AMERICIUM
Symbol: Am; Atomic Number 95; Atomic Weight 243.0614; an inner-transition, actinide series, radioactive man-made element; electron configuration: [Rn]86 5Æ’66d17s2, partially filled Æ’-orbitals; valence 2, 3, 4, 5 or 6Â Â
| Isotopes | Half-life | Decay Mode |
| Am-237 | 1.22 hr. | Orbital electron emission |
| Am-238 | 1.63 hr. | Orbital electron emission |
| Am-239 | 11.90 hr. | Orbital electron emission |
| Am-240 | 50.90 hr. | Orbital electron emissionAlpha emission |
| Am-241 | 432.2 yr. | Alpha emission |
| Am-242 | 16.01 hr. | Beta emission (83%) Orbital electron emission (17%) |
| Am-242 | ~141 yr. | Isomeric transition (isomer) |
| Am-243 | 7,370 yr. | Alpha emission |
| Am-244 | 10.1 hr. | Beta emission |
| Am-244 | 26 min. | Beta emission (isomer) |
| Am-245 | 2.05 hr. | Beta emission |
| Am-246 | 39 min. | Beta emission |
| Am-246 | 25 min. | Beta emission (isomer) |
| Am-247 | ~22 min. | Beta emission |
Occurrence
Americium does not occur in nature. It is a man-made element produced in nuclear reactors.
Uses
One of its isotopes, Am-241, is a portable source for gamma radiography; also a source of ionization for smoke detectors. In the glass industry, it is used as a radioactive glass thickness gage. Other isotopes do not have much commercial application.Â
Physical Properties
White lustrous metal when freshly prepared; turns silvery; exists in two forms: as a double hexagonal closed-packed alpha form, and a closed-packed cubic structure known as beta form; melts at 994°C; more volatile than its neighbor elements, plutonium or curium; vaporizes at 2,607°C; density 13.67 g/cm3; soluble in dilute acids.
Production
Am-241 may be prepared in a nuclear reactor as a result of successive neutron capture reactions by plutonium isotopes:
Pu-241 isotope undergoes β-decay forming Am-241:
Am-241 obtained as a decay product in the above nuclear reaction (over a period of years), can be separated by extraction. Am-242 and Am-243 isotopes can be prepared from Am-241 by neutron bombardments:
Also, Am-243 can be made from Pu-242, which can be prepared either by very intense neutron irradiation of Pu-239, or from Am-241; resulting from successive neutron-capture reactions.
The Pu-242 obtained in the nuclear reaction is separated by chemical extraction. Americium metal can be prepared from its dioxide by reducing with lanthanum metal at high temperature in a vacuum.
or from its fluoride by reducing the latter with Ba vapors at 1,100°C to 1,200°C:
The metal is soluble in a melt of its trihalide salts. Americium may be separated from other elements, particularly from the lanthanides or other actinide elements, by techniques involving oxidation, ion exchange and solvent extraction. One oxidation method involves precipitation of the metal in its trivalent state as oxalate (controlled precipitation). Alternatively, it may be separated by precipitating out lanthanide elements as fluorosilicates leaving americium in the solution. Americium may also be oxidized from trivalent to pentavalent state by hypochlorite in potassium carbonate solution. The product potassium americium (V) carbonate precipitates out. Curium and rare earth metals remain in the solution. An alternative approach is to oxidize Am3+ to AmO22+ in dilute acid using peroxydisulfate. AmO22+ is soluble in fluoride solution, while trivalent curium and lanthanides are insoluble.
Ion exchange techniques have been widely applied in the separation process. In the large-scale ammonium thiocyanate process, the metal is retained on strong base anion exchanger; thus, separating it from the lighter lanthanide elements which are not strongly absorbed on the resin.
Americium and other actinide elements may be separated from lanthanides by solvent extraction. Lithium chloride solution and an eight to nine carbon tertiary amine are used in the process. Americium is then separated from curium by the above methods.
Chemical Reactions
The metal forms its oxide, AmO on its surface in contact with air or oxygen. Similarly, reaction with hydrogen forms the hydride, AmH2.
Divalent Am2+ is less stable than the corresponding divalent lanthanide elements. It has not been found in aqueous solutions, even after treatment with strong reducing agents.
Am3+ is the most stable oxidation state of the metal. In trivalent state, its properties are similar to europium. Am3+ reacts with soluble fluoride, hydroxide, phosphate, oxalate, iodate and sulfate of many metals forming precipitates of these anions; e.g., Am(OH)3, Am(IO3)3, etc.
No stable divalent salt is known. However, Am2+ has been detected in CaF2 matrix (0.1% Am) by paramagnetic resonance spectrum at low temperature. Its formation is attributed to the reduction of Am3+ by electrons in the lattice set free by the effects of alpha particle emission.
Trivalent Am3+ ions occur in aqueous acid solution. The solution has a pink color and the ion exists as a hydrated species. Reactions with halide salts or the acids produce trihalides.
In solution Am4+ ion is not so stable, slowly reducing to trivalent Am3+. However, simple and also complex tetravalent compounds of americium are known. Some examples are Am(OH)4, AmF4, LiAmF8 , and K2AmF4. Am(OH)4 is stable in basic solution and results from the oxidation of Am(OH)3 by hypochlorite ion.
All pentavalent americium compounds are complex salts. Examples are KAmO2CO3, KAmO2F2 and Li3AmO4. These are formed upon oxidation of Am3+ . For example, Am3+ reacts with hypochlorite ion in hot K2CO3, precipitating KAmO2CO3 as a crystalline solid.
No simple hexavalent americium compound is known. All Am6+ compounds are complex salts containing oxygen. Examples are Li6AmO6, NaAmO2AC3 (Ac is acetate ion), AmO2F2 and Ba3AmO6. Hexavalent americium ion is a strong oxidizing agent and is reduced to AmO+2 in oxidation-reduction reactions. Am ion in higher oxidation states is reduced to Am3+ by Am-241 alpha radiation.
Safety Precautions
Am emits alpha and gamma radiation. The alpha decay of the isotope Am-241 is three times as active as radium and is associated with 59 KeV gamma radiation, which is a serious health hazard. The alpha energies of Am-241 and Am-243, the two longest lived isotopes, are 5.48 and 5.27 MeV, respectively, accompanied with gamma rays. Therefore, a totally enclosed storage system using x-ray glass should be used, maintaining a slight negative pressure.
Tags: AMERICIUM, Americium Am, Atomic Number 95, Atomic Weight 243.0614, inner-transition, Symbol
