Thallium is a chemical element; it has symbol Tl and atomic number 81. It is a silvery-white post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861, in residues of sulfuric acid production. Both used the newly developed method of flame spectroscopy, in which thallium produces a notable green spectral line. Thallium, from Greek θαλλός, thallós, meaning "green shoot" or "twig", was named by Crookes. It was isolated by both Lamy and Crookes in 1862, Lamy by electrolysis and Crookes by precipitation and melting of the resultant powder. Crookes exhibited it as a powder precipitated by zinc at the International Exhibition, which opened on 1 May that year. Thallium tends to form the +3 and +1 oxidation states. The +3 state resembles that of the other elements in group 13 (boron, aluminium, gallium, indium). However, the +1 state, which is far more prominent in thallium than the elements above it in the periodic table, recalls the chemistry of alkali metals. Thallium(I) ions are found geologically mostly in potassium-based ores and (when ingested) are handled in many ways like potassium ions (K+) by ion pumps in living cells. Commercially, thallium is produced not from potassium ores, but as a byproduct from refining of heavy-metal sulfide ores. Approximately 65% of thallium production is used in the electronics industry and the remainder is used in the pharmaceutical industry and in glass manufacturing. It is also used in infrared detectors. The radioisotope thallium-201 (as the soluble chloride TlCl) is used in small amounts as an agent in a nuclear medicine scan, during one type of nuclear cardiac stress test. Soluble thallium salts (many of which are nearly tasteless) are highly toxic and they were historically used in rat poisons and insecticides. Because of their nonselective toxicity, use of these compounds has been restricted or banned in many countries. Thallium poisoning usually results in hair loss. Because of its historic popularity as a murder weapon, thallium has gained notoriety as "the poisoner's poison" and "inheritance powder" (alongside arsenic).
Characteristics A thallium atom has 81 electrons, arranged in the electron configuration [Xe]4f145d106s26p1; of these, the three outermost electrons in the sixth shell are valence electrons. Due to the inert pair effect, the 6s electron pair is relativistically stabilised and it is more difficult to get these involved in chemical bonding than it is for the heavier elements. Thus, very few electrons are available for metallic bonding, similar to the neighboring elements mercury and lead. Thallium, then, like its congeners, is a soft, highly electrically conducting metal with a low melting point, of 304 °C. A number of standard electrode potentials, depending on the reaction under study, are reported for thallium, reflecting the greatly decreased stability of the +3 oxidation state:
Tl3+ + 3e− ⇌ Tl +0.73 V Tl+ + e− ⇌ Tl -0.336 V Thallium is the first element in group 13 where the reduction of the +3 oxidation state to the +1 oxidation state is spontaneous under standard conditions. Since bond energies decrease down the group, with thallium, the energy released in forming two additional bonds and attaining the +3 state is not always enough to outweigh the energy needed to involve the 6s-electrons. Accordingly, thallium(I) oxide and hydroxide are more basic and thallium(III) oxide and hydroxide are more acidic, showing that thallium conforms to the general rule of elements being more electropositive in their lower oxidation states. Thallium is malleable and sectile enough to be cut with a knife at room temperature. It has a metallic luster that, when exposed to air, quickly tarnishes to a bluish-gray tinge, resembling lead. It may be preserved by immersion in oil. A heavy layer of oxide builds up on thallium if left in air. The metal also reacts with water, forming thallium hydroxide. Sulfuric and nitric acids dissolve thallium rapidly to make the sulfate and nitrate salts, while hydrochloric acid forms an insoluble thallium(I) chloride layer.
Isotopes
Thallium has 41 known isotopes with atomic masses from 176 to 216. 203Tl and 205Tl are the only stable isotopes and make up all natural thallium. The five short-lived isotopes 206Tl through 210Tl inclusive occur in nature, but only as part of the natural decay chains of heavier elements. 204Tl is the most stable radioisotope, with a half-life of 3.78 years; the next most stable are 202Tl (half-life 12.31 days) and 201Tl (half-life 3.0421 days). It is made by the neutron activation of stable thallium in a nuclear reactor. The isotope 201Tl is useful in nuclear medicine; it decays by electron capture, emitting X-rays (~70–80 keV), and gamma rays of 135 and 167 keV; therefore, it has good imaging characteristics without an excessive patient-radiation dose. It is the most popular isotope used for thallium nuclear cardiac stress tests.
Compounds
Thallium(III) Thallium(III) compounds resemble the corresponding aluminium(III) compounds. They are moderately strong oxidizing agents and are usually unstable, as illustrated by the positive reduction potential for the Tl3+/Tl couple. Some mixed-valence compounds are also known, such as Tl4O3 and TlCl2, which contain both thallium(I) and thallium(III). Thallium(III) oxide, Tl2O3, is a black solid which decomposes above 800 °C, forming the thallium(I) oxide and oxygen. The simplest possible thallium compound, thallane (TlH3), is too unstable to exist in bulk, both due to the instability of the +3 oxidation state as well as poor overlap of the valence 6s and 6p orbitals of thallium with the 1s orbital of hydrogen. The trihalides are more stable, although they are chemically distinct from those of the lighter group 13 elements and are still the least stable in the whole group. For instance, thallium(III) fluoride, TlF3, has the β−BiF3 structure rather than that of the lighter group 13 trifluorides, and does not form the TlF4− complex anion in aqueous solution. The trichloride and tribromide disproportionate just above room temperature to give the monohalides, and thallium triiodide contains the linear triiodide anion (I−3) and is actually a thallium(I) compound. Thallium(III) sesquichalcogenides do not exist.
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