Thallium

"Thallium ( /ˈθæliəm/ thal-ee-əm) is a chemical element with symbol Tl and atomic number 81. This soft gray poor metal is not found free in nature. When isolated, it 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, thallos, meaning 'a green shoot or twig,' was named by Crookes. It was isolated by electrolysis a year later, by Lamy."

"Thallium tends to oxidize to the +3 and +1 oxidation states as ionic salts. The +3 state resembles that of the other elements in thallium's group (boron, aluminum, gallium, indium). However, the +1 state, which is far more prominent in thallium than the elements above it, recalls the chemistry of alkali metals, and 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."