temperature, °C.	Specific Heat.	Atomic Heat.
28	0.0772	5.78
-39	0.0736	5.51
-136	0.0619	4.64
-223 approx.	0.0258	1.93

According to Jannettaz, the thermal conductivity perpendicular to the (111) face of a crystal of metallic arsenic is nearly twice as great as it is parallel to the chief axis. Little gives the coefficient of thermal conductivity in absolute units at 20° C. to be 3.68×10⁶.

When heated under ordinary pressure, arsenic does not melt; the metallic form volatilises at a dull red heat, while amorphous arsenic does so at a lower temperature. Even at ordinary temperature the element possesses an appreciable vapour pressure, as may be shown by enclosing pieces of arsenic and silver in a vessel, but not in contact with each other; after some months the silver is found to be coated with a film of arsenide. Vapour pressure measurements of grey metallic arsenic and its liquid at temperatures up to 853° C. have been made; the results, expressed in atmospheres, are as follows:

Temp., °C.	Vapour Press. (Solid As).
450	0.026
500	0.076
550	0.222
604	0.785
615.5	0.997
658	2.392
697	4.85
741	9.7
772	16.9
790	22.3
815	33.6

Vapour Pressure of Metallic Arsenic and its Liquid.

By extrapolation of the vapour pressure curves Horiba showed the melting point to be 817° to 818° C. at the corresponding pressure of 35.8 atm. Johnston deduced the following boiling temperatures at low pressures -

p (mm. Hg)	10-3	10-2	10-1	1	10	50	100	760
B.pt., °C	220	260	310	360	430	490	510	610

and calculated the molar heat of vaporisation to be 30.5 Cal. The molar heat of sublimation was calculated by Horiba to be 33.6 Cal., the molar heat of fusion 22.4 Cal., and the molar heat of vaporisation of the liquid 11.2 Cal. The boiling point of arsenic was determined by Mott to be 616° C.

Much difficulty has been encountered in determining the melting point of arsenic. Landolt4 in 1859 by prolonged heating to dull redness in a sealed glass tube encased in one of iron obtained liquid globules, and by using similar methods Mallet decided that arsenic melted at a temperature between the melting points of antimony (629° C.) and silver (960° C.). Jonker stated that there was no sign of fusion at 800° C. In 1911, however, Jolibois succeeded in fusing arsenic in a quartz tube and by means of a thermocouple determined the melting point to be 850 ± 10° C. A number of subsequent determinations have been made, with the following results: 814.5 ± 0.5° C.; 817° C.; 818° C.; the two latter values agree with Horiba's value given above, but are somewhat lower than Laschtschenko's value, 822° C. From the crystal lattice data it has been deduced that liquid arsenic should expand during solidification by 5.1 per cent.

The vapour density of arsenic varies with the temperature. At about 600° C. it approximates to the value 10.38 required by As₄. Thus Deville and Troost obtained the values 10.6 at 563° C. and 10.2 at 720° C. As the temperature rises the vapour density decreases and approaches the value for As₂ (5.19); thus the following values have been obtained: 5.543 at 1714° C., 5.451 at 1736° C.; and also 5.4 at 1700° C. Preuner and Brockmoller devised a spiral quartz manometer which enabled them to obtain accurate readings of gas pressures at temperatures from 800° to 1200° C., and so obtained the isothermal curves of arsenic at intervals of 100° between these two temperatures. From their results they concluded that the vapour consists of a mixture of tetratomic, diatomic and monatomic molecules.

The refractive index of arsenic vapour for light of wavelength 5893 A. is 1.001550, and for 5460 A. 1.001580. Calthrop observed that the relation between the refractivity and the atomic volume does not correspond with the values obtained for nitrogen and phosphorus, and that whereas the curve obtained by plotting refractivity against atomic volume for elements of a given group is usually a straight line, the value for arsenic is much too low. This is shown by the following data, the estimates of atomic radii being those of Bragg:

	Atomic Radius (A.) r	Atomic Volume (cubic A.) 4/3πr3	Refractivity (μD - 1)×10-6	Refractivity/ Atomic Vol.
N	0.65	1.152	297.1	257.9
P	1.03	4.577	1212.0	264.8
As	1.26	8.383	1552.0	185.1

From the determination of the molecular refractions of a large number of organic compounds containing tervalent arsenic, the atomic refraction of arsenic in each compound has been calculated, the values obtained varying from 9.2 to 14.39. Hydrogen, chlorine and alkyl groups in an arsine exert about the same influence on the atomic refraction of arsenic, but replacement of any of these by aryl groups causes an increase in the atomic refraction. The opposite effect results from substitution by a cyanide, oxalate or alkoxyl radical.

