Actinide

The actinide or actinoid (IUPAC nomenclature) series encompasses the 14 chemical elements with atomic numbers from 90 to 103, thorium to lawrencium.[1][2][3] The actinide series derives its name from the group 3 element actinium which can be included in the series for the purpose of comparisons. Only thorium and uranium occur in usable quantities in nature. The other actinides are man-made elements. The actinides are usually considered to be f-block elements. The actinides show much more variable valency than the lanthanoids. All actinides are radioactive.

History

Prior to 1945, it was generally thought, following Mendeleev, that thorium and uranium were transition metals in groups 4 and 6 respectively. The assumption was that the transuranium elements would also have the properties of transition metals. However, Charles Janet proposed in 1928 that, starting with actinium, there would be 14 elements corresponding with the lanthanides. The transuranium elements were first synthesized as part of the Manhattan Project in around 1944. Glenn T. Seaborg, the principal investigator, found that americium and curium did not have the properties to be expected of transition elements.[4] In 1945, he went against the advice of colleagues and, without knowing of Janet, adopted his proposal, which thus became the most significant change to the periodic table to have been accepted universally by the scientific community: that the actinide elements belong to a new series, similar to the lanthanide series in that the valence electrons would be situated in f orbitals. This is also in accord with the Aufbau principle which predicts that 5f orbitals will be filled before 6d orbitals.
Chemistry
Atomic No. 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
Name Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
Atoms 7s26d1 7s26d2 7s25f26d1 7s25f36d1 7s25f46d1 7s25f6 7s25f7 7s25f76d1 7s25f9 7s25f10 7s25f11 7s25f12 7s25f13 7s25f14 7s25f147p1

Some actinide atoms have electrons in 6d orbitals, but in compounds the 6s electrons and any d electrons are lost, leaving the ions with an electronic configuration [Rn]5fn. In this respect the actinides are similar to the lanthanoids, with only f electrons in the valence shell in compounds. There is also a similarity in the fact that the maximum oxidation state of the later actinides is +3. However, the early actinides, Th and U can lose all their valence electrons to achieve a maximum oxidation state of 4 and 6 respectively. Historically this led to some debate as to whether thorium, and uranium should be considered as d-block elements, that is, with thorium in group 4, below hafnium, and uranium in group 6, below tungsten. The chemistry of these elements does in fact follow the trends expected with increasing atomic number going down those groups, taking into account the effects of the lanthanide contraction. Pu can also lose all its valence electrons, as in [PuO5]3-.

The highest oxidation states of U, Np and Pu occur in covalent, mostly oxo- and fluoro compounds. For example, UF6 (mp 64oC) is sufficiently volatile to be used in gaseous diffusion or gas centrifuge isotope separation plants. All uranium (VI) compounds, apart from fluoro complexes and UO2, contain the linear "uranyl" group, UO22+. Between 4 to 6 ligands can be accommodated in an equatorial plane perpendicular to the uranyl group. The uranyl group acts as a hard acid and form stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands. NpO22+ and PuO22+ are also the common form of Np and Pu in the +6 oxidation state.

Compound in the +5 and +4 oxidation states are also predominantly covalent. A notable feature of complexes of actinides in the +4 oxidation state is that they can achieve coordination numbers as high as 11. Compounds in the +3 oxidation state are semi-covalent. For example the trichlorides crystallize with ionic-type structures, but with clear evidence for some covalent bonding. Compounds of Th(III) and U(III) are very strong reducing agents, but the reducing power decreases from left to right along the actinide series, in line with the decreasing size.
The actinide contraction
Radii of 6-coordinate actinides in various oxidation states.[5]

