Actinium is a radioactive metal. It is the third element in row 7 of the periodic table. It is considered often as the first of the actinides, a group of “inner transition metals”. Quite recently discovered, it has been a useful metal in laboratory research over the past century.
History and Discovery
It was discovered in 1899 by the French chemist André-Louis Debierne In some of the pitchblende residues left after French physicists Pierre and Marie Curie had extracted radium from them. Pitchblende is a naturally occurring uranium oxide. The word actinium is gotten from the Greek word, aktinos which means “ray”. In 1902, it was also discovered independently by German chemist, Friedrich Oskar Giesel who had not heard of Debierne’s earlier discovery. Giesel suggested the name, emanium which means to “give off rays”. Debierne’s name was adopted because he discovered actinium first. A group of 15 similar elements between Actinium and Lawrencium in the periodic table form the actinide series, a name gotten from actinium. Its symbol is Ac.
|Periodic Table Classification||Group 3
|State at 20C||Solid|
|Electron Configuration||[Rn] 6d1 7s2|
|Electron Shell||2, 8, 18, 32, 18, 9, 2|
|Density||10.07 g.cm-3 at 20°C|
|Atomic Mass||227.00 g.mol -1|
|Electronegativity according to Pauling||1.1|
Actinium is a rare earth metal. It is found in trace amounts in uranium ore and sometimes thorium ore. Its low natural concentration and similarity to lanthanides which are present in large quantities make the separation of actinium from the ore very difficult, time consuming, and impractical. Ion chromatography and solvent extraction are commonly used for separation. Instead, actinium is prepared by bombarding radium with neutrons in a nuclear reactor. This method can be used to prepare actinium metal of 98% purity. Actinium’s estimated crustal abundance is about 5.5˟10-10 milligrams per kilogram.
It has close similarities to that of lanthanum. It is solid, silvery-white in colour, has a density of 10g/cm3, it has a boiling point of 3200 ˚C. It has a melting point of 1050 ˚C. It has a heat of fusion of 14KJ, a heat of vaporization of 400Kj/mol. It has a molar heat capacity of 27.2j/ (mol.k). Its flammability is not known.
Actinium is located in period seven and group three of the periodic table. It lies just below lanthanum and their chemical behaviour is similar. Actinium is the first of the actinides. There may be a slight disagreement because using their electronic structure, actinides are characterized by the gradual filling of the 5f subshell but actinium has no electrons in its 5f subshell so it may also fit with group three elements.
Actinium reacts rapidly with oxygen and moisture in the air forming a white coat of the oxide of the metal. This stops further oxidation. Actinium exists in the +3 oxidation state like most actinides and its ions are colourless in solutions.
It forms halides. Actinium trifluoride can be produced in solution or in solid reaction. In solution, it is carried out at room temperature by adding hydrofluoric acid to a solution containing actinium ions. Actinium trichloride is gotten by reacting actinium oxalate or hydroxide with carbon tetrachloride vapours at a temperature above 960˚C.
It also forms hydrides. Actinium hydride is obtained by reducing actinium trichloride with potassium at 300˚C. Actinium sulphide can be produced by heating actinium oxalate with hydrogen sulphide vapours at 1400˚C for a few minutes. This results in a black compound being produced.
Significance and Uses
- Actinium is scarce and costs high. It has no significant industrial use however, listed below are some of its uses.
- It is used in radio immunotherapy. Actinium-225 has a half-life of 10 days.
- When it decays, it emits alpha particles. This can thus cause high-energy radiation to a tumor.
- It is used in laboratory research.
- Actinium-225 is used in medicine to produce 213Bi in a reusable generator.
- A very good source of neutrons. This is because actinium is about 150 times as radioactive as radium.
- Some research studies show the effectiveness of actinium-225 in the treatment of prostate cancer.
It is very toxic due to its radioactivity. Ingesting just a little amount of actinium-227 presents with a serious health hazard. Accidental exposure to this element has been linked to complications in pregnant women, congenital anomalies, fertility problems, damage to the gastrointestinal system, damage to the immune system, carcinogenesis, etc.
An experiment was carried out with actinium trichloride administered intravenously to rats. It was discovered that about 33% of actinium is deposited into the bones and about 50% into the liver. Its toxicity is slightly lower than that of americium and plutonium.
Isotopes of Actinium
Actinium has about a 32 isotopes with all being radioactive. It has 7 isomers. The two naturally occurring isotopes are actinium-227 and actinium-228. The half-lives of actinium-227 and actinium-228 are 21.77 years and 6.13 hours respectively. The remaining radioactive isotopes have half-lives that are less than 10 hours and many of those even have half-lives that are less than a minute. The isotope of actinium with the shortest half-life is actinium-217 which decays through alpha decay and electron capture. It has a half-life of 69ns. Actinium has no stable isotopes. The most common and most stable isotope is actinium-227, a decay product of uranium-235. The isotopes of actinium are within a range in atomic weight of 206amu to 236amu. Purified actinium-227 is comes in equilibrium with its decay products (thorium-227 and Francium-223) after 185 days.
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