Certain organic compounds containing arsenic which are optically active appear to owe their activity to an asymmetric tervalent arsenic atom.

When placed in the light from a mercury lamp, arsenic exhibits a photoelectric effect, emitting electrons; the longest effective wavelength is A 2360. At 1100° to 1150° C. a resonance series is excited in the vapour of arsenic by each of the mercury lines λλ 2483, 2536, 2654 and 2804; the fundamental frequency is apparently 410 cm.^-1, which gives as the distance As to As in the diatomic molecules 1.94 A., or 77 per cent, of the distance in crystalline arsenic.

Arsenic vapour incident on sodium chloride crystals gives a weak specular beam with much diffuse scattering. Arsenic is also diffusely reflected from crystals of fluorite or orthoclase. Arsenic layers on glass, of such thickness that 10 to 15 per cent, of incident light is transmitted at room temperature, are transparent at liquid air temperatures. Thick layers deposited at liquid air temperatures are black, but on warming become successively, in abrupt changes, deep red, bright yellow, and finally the usual steel grey of metallic arsenic.

The ionisation potential for electrons in arsenic vapour has been calculated to be 9.04 volts; the value previously accepted was 11.54 volts. The inelastic collision potential is 4.69 volts, and the resonance potential 4.7 volts. The electrical conductivity of metallic arsenic at 0° C. is 0.00285 mho. The yellow and amorphous forms do not conduct electricity appreciably. The specific resistance of grey arsenic has been determined at various temperatures as follows:

Temperature, ° C	Cold	190°	200°	220°	240°	255°
Resistance, ohms	∞	40,000	30,000	15,000	7000	4100

The effect on the resistivity of maintaining the element at 260° C. is shown in the following results:

After 20 minutes, resistance was 3400 ohms
After 70 minutes, resistance was 1000 ohms
After 90 minutes, resistance was 250 ohms
After 170 minutes, resistance was 11 ohms

According to Matthiessen and von Bose, the electrical resistance at any temperature 6 between 12° and 100° C. may be obtained from the equation

R_θ = ₀(R1 – 0.00389960 + 0.058879θ²)

The pressure coefficient of resistance of arsenic is negative. Little gives the specific resistance at 20° C. as 46,000 ohms and the temperature coefficient of resistance as –0.00435 per degree.

The electrical conductivity of a pure arsenic crystal has been measured at temperatures down to 2.42° Abs. The resistance-temperature curve is similar to those of pure metals. There is evidence of definite residual resistance being maintained at low temperatures, but arsenic does not exhibit the abnormally high residual resistance shown by bismuth, nor does it show superconductivity. The resistance is by no means proportional to the absolute temperature. It has been estimated that the electrical resistance of liquid arsenic at the melting point is about 0.4 of that of the solid phase.

The single potential of arsenic in various solutions has been determined. The electrodes used were made in various ways, but the most reliable results were obtained with a solid stick of arsenic; electrodes made by electroplating arsenic on other metals were unsatisfactory. The following combination was used -

Hg|HgCl in 0.5N KCl|Solution of Electrolyte|As

and taking -0.56 for the value of the calomel electrode, the following values were obtained with standard solutions of various electrolytes:

Single potentials of Arsenic in solutions of electrolytes

Electrolyte	Concentration	Volts
AsCl3	1 g. equiv. per l	-0.554
AsI3	1 g. equiv. per l	-0.540
NaCl saturated with As2O3	1 g. equiv. per l	-0.365
KCl saturated with As2O3	1 g. equiv. per l	-0.365
K3AsO4	0.5 g. equiv. per l	-0.381
Na3AsO4	0.5 g. equiv. per l	-0.381
Na3AsO3	0.5 g. equiv. per l	-0.054

