Thulium is a chemical element with the symbol Tm and atomic number 69. It is the thirteenth and third-last element in the lanthanide series. Like the other lanthanides, the most common oxidation state is +3, seen in its oxide, halides and other compounds; however, the +2 oxidation state can also be stable. In aqueous solution, like compounds of other late lanthanides, soluble thulium compounds form coordination complexes with nine water molecules. In 1879, the Swedish chemist Per Teodor Cleve separated from the rare earth oxide erbia another two previously unknown components, which he called holmia and thulia; these were the oxides of holmium and thulium, respectively. A relatively pure sample of thulium metal was first obtained in 1911. Thulium is the second-least abundant of the lanthanides, after radioactively unstable promethium which is only found in trace quantities on Earth. It is an easily workable metal with a bright silvery-gray luster. It is fairly soft and slowly tarnishes in air. Despite its high price and rarity, thulium is used as the radiation source in portable X-ray devices, and in some solid-state lasers. It has no significant biological role and is not particularly toxic. Properties Physical properties Pure thulium metal has a bright, silvery luster, which tarnishes on exposure to air. The metal can be cut with a knife, as it has a Mohs hardness of 2 to 3; it is malleable and ductile. Thulium is ferromagnetic below 32K, antiferromagnetic between 32 and 56K, and paramagnetic above 56K. Thulium has two major allotropes: the tetragonal α-Tm and the more stable hexagonal β-Tm. Chemical properties Thulium tarnishes slowly in air and burns readily at 150°C to form thulium(III) oxide: Thulium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form thulium hydroxide: Thulium reacts with all the halogens. Reactions are slow at room temperature, but are vigorous above 200°C: (white) (yellow) (white) (yellow) Thulium dissolves readily in dilute sulfuric acid to form solutions containing the pale green Tm(III) ions, which exist as complexes: Thulium reacts with various metallic and non-metallic elements forming a range of binary compounds, including , , , , , , , , , and . Like most lanthanides, the +3 state is most common and is the only state observed in thulium solutions. Thulium exists as a ion in solution. In this state, the thulium ion is surrounded by nine molecules of