What is Thallium?
{{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