Using the scale in which hydrogen is zero, the electrode potential of arsenic in contact with normal arsenious chloride is -0.27 volt. Using an electrode formed by plating arsenic on copper, Marquis made a series of potential measurements in an alcoholic solution of arsenious chloride and from his results concluded that arsenic should be placed between hydrogen and copper in the electromotive series; the nature of the electrolyte used, however, influences the relative positions of metals in the series, and arsenic is generally placed between bismuth and copper thus: Pb, H, Sb, Bi, As, Cu, Hg, Ag, Pd. Thus arsenic is able to replace copper, mercury and silver from solutions of their salts. The reaction which takes place in these cases may be represented thus -

6MX + 2As + 3H₂O = 6M + 6HX + As₂O₃

M being a univalent metal and X a univalent acid radical. Arsenic may similarly be replaced by metals, but the replacement cannot always be predicted from the position of arsenic in the electrochemical series, this power of replacement being a highly specific property which depends on the nature of the metal and the conditions of the experiment. Such reactions in liquid ammonia solutions show that in the following series an element will in general replace any subsequent element, if it is present in an analogous anion: I, S, Se, Te, As, Sb, Sn, Bi, Pb.

The overvoltage of hydrogen on an arsenic electrode has been determined by measuring the back e.m.f. or polarisation of a cell consisting of a platinum anode and an arsenic cathode in normal sulphuric acid. By the open-circuit method the value obtained was 0-379 volt, and by the closed-circuit method 0.478 volt. With 2N sulphuric acid the value at 25° C. is 0.369 ± 0.014 volt. According to Grube and Kleber, the electrolytic discharge of hydrogen ions at an arsenic cathode yields arsine as the primary product, and gaseous hydrogen is formed by decomposition of this hydride. Lloyd, however, studied the electrolysis of sulphuric, hydrochloric, phosphoric, oxalic and tartaric acids, using various concentrations and employing a cathode of polished compact arsenic, with the following conclusions: Initial polarisation at low c.d.'s yields a high overpotential which is not maintained at high c.d.'s. A maximum is reached so long as no arsine is produced; in the event of the presence of arsine there is a decrease in the overpotential. The percentage of arsine produced at a fixed c.d. is approximately the same for the different acids and does not greatly vary with the concentration.

The cathodic deposition of arsenic at the dropping mercury cathode has been studied and appears to be complex both in acid and in alkaline solutions; the polarisation curves do not show reversible shifts. From acid solutions the deposition of antimony or bismuth proceeds reversibly.

The thermal e.m.f. of arsenic against copper is + (7.91t + 0.051t²) microvolts between 0° and 170° C.; that of arsenic against lead is -13.56 microvolts.

Arsenic exhibits triboelectricity, becoming negatively charged when rubbed on glass under suitable conditions.

From measurements of the magnetic susceptibilities of a large number of arsenic compounds it is found that combined arsenic has two atomic susceptibilities, according to the degree of saturation of its compounds, and the results support the rule that the logarithm of the atomic susceptibility is a function of the atomic number in each natural family. This is the case for the quinquevalent group P, As, Sb, and for the tervalent group As, Sb, Bi.

From the results of an investigation of thermomagnetic and galvanomagnetic effects in arsenic, Little has recorded the following coefficients in absolute e.m.u. at 20° C.:

Specific resistance	4.60×104
Thermal conductivity	3.68×106
Peltier heat against lead	3.80×105
Thomson heat	3.33×103
Hall coefficient	4.52×10-2
Nernst coefficient	2.25×10-3
Ettingshausen coefficient	1.75×10-7
Righi-Leduc coefficient	4.15×10-7

None of the coefficients varies with the strength of the magnetic field. When a plate of arsenic was subjected to a temperature gradient of 10° per cm., a field of 8000 gauss caused a fall in temperature of 0.4°. The value of the coefficient provisionally defined by the equation

Temperature change = Coefficient×(Temperature gradient)²×(Field strength)²

is, for arsenic, -6.25×10^-10at 20° C.

Arsenic may be obtained in a radioactive state by bombardment with deuterons or neutrons. The activity is accompanied by the emission of γ-rays, β-particles and a few positrons.