Physical Properties of Thallium

Thallium is a bluish-white metal. It is so soft that it can be scratched with the finger-nail and cut with a knife. When freshly cut it exhibits a bright metallic lustre. The metal is extremely malleable, but has little tenacity. Its structure is crystalline, and when a bar of thallium is bent it emits a sound resembling the " cry of tin." Thallium exists in two modifications which are enantiotropic; the transition-point is 226° C., and is lowered 6° for an increase of pressure of 3000 kilos, per sq. cm. Accordingly, as the temperature is raised through 226° a contraction in volume occurs, amounting to 0.000044 c.c. per gram. The density of thallium is 11.85 at 20° C., the coefficient of expansion (0° to 100°) is 0.00003135, and the compressibility coefficient is 2.33×10^-6 per atmosphere.

Thallium melts at 302° ± 1° C., the volume increasing by 3.1 per cent, during the change. The latent heat of fusion is 7.2 cals. per gram. In vacuo, thallium commences to volatilise at 174°, and boils at 818° under a pressure of 15 cm. of its own vapour. It is said to boil at 1515° under normal pressure; von Wartenberg, however, gives the following values for the vapour pressure of thallium : -

Temperature, °C	634°	783°	970°
Vapour pressure mm. of Hg	0.056	1.329	24.31

and estimates the boiling-point to be 1306°. The mean specific heat of thallium between 20° and 100° is given as 0.0325, 0.0336, and 0.0326 by Lamy, Regnault, and Schmitz respectively; between -188° and +20° the mean value is given as 0.0300 by Schmitz and 0.0296 by Richards and Jackson. The specific heat is 0.0277 at –177.3° and 0.0143 at -250.1° C. The electrical conductivity (σ) of thallium (in reciprocal ohms per cm. cube) at various temperatures is as follows : -

Temperature, °C	-180°	-80°	-40°	0°	+20°	+60°	+100°
σ×104	20.11	8.54	6.83	5.67	5.28	4.54	4.04

The specific refraction of thallium in its salts is (Gladstone and Dale's formula; H_α line) 0.106, and the atomic refraction is therefore 21.6. Thallium and its compounds are diamagnetic (Lamy), the magnetic susceptibility of thallium at the laboratory temperature being –2.9×10^-6 c.g.s. electromagnetic units per unit volume.

In appearance, hardness, elasticity, density, fusibility, specific heat, and electrical conductivity it will be noticed that thallium closely resembles lead.

The flame, arc, and spark spectra of thallium are characterised in the visible region by the green ray λ5350.7, the observation of which led to the discovery of the element by Crookes. The most intense lines in the arc and spark spectra are the following (Exner and Haschek): -

arc: 2709.33, 2767.96, 2918.42, 2921.66, 3229.89, 3519.37, 3529.53, 3775.89, 5350.70.

spark: 3519.37, 3529.53, 3775.89, 5350.70.

The most persistent lines in the spark spectrum, and therefore the lines that should be looked for when seeking traces of thallium, are (Exner and Haschek's wave-lengths) 2298.25, 2530.94, 2768.00,* 3091.88, 3519.35,* 3775.89,* and 5350.69,* those asterisked being the most sensitive.

Commercial thallium, when spectroscopically examined, is found to contain traces of lead, tin, copper, and aluminium.

Thallium does not unite with hydrogen. With each of the halogens it combines directly. When heated in oxygen, thallium is converted into thallous or thallic oxide, according to circumstances. Ozone readily attacks it at ordinary temperature, producing thallic oxide. Thallium oxidises in air, slowly at the ordinary temperature, rapidly at 100°. The oxidation also occurs under water containing dissolved air, so that thallium is best preserved under air-free water or glycerol. At a red heat, thallium decomposes water. Thallium combines directly with sulphur, selenium, tellurium, phosphorus, arsenic, and antimony, but not with carbon, silicon, and boron. It is unacted upon by carbon dioxide at 300°. Thallium is insoluble in liquid ammonia.

Thallium dissolves in mineral acids with the formation of thallous salts. The action is very slow with hydrochloric acid, quicker with sulphuric acid, and rapid with nitric acid. Except in the cases of nitric and concentrated sulphuric acids, hydrogen is evolved. Thallium reduces potassium permanganate at the ordinary temperature.

In accordance with its position in the electromotive series, thallium readily displaces gold, silver, mercury, copper, and lead from aqueous solutions of their salts.

Thallium is employed in the manufacture of certain kinds of optical glass of high refractive index, since the salts of thallium are characterised by their unusually high refracting power.

Thallium readily forms alloys with many other metals. The following inter-metallic compounds have been described, containing thallium and a metal of Group I. or II.: -

NaTl, m.p. 306°; Tl₃Mg₈, m.p. 413°; CaTl₃, no m.p.; KTl, m.p. 335°; TlMg₂, no m.p.; CaTl, m.p. 969°; Hg₂Tl, m.p. 15°; Tl₂Mg₃, no m.p.; Ca₃Tl₄, no m.p.;

Thallium is miscible with gold, silver, and cadmium, and with silver forms mixed crystals containing 0 to 10 per cent, of thallium. It is only partially miscible with copper and zinc, and is practically non-miscible with aluminium.

Thallium readily dissolves in mercury. A liquid amalgam containing 8.5 per cent, of thallium may be used in thermometers down to –60°.

Alloys of thallium and the metals of Groups IV. to VIII. in the Periodic Table are mentioned in the corresponding volumes of this series.