The actinide family of elements (actinoid series) are the 15 metallic elements with atomic numbers from 89 to 103. The actinoid series derives its name from the first element in the series, actinium, chemical symbol Ac.
What is the Actinide family of elements?
The actinide family of elements comprise 15 metallic elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide (actinoid) series is named by the first element in the series, actinium, Ac.
Definition of an Actinide Element
Actinides are f-block elements, (lawrencium is uniquely, a d-block element). The family is mainly associated with the filling of the 5f electron shell, although in the ground state many have configurations involving the filling of the 6d shell due to inter-electronic repulsion. They all have very large atomic and ionic radii. Actinium and some actinides (after americium) act like the lanthanides, the elements thorium, protactinium, and uranium are like transition metals.
Thorium and uranium occur naturally in high quantities. The decay of uranium produces actinium and protactinium, and atoms of neptunium and plutonium are occasionally produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements. Analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium, einsteinium and fermium.
Actinides in the Periodic Table (Discovery and History)
Transuranium elements were suggested in 1934, by Fermi. Research at the time was based upon the fact that that they were regular elements in the 7th period, with thorium, protactinium and uranium corresponding to 6th-period hafnium, tantalum and tungsten, respectively. Synthesis changed this perspective. Seaborg suggested an actinide hypothesis based on curium being unable to exhibit an oxidation state above +4.
There are two methods of producing isotopes of transplutonium elements. The first, irradiation of the lighter elements with neutrons and the second, irradiation with accelerated charged particles. The first is crucial as only neutron irradiation using nuclear reactors allows the production of sizeable amounts of synthetic actinides. It is limited to relatively light elements. The advantage of the second method is that elements heavier than plutonium, can be obtained, which are not formed during neutron irradiation.
In 1962, the United States tried to produce transplutonium isotopes using underground nuclear explosions. Samples of rock were extracted from the blast area immediately after the test to study the explosion products, but no isotopes with mass number greater than 257 could be detected. This non-observation was attributed to spontaneous fission owing to the large speed of the products and to other decay channels, such as neutron emission and nuclear fission.
Actinide properties and isotopes
32 isotopes of actinium and 8 isotopes were identified by 2016. Three isotopes, 225Ac, 227Ac and 228Ac, were found in nature and are used in applications. Actinium-225 was first discovered as a decay product of uranium-233, it is an α-emitter with a half-life of 10 days. Actinium-227 occurs in all uranium ores in small quantities. Actinium-228 is a member of the radioactive thorium series formed by the decay of 228Ra. In one tonne of thorium there is 5×10−8 gram of 228Ac.
There are 31 known isotopes of thorium ranging in mass number from 208 to 238. 232Th, whose half-life of 1.4×1010 years. The next longest-lived is 230Th, an intermediate decay product of 238U with a half-life of 75,400 years. Several other thorium isotopes have half-lives over a day.
28 isotopes of protactinium are known with mass numbers 212–239 as well as three excited isomeric states. Only 231Pa and 234Pa have been found in nature. The most important isotopes are 231Pa and 233Pa, which is an intermediate product in obtaining uranium-233 and is the most affordable among artificial isotopes of protactinium.
There are 26 known isotopes of uranium, having mass numbers 215–242 (except 220 and 241). Three of them, 234U, 235U and 238U, are present in nature. 233U, is a final product of transformation of 232Th irradiated by slow neutrons. 233U has a much higher fission efficiency by low-energy (thermal) neutrons, compared e.g. with 235U.
There are 24 isotopes of neptunium with mass numbers of 219, 220, and 223–244; they are all highly radioactive. The most popular among scientists are long-lived 237Np , half-life at 2.20×106 years.
Eighteen isotopes of americium are known with mass numbers from 229 to 247 (with the exception of 231). The most important are 241Am and 243Am, which are alpha-emitters and also emit soft, but intense γ-rays; both of them can be obtained in an isotopically pure form.
