Thulium was discovered in 1879. It is slowly tarnished in air but is more resistant to oxidation than other rare earth metals. Thulium is used as radiation source in portable X-ray devices.
History and Discovery
Thulium was discovered in 1879 by Per Teodor Cleve. He followed the same method of Carl Gustaf Mosander to find the impurities in the oxides of rare earth element. He found two substance one brown and other one is green in color and named the brown one holmium and green substance called thulia later called thulium after Thule, an Ancient Greek place name popular and associated with Scandinavia. Earlier its atomic symbol was Tu but it was changed to Tm [1]. Through repetition of bromate fractional crystallization Charles James manufactured first pure thulium in 1911. Commercially it was first prepared in late 1950s through ion exchange separation technology.
Thulium
| Periodic Table Classification | Group n/a Period 6 |
|---|---|
| State at 20C | Solid |
| Color | Silvery gray |
| Electron Configuration | [Xe] 4f13 6s2 |
| Electron Number | 69 |
| Proton Number | 69 |
| Electron Shell | 2, 8, 18, 31, 8, 2 |
| Density | 9.32 g.cm-3 at 20°C |
| Atomic number | 69 |
| Atomic Mass | 168.93 g.mol -1 |
| Electronegativity according to Pauling | 1.25 |
Occurrence
Thulium is second least abundant element of the lanthanide series after promethium. Its abundance in earth crust is about 0.5 mg/kg by weight and 50 parts per million by moles. It makes up to 0.5 parts per million of soil and 250 parts per quadrillion of seawater. Thulium is about 200 parts per trillion by weight in the solar system and 1 part per trillion by moles [2]. Naturally thulium is not present in pure form but it is present in small quantities in minerals along with other rare earth elements. Mostly it is present in minerals which contain yttrium and gadolinium, particularly it occurs in the mineral gadolinite. It is also found in the minerals monazite, xenotime and euxenite. The main deposits of thulium main deposits are present in China; however, it is also present in Australia, Brazil, Greenland, India, Tanzania and the United States.
Physical Characteristics
Thulium in pure form has silvery luster and is a bright metal. Thulium metal can be cut with knife and has ability of deformation. It has two allotropes: tetragonal α-Tm and the stable hexagonal β-Tm. It is ferromagnetic at temperature below 32 K and antiferromagnetic between 32 and 56 K. Thulium is paramagnetic above 56 K. Thulium has chemical symbol Tm with atomic number 69. Its atomic weight is 168.934. Thulium melting point is 1545OC and boiling point is 1950OC. Its density is very high and at room temperature it is about 9.32 g/cm3 [3].
Chemical Characteristics
Thulium is a reactive metal. When exposed to air and water it is tarnished slowly. It speedily burns at 150OC and form thulium (lll) oxide. It is electropositive and react slowly with cold water and quickly with hot water and formed thulium hydroxide. It also react slowly with halogens at room temperature but vigorously above 200OC to form TmF3 (white), TmCl3 (yellow), TmBr3 (white) and TmI3 (yellow). Thulium dissolves in dilute sulfuric acid and form pale green Tm (lll) ions. It reacts with metallic and non-metallic elements and formed binary compounds like: TmN, TmS, TmC2, TmH2, TmH3, TmSi2, TmB4. Thulium mostly exist in +2 and +3 oxidation states. Tm3+ ions shows bright blue luminescence. Thulium oxide called ‘’thulia’’ (Tm2O3). It react with hydrogen chloride and produce hydrogen gas and thulium chloride. It also reacts with nitric acid and form thulium nitrate [4].
Significance and Uses
- Thulium is used in portable X-rays device that has been bombarded in a nuclear reactor as a radiation source.
- Holmium- chromium-thulium doped in YAG (yttrium aluminum garnet) is an active laser medium. It has high efficiency.
- Thulium-170 is used for cancer treatment via brachytherapy (radiotherapy).
- Thulium-170 is also used as popular radioisotopes for use in industrial radiography.
- It is also used in high temperature super conductors like yttrium.
- It has potential to use in ferrites (ceramic material: mixing iron (lll) oxide with small portion of metallic mineral), ceramic magnetic material which are used in microwave equipment.
- Like scandium it is also used in arc lighting.
- Thulium is also used in euro banknotes against counterfeiting (fake or unauthorized replicas).
- Thulium- doped calcium sulfate is used in personal radiation dosimeters.
Health Effects
Thulium in the form of soluble salts are slightly toxic but insoluble are nontoxic. Thulium is present in the liver, kidneys and bones in high amount. Thulium in the form of dust and powder are toxic when inhaled or ingested. Thulium in powdered form can be explosive. Its radioactive isotopes can cause radiation poisoning.
Isotopes of Thulium
Thulium isotopes range in atomic masses from 145 to 179. Thulium-169 is considered the stable isotope. Thulium-171 is also longest lived isotope and has half-life of 1.92 years. Thulium-170 has 128.6 days. Other isotopes have half- lives of few minutes or may be less.
REFERENCES
[1].Eagleson, Mary (1994).Concise Encyclopedia Chemistry. Walter de Gruyter. p.1061.ISBN978-3-11-011451-5.
[2].Thulium. Chemicool.com. Retrieved on 2013-03-29.
[3].https://en.wikipedia.org/wiki/Thulium
[4].“Chemical reactions of Thulium”. Webelements. Retrieved2009-06-06.
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