Editing Thallium (section)

==Characteristics==
A thallium atom has 81 electrons, arranged in the electron configuration [Xe]4f<sup>14</sup>5d<sup>10</sup>6s<sup>2</sup>6p<sup>1</sup>; of these, the three outermost electrons in the sixth shell are valence electrons. Due to the [[inert pair effect]], the 6s electron pair is relatively stabilized and is more inert than in other heavier elements. Thus, very few electrons are available for metallic bonding, similar to the neighboring elements [[mercury (element)|mercury]] and [[lead]]. Thallium, then, like its congeners, is a soft, highly electrically conducting metal with a low melting point, of 304&nbsp;°C.<ref name="Greenwood222">Greenwood and Earnshaw, pp. 222–224</ref>

A number of standard electrode potentials, depending on the reaction under study,<ref>{{RubberBible92nd|page=8.20}}</ref> are reported for thallium, reflecting the greatly decreased stability of the +3 oxidation state:<ref name="Greenwood222" />
{|
|-
| +0.73 ||Tl<sup>3+</sup> + 3 e<sup>−</sup>|| ↔ Tl
|-
| −0.336 ||Tl<sup>+</sup> + e<sup>−</sup>|| ↔ Tl
|}

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.<ref name="Greenwood222" /> 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.<ref name="Greenwood224">Greenwood and Earnshaw, pp. 224–7</ref> 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.<ref name="Greenwood224" />

Thallium is [[Ductility|malleable]] and [[Sectility|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. In the presence of water, thallium [[hydroxide]] is formed. [[Sulfuric acid|Sulfuric]] and [[nitric acid]]s dissolve thallium rapidly to make the [[thallium(I) sulfate|sulfate]] and [[thallium(I) nitrate|nitrate]] salts, while [[hydrochloric acid]] forms an insoluble [[thallium(I) chloride]] layer.<ref name="HollemanAF">{{cite book|publisher = Walter de Gruyter|date = 1985|edition = 91–100|pages = 892–893|isbn = 978-3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|first = Arnold F.|last = Holleman|author2 = Wiberg, Egon|author3 = Wiberg, Nils|chapter =Thallium|language = de}}</ref>

===Isotopes===
{{main|Isotopes of thallium}}
Thallium has 41 [[isotope]]s which have [[atomic mass]]es that range from 176 to 216. <sup>203</sup>Tl and <sup>205</sup>Tl are the only [[stable isotope]]s and make up nearly all of natural thallium. The five short-lived isotopes <sup>206</sup>Tl through <sup>210</sup>Tl inclusive occur in nature, as they are part of the natural [[decay chain]]s of heavier elements. <sup>204</sup>Tl is the most stable [[radioisotope]], with a [[half-life]] of 3.78 years.<ref name="Audi">{{NUBASE 2003}}</ref> It is made by the [[neutron activation]] of stable thallium in a [[nuclear reactor]].<ref name="Audi" /><ref>{{cite web|url = http://www-pub.iaea.org/MTCD/publications/PDF/te_1340_web.pdf|title = Manual for reactor produced radioisotopes|publisher = [[International Atomic Energy Agency]]|date = 2003|access-date = 2010-05-13|archive-date = 2011-05-21|archive-url = https://web.archive.org/web/20110521072530/http://www-pub.iaea.org/MTCD/publications/PDF/te_1340_web.pdf|url-status = live}}</ref> The most useful radioisotope, <sup>201</sup>Tl (half-life 73 hours), decays by electron capture, emitting X-rays (~70–80&nbsp;keV), and photons of 135 and 167&nbsp;keV in 10% total abundance;<ref name="Audi" /> therefore, it has good imaging characteristics without an excessive patient-radiation dose. It is the most popular isotope used for thallium nuclear [[cardiac stress test]]s.<ref>{{cite book|chapter-url = https://books.google.com/books?id=CqQgnHrDxrUC&pg=PA173|chapter = Detection, Evaluation, and Risk Stratification of Coronary Artery Disease by Thallium-201 Myocardial Perfusion Scintigraphy 155|first1 = Jamshid|last1 = Maddahi|first2 = Daniel|last2 = Berman|title = Cardiac SPECT imaging|edition = 2nd|publisher = Lippincott Williams & Wilkins|date = 2001|isbn = 978-0-7817-2007-6|pages = 155–178|access-date = 2016-09-26|archive-date = 2017-02-22|archive-url = https://web.archive.org/web/20170222122246/https://books.google.com/books?id=CqQgnHrDxrUC&pg=PA173|url-status = live}}</ref>