Among 19 isotopes of curium, ranging in mass number from 233 to 251, the most accessible are 242Cm and 244Cm; they are α-emitters, but with much shorter lifetime than the americium isotopes. More long-lived isotopes of curium (245–248Cm, all α-emitters) are formed as a mixture during neutron irradiation of plutonium or americium. Upon short irradiation, this mixture is dominated by 246Cm, and then 248Cm begins to accumulate. Both of these isotopes, especially 248Cm, have a longer half-life (3.48×105 years) and are much more convenient for carrying out chemical research than 242Cm and 244Cm, but they also have a rather high rate of spontaneous fission.
Seventeen isotopes of berkelium were identified with mass numbers 233–234, 236, 238, and 240–252. Only 249Bk is available in large quantities; it has a relatively short half-life of 330 days and emits mostly soft β-particles, which are inconvenient for detection. Its alpha radiation is rather weak (1.45×10−3% with respect to β-radiation), but is sometimes used to detect this isotope.
The 20 isotopes of californium with mass numbers 237–256 are formed in nuclear reactors; californium-253 is a β-emitter and the rest are α-emitters. The isotopes with even mass numbers (250Cf, 252Cf and 254Cf) have a high rate of spontaneous fission, especially 254Cf of which 99.7% decays by spontaneous fission. Californium-249 has a relatively long half-life (352 years), weak spontaneous fission and strong γ-emission that facilitates its identification. 249Cf is not formed in large quantities in a nuclear reactor because of the slow β-decay of the parent isotope 249Bk and a large cross section of interaction with neutrons, but it can be accumulated in the isotopically pure form as the β-decay product of 249Bk.
Among the 18 known isotopes of einsteinium with mass numbers from 240 to 257, the most affordable is 253Es. It is an α-emitter with a half-life of 20.47 days, a relatively weak γ-emission and small spontaneous fission rate as compared with the isotopes of californium.
Twenty isotopes of fermium are known with mass numbers of 241–260. 254Fm, 255Fm and 256Fm are α-emitters with a short half-life (hours), which can be isolated in significant amounts. 257Fm can accumulate upon prolonged and strong irradiation. All these isotopes are characterised by high rates of spontaneous fission.
Among the 17 known isotopes of mendelevium (mass numbers from 244 to 260), the most studied is 256Md, which mainly decays through the electron capture (α-radiation is ≈10%) with the half-life of 77 minutes. Another alpha emitter, 258Md, has a half-life of 53 days. Both these isotopes are produced from rare einsteinium (253Es and 255Es respectively), that therefore limits their availability.
Long-lived isotopes of nobelium and isotopes of lawrencium (and of heavier elements) have relatively short half-lives. For nobelium, 11 isotopes are known with mass numbers 250–260 and 262. The chemical properties of nobelium and lawrencium were studied with 255No (t1/2 = 3 min) and 256Lr (t1/2 = 35 s).
Actinides list of elements and origin
Actinides found in nature
Thorium and uranium are found in the greatest quantity in nature. Uranium oxides in the mineral uraninite, which is also called pitchblende. 238U (99.27%), 235U (0.72%) and 234U (0.0054%). The worldwide production of uranium in 2009 amounted to ovr 50k tonnes and nearly 30% was extracted from Kazakhstan.
Thorium minerals found are thorianite (ThO2) and thorite (ThSiO4) and are in abundance in the United States (440,000 tonnes), Australia and India (~300,000 tonnes each).
237Np is present in nature in negligible amounts produced as intermediate decay products of other isotopes. The upper limit of abundance of the longest-living isotope of plutonium, 244Pu, is 3×10−20%. Most plutonium is synthetically man made.
