Thorium is a slightly radioactive element discovered by Jons Jacob Berzelius in 1828. It is three times more abundant than Uranium and can meet the world’s energy demand by being used as fuel in nuclear reactors.
History and Discovery
In 1828, an amateur mineralogist named Morten Thrane Esmark found a black mineral on an island, in Norway. He found it interesting and sent it to his father who forwarded it to, a Swedish chemist, Jons Jacob Berzelius for examination. Berzelius managed to isolate the new element thorium from the sent mineral sample. Thorium was named after a Germanic god called Thor, the god of thunder, and the mineral from which it was extracted is now known as thorite [1]. Thorium was isolated in metallic form in 1914 by Dutch entrepreneurs Lodewijk Hamburger and Dirk Lely Jr. It was used in gas mantle till electricity became extensively available around the world. Thorium was discovered to be radioactive in 1898 by German Chemist Gerhard Carl Schmidt and by Marie Curie, two years after the discovery of radioactivity of uranium. Radioactive decay of natural thorium is the major contributor to Earth’s internal heat.
Thorium
| Periodic Table Classification | Group n/a Period 7 |
|---|---|
| State at 20C | Solid |
| Color | Silvery, often with black tarnish |
| Electron Configuration | [Rn] 6d2 7s2 |
| Electron Number | 90 |
| Proton Number | 90 |
| Electron Shell | 2, 8, 18, 32, 18, 10, 2 |
| Density | 11.72 g.cm-3 at 20°C |
| Atomic number | 90 |
| Atomic Mass | 232.04 g.mol -1 |
| Electronegativity according to Pauling | 1.30 |
Occurrence
Thorium is a primordial element which still naturally occurs in large quantities in the earth’s crust. It is found in minor quantity in soil and most rocks. It is a radioactive element, like uranium, but it is three times more abundant than uranium. Around four-fifth of the thorium produced at the time of formation of the earth still exists due to its long half-life [2]. It is the rarest primordial element in the universe as it is only produced during r process (core collapse supernovae).
Physical Characteristics
Thorium is a bright silvery metal and corrodes to black when exposed to air. Thorium belongs to actinide series in the periodic table. It makes a face centred cubic crystal structure at room temperature. Thorium is very ductile in pure form and can be forged easily. It is malleable and its bulk modulus is same as that of tin. Thorium is less dense when compared to its nearby elements, for instance, uranium. It has a high melting point of 1750 degree centigrade and a fifth highest boiling point (of 4788°C) amongst the known elements’ boiling point. It is represented by symbol Th and has atomic number 90.
Chemical Characteristics
Thorium is a weakly radioactive element. It is electropositive element with four valence electrons. It is quite reactive and can ignite spontaneously if exposed to air when in finely divided form and forms thorium dioxide. Thorium dioxide is used as refractory material because it has very high melting point. At standard temperature and pressure thorium reacts with water.
Significance and Uses
- Thorium element is abundant and can satisfy world’s energy demand. It is used as fuel in nuclear reactors, as a replacement for uranium.
- Thorium is used as alloying element in making welding electrodes.
- It is added to improve the mechanical strength of magnesium.
- Thorium samples are purified to extract daughter nuclides which are used in cancer therapy.
- Thorium is used to mark impurities in evacuated tubes because of its reactivity with air.
Health Effects
Thorium decays relatively slowly and emits alpha radiation which cannot penetrate human skin. Hence exposure to small amount of thorium is considered safe. The decay products of thorium include radium and radon which are dangerous radionuclides. Exposure to thorium containing dust can cause lung cancer, blood cancer, and liver or pancreas diseases. Some thorium compounds are toxic. Thorium metal ignites spontaneously in air so should be handled with care.
Isotopes of Thorium
All isotopes of thorium are unstable. Th-232 is the most stable and abundantly existing isotope with half-life approximately equal to the age of universe and about three times the age of the earth. Thirty radioisotopes of thorium have been characterized, their atomic masses ranging from 209 to 238. The half-life of these radioactive isotopes ranges from thousands of years to less than ten minutes [3].
REFERENCES
[1]. Thomson, T. (1831). A System of Chemistry of Inorganic Bodies. 1. Baldwin & Cradock and William Blackwood. p. 475
[2]. Audi, G.; Bersillon, O.; Blachot, J.; et al. (2003). “The NUBASE evaluation of nuclear and decay properties” (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729….3A. doi:10.1016/j.nuclphysa.2003.11.001. Archived from the original (PDF) on 24 July 2013.
[3]. Ikezoe, H.; Ikuta, T.; Hamada, S.; et al. (1996). “alpha decay of a new isotope of 209Th”. Physical Review C. 54 (4): 2043–2046. Bibcode:1996PhRvC..54.2043I. doi:10.1103/PhysRevC.54.2043.