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{{Infobox thallium}}
'''Thallium''' is at [[chemical element]], it has symbol '''Tl''' and [[atomic number]] 81. Its [[relative atomic mass|standard atomic weight]] is 204.4. It is found in [[Group 15]] of the periodic table. Thallium is a soft, heavy and gray metal, but can look red due to [[oxidation]]. Thallium and its compounds are extremely toxic, even more than [[cyanide]] and [[arsenic]].

==Properties==
===Physical properties===
Thallium is a soft, [[malleability|malleable]], grayish [[post-transition metal]]. It can be cut with a knife at [[room temperature]]. It melts at a low [[temperature]], 304&nbsp;°C., which is typical of a post-transition metal. Thallium has 25 known [[isotope]]s and two stable (non[[radioactive]]) ones. It is extremely [[toxic]].

===Chemical properties===
[[File:Thallium rod corroded.jpg|thumb|left|A corroded thallium rod]]
Thallium is a moderately reactive metal. It corrodes easily in air with a color similar to [[lead]]. If it is kept in air for a long time, a large amount of [[thallium(I) oxide]] will build up. It corrodes in the presence of water to make the hydroxide. It burns with a greenish flame. It reacts with most [[acid]]s.

===Chemical compounds===
Thallium makes [[chemical compound]]s in two [[oxidation state]]s: +1 and +3. The +1 state is more common and less reactive. Its chemical compounds are very similar to [[potassium compounds|potassium]] or [[silver compounds]]. It makes a [[thalliuM(I) hydroxide|hydroxide]] that in a strong [[base (chemistry)|base]] when dissolved in water. Most other [[transition metal]] and post-tranansition metal [[hydroxide]]s do not dissolve in water. This reacts with [[carbon dioxide]] to make [[thallium(I) carbonate]], which is also water-soluble and very heavy. It is the only heavy metal carbonate that can dissolve in water. Other compounds are similar to [[silver compounds]]. [[Thallium(I) bromide]] turns yellow when exposed to light, similar to [[silver(I) bromide]]. [[Thallium(I) sulfide]] is black, similar to [[silver(I) sulfide]]. The +3 state compounds are [[oxidizing agent]]s. The black oxide, [[thallium(III) oxide]] and the hydroxide, [[thallium(III) hydroxide]], are the only stable +3 compounds. They break down to oxygen and thallium(I) oxide when heated. Thallium and its compounds are rare because they are toxic and [[pollution|polluting]].
[[File:Thallium(I) chloride.jpg|thumb|left|Thallium(I) chloride]]

====+1 compounds====
+1 compounds are quite unreactive. It is the more common oxidation state. They are made when thallium dissolves in [[acid]]s or [[corrode]]s in air.
* [[Thallium(I) bromide]], yellowish solid, turns black when in light
* [[Thallium(I) carbonate]], colorless solid, dissolves in water
* [[Thallium(I) chloride]], colorless solid, does not dissolve in water
* [[Thallium(I) fluoride]], colorless solid, dissolves in water
* [[Thallium(I) hydroxide]], yellow solid, strong base, dissolves in water
* [[Thallium(I) iodide]], yellow or red solid, does not dissolve in water
* [[Thallium(I) oxide]], black solid, dissolves in water
* [[Thallium(I) sulfate]], colorless solid, dissolves in water
* [[Thallium(I) sulfide]], black solid, does not dissolve in water

====+3 compounds====
+3 compounds are [[oxidizing agent]]s. They are quite rare.
* [[Thallium(III) halide]], the thallium(III) fluoride, chloride, and bromide
* [[Thallium(III) hydroxide]], white solid
* [[Thallium(III) oxide]], white solid, [[semiconductor]]

==History==
Thallium was found by [[spectroscopy]] in 1861 by a bright green line in its [[spectrum]]. The main use for thallium, rat poison, was banned in many countries in the 1970s. Thallium was also used to poison people, similar to the more popular [[arsenic]].<ref>{{cite book|title = The Boron Elements: Boron, Aluminum, Gallium, Indium, Thallium| page = 14| first =Heather|last = Hasan|year = 2009| isbn = 9781435853331|publisher = Rosen Publishing Group}}</ref>

==Occurrence==
[[File:Hutchinsonite-169844.jpg|thumb|left|A mineral that has thallium in it]]
Thallium is found most in certain [[clay]]s and [[granite]]s. It cannot be gotten easily from these, though. Thallium is normally gotten from the waste after other [[ore]]s like [[galena]] are processed.<ref name="Vira">{{cite journal|doi = 10.1016/j.envint.2004.09.003|title = Thallium: a review of public health and environmental concerns|year = 2005|last1 = Peter|first1 = A|last2 = Viraraghavan|first2 = T|journal = Environment International|volume = 31|pages = 493–501|pmid = 15788190|issue = 4}}</ref> [[Hutchinsonite]] is another mineral that has thallium in it.

==Preparation==
When lead and [[zinc]] are taken from their [[ore]]s, many [[wikt:impurity|impurities]] are left behind. [[Sulfuric acid]] is used to dissolve the thallium from it as thallium(I) sulfate. Then the thallium(I) sulfate is [[electrolysis|electrolyzed]] to make thallium metal.

==Uses==
It is used in [[rodenticide|rat poisons]] and [[insecticide]]s. The use of thallium as a poison has been reduced or banned in many countries because these countries think that thallium might cause [[cancer]]s. It is also used in [[infrared detector]]s. It has been used in some [[murder]]s. Like [[arsenic]], the use of thallium in murders has given it the name "[[inheritance]] powder". Thallium compounds are used in glass for [[infrared]] light. Thallium was also used to kill skin infections, but it is too toxic to be used for that now. A [[superconductor]] that can work at higher temperatures than normal ones do uses thallium. A radioactive thallium isotope was used for nuclear scans. An alloy of thallium and [[mercury (element)|mercury]] has a low freezing temperature and is a liquid. A very dense solution of a thallium compound was used to test minerals for [[specific gravity]], but it is too toxic for use.

==Safety==
Thallium is extremely toxic, even touching it is dangerous. Many of its [[salt]]s easily dissolve. Some are colorless, tasteless, and odorless, but are very toxic. Some think that it is a [[carcinogen]]. Thallium can be a pollutant if the thallium waste from metal processing is washed away.

==References==
<references/>

{{Periodic Table}}

[[Category:Post-transition metals]]