Chemical properties of actinides
Lanthanides and Actinides
Actinides are highly reactive with halogens and chalcogens. They are more reactive than Lanthanides. Actinides are also prone to hybridisation because of the similarity of the electron energies at the 5f, 7s and 6d shells. Most actinides exhibit a larger variety of valence states, and the most stable are +6 for uranium, +5 for protactinium and neptunium and +4 for thorium.
Actinium is chemically similar to lanthanum, which is explained by their similar ionic radii and electronic structures. Like lanthanum, actinium almost always has an oxidation state of +3 in compounds, but it is less reactive. Among other trivalent actinides Ac3+ has the least tendency to hydrolyse in aqueous solutions.
The lack of electrons on 6d and 5f orbitals means the tetravalent thorium compounds appear colourless. At pH < 3, the solutions of thorium salts are dominated by the cations [Th(H2O)8]4+. The distinctive ability of thorium salts is their high solubility both in water and polar organic solvents.
Protactinium exhibits two valence states; the +5 is stable, and the +4 state easily oxidises to protactinium(V). Tetravalent protactinium is chemically similar to uranium(IV) and thorium(IV). Protactinium forms soluble carbonates. The hydrolytic properties of pentavalent protactinium are close to those of tantalum(V) and niobium(V). The complex chemical behaviour of protactinium is a consequence of the start of the filling of the 5f shell in this element.
Uranium has a valence from 3 to 6, the last being most stable. Many compounds of uranium(IV) and uranium(VI) are non-stoichiometric. Uranium(VI) compounds are weak oxidants. Most of them contain the linear uranyl group, UO2+2. Between 4 and 6 ligands can be accommodated in an equatorial plane perpendicular to the uranyl group. The uranyl group acts as a hard acid and forms stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands. Uranium(IV) compounds exhibit reducing properties, e.g. they are easily oxidised by oxygen. Uranium(III) is a very strong reducing agent. Owing to the presence of d-shell, uranium forms organometallic compounds, such as UIII(C5H5)3 and UIV(C5H5)4.
Neptunium has valence states from 3 to 7. The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds. Neptunium metal is very reactive.
Plutonium also exhibits valence states between 3 and 7 inclusive, and thus is chemically similar to neptunium and uranium. It is highly reactive, and quickly forms an oxide film in air. Hydrolysis reactions of plutonium ions of different oxidation states are quite diverse. Plutonium(V) can enter polymerisation reactions.
The largest chemical diversity among actinides is observed in americium, which can have valence between 2 and 6. Divalent americium is obtained only in dry compounds and non-aqueous solutions (acetonitrile). Oxidation states +3, +5 and +6 are typical for aqueous solutions. Tetravalent americium forms stable solid compounds (dioxide, fluoride and hydroxide) as well as complexes in aqueous solutions. The most stable valence of americium is 3 in the aqueous solutions and 3 or 4 in solid compounds.
Valence 3 is dominant in all subsequent elements up to lawrencium (with the exception of nobelium). Curium can be tetravalent in solids (fluoride, dioxide). Berkelium, along with a valence of +3, also shows the valence of +4, more stable than that of curium; the valence 4 is observed in solid fluoride and dioxide. The stability of Bk4+ in aqueous solution is close to that of Ce4+. Only valence 3 was observed for californium, einsteinium and fermium. The divalent state is proven for mendelevium and nobelium, and in nobelium it is more stable than the trivalent state. Lawrencium shows valence 3 both in solutions and solids.
Actinides react with halogens forming salts MX3 and MX4. The first berkelium compound, BkCl3, was synthesised in 1962. Like the halogens of rare earth elements, actinide chlorides, bromides, and iodides are water-soluble, and fluorides are insoluble. Actinide hexafluorides have properties close to anhydrides. They hydrolyse forming AnO2F2. The pentachloride and black hexachloride of uranium were synthesised, but they are both unstable.
Action of acids on actinides yields salts, and if the acids are non-oxidising then the actinide in the salt is in low-valence state:
- U + 2H2SO4 → U(SO4)2 + 2H2
- 2Pu + 6HCl → 2PuCl3 + 3H2
However, in these reactions the regenerating hydrogen can react with the metal, forming the corresponding hydride. Actinide salts can also be obtained by dissolving the corresponding hydroxides in acids. Nitrates, chlorides, sulfates and perchlorates of actinides are water-soluble. Salts of high-valence actinides easily hydrolyse. Colourless sulfate, chloride, perchlorate and nitrate of thorium transform into basic salts with formulas Th(OH)2SO4 and Th(OH)3NO3. The solubility and insolubility of trivalent and tetravalent actinides is like that of lanthanide salts.
Actinides with oxidation state +6, except for the AnO22+-type cations, form [AnO4]2−, [An2O7]2− and other complex anions. For example, uranium, neptunium and plutonium form salts of the Na2UO4 (uranate) and (NH4)2U2O7 (diuranate) types. In comparison with lanthanides, actinides more easily form coordination compounds, and this ability increases with the actinide valence. Thorium also forms the corresponding sulfates (for example Na2SO4·Th(SO4)2·5H2O), nitrates and thiocyanates. Salts with the general formula An2Th(NO3)6·nH2O are of coordination nature, with the coordination number of thorium equal to 12. Even easier is to produce complex salts of pentavalent and hexavalent actinides. The most stable coordination compounds of actinides – tetravalent thorium and uranium – are obtained in reactions with diketones, e.g. acetylacetone.
Actinides have some applications, such as in smoke detectors (americium) and gas mantles (thorium), they are mostly used in nuclear weapons and as fuel in nuclear reactors. One property of actinides is to release enormous energy in nuclear reactions, which under certain conditions may stimulates self-sustaining chain reactions.
The most important isotope for nuclear power applications is uranium-235. This isotope strongly absorbs thermal neutrons releasing much energy. One fission act of 1 gram of 235U converts into about 1 MW day. Of importance, is that 23592U emits more neutrons than it absorbs at critical mass, 23592U enters into a self-sustaining chain reaction. Typically, uranium nucleus is divided into two fragments with the release of 2–3 neutrons, for example:
- 23592U + 1 0n ⟶ 115 45Rh + 118 47Ag + 31 0n
Other promising actinide isotopes for nuclear power are thorium-232 and its product from the thorium fuel cycle, uranium-233.
Thorium is added into multicomponent alloys of magnesium and zinc. So the Mg-Th alloys are light and strong, but also have high melting point and ductility and thus are widely used in the aviation industry. The relative content of thorium and uranium isotopes is widely used to estimate the age of various objects, including stars.
The major application of plutonium has been in nuclear weapons. Plutonium-based designs allow reducing the critical mass to about a third of that for uranium-235. The Fat Man plutonium bombs used explosive compression of plutonium to obtain significantly higher densities than normal, combined with a central neutron source to begin the reaction. 6.2 kg of plutonium was needed for an explosive yield equivalent to 20 kilotons of TNT.
Plutonium-238 is potentially more efficient isotope for nuclear reactors, since it has smaller critical mass than uranium-235, but it continues to release much thermal energy (0.56 W/g) by decay even when the fission chain reaction is stopped by control rods. Its application is limited by its high price. This isotope has been used in thermopiles and water distillation systems of some space satellites and stations. The decay of plutonium-238 produces relatively harmless alpha particles and is not accompanied by gamma-irradiation. Therefore, this isotope (~160 mg) is used as the energy source in heart pacemakers.
Actinium-227 is used as a neutron source. Its high specific energy (14.5 W/g) and the possibility of obtaining significant quantities of thermally stable compounds are attractive for use in long-lasting thermoelectric generators for remote use. 228Ac is used as an indicator of radioactivity in chemical research, as it emits high-energy electrons (2.18 MeV) that can be easily detected. 228Ac-228Ra mixtures are widely used as an intense gamma-source in industry and medicine.
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