The size of the actinides decreases with increasing atomic number. This is the normal periodic trend and is similar to the lanthanide contraction. The trend is shown in each of the oxidation states +3, +4 and +5.
Colour and magnetism
Approximate colours of actinide ions in aqueous solution[6][7] Oxidation state 89 90 91 92 93 94 95 96 97 98 99
+3 Ac3+ Th3+ Pa3+ U3+ Np3+ Pu3+ Am3+ Cm3+ Bk3+ Cf3+ Es3+
+4 Th4+ Pa4+ U4+ Np4+ Pu4+ Am4+ Cm4+ Bk4+ Cf4+
+5 PaO2+ UO2+ NpO2+ PuO2+ AmO2+
+6 UO22+ NpO22+ PuO22+ AmO22+
+7 NpO23+ PuO23+ [AmO6]5-

The colours of the actinide ions in the lower oxidation states are due to f-f transitions. In the high oxidation states there may also be charge-transfer transitions. There is strong spin-orbit coupling, but weak crystal field splitting, so there is little colour variation in compounds of a given element in a given oxidation state. Transitions between 5f and 6d orbitals can be observed in the ultraviolet region of the spectrum. Magnetic moments of the paramagnetic species are far from spin-only values.
Organometallic chemistry

An organometallic compound of an actinide is known as an organoactinide. The organometallic chemistry of the actinides is not extensive. Uranocene, U(C8H8)2, is particularly interesting for the presence of the planar, Huckel rule aromatic cyclooctatrenyl anion, analogous to the cyclopentadenyl ion found in ferrocene. The formation of this compound is helped by the relative large size the U4+ ion.
Chemical aspects of radioactivity

All actinides are radioactive. Protactinium and all isotopes of the elements following uranium (the trans-uranium elements) are man-made elements and have half-lives much less than the age of the earth and are not found in usable quantity in nature. Uranium and thorium are weak alpha emitters with very long half-lives and can be handled with minimum radiological protection procedures.

Elements beyond einsteinium have not been synthesized in sufficient quantity to make detailed studies of their chemistry.

Radioactive decay is a significant source of heat, so temperature control is an issue with the trans-uranium elements. Also, emitted alpha particles can act as oxidizing agents. For example,

He2+ + H2O → 2H+ + 1/2 O2 + He

Occurrence
Actinides Half-life Fission products
244Cm 241Pu f 250Cf 243Cmf 10–30 y 137Cs 90Sr 85Kr
232U f 238Pu f is for
fissile 69–90 y 151Sm nc➔
4n 249Cf f 242Amf 141–351 No fission product
has half-life 102
to 2×105 years
241Am 251Cf f 431–898
240Pu 229Th 246Cm 243Am 5–7 ky
4n 245Cmf 250Cm 239Pu f 8–24 ky
233U f 230Th 231Pa 32–160
4n+1 234U 4n+3 211–290 99Tc 126Sn 79Se
248Cm 242Pu 340–373 Long-lived fission products
237Np 4n+2 1–2 my 93Zr 135Cs nc➔
236U 4n+1 247Cmf 6–23 107Pd 129I
244Pu 80 my >7% >5% >1% >.1%
232Th 238U 235U f 0.7–12by fission product yield

Only thorium and uranium occur naturally in the Earth's crust in anything more than trace quantities. Protactinium and actinium, which are both decay products of uranium, are the only remaining actinides that were discovered in nature before they were synthesized. Neptunium and plutonium have also been known to show up naturally in trace amounts in uranium ores as a result of decay or bombardment, but this was only discovered after they were synthesized. The remaining actinides were synthesized in particle colliders or nuclear reactors, and none of them has been found to occur naturally on earth. Actinides beyond californium possess exceedingly short half-lives.

Isotopes of all of the transuranium elements up to and including fermium can be produced by rapid neutron bombardment of lighter nuclides. The nuclei created have an excess of neutrons. β-decay occurs with a neutron decaying to a proton and an electron, increasing the atomic number in the process. Conditions suitable for the synthesis of transuranium elements occur in supernovae. These elements may also be produced in specialized nuclear reactors. They may be created in a nuclear explosion and come to earth as nuclear fallout from an atmospheric test explosion. The heavier elements may be synthesized by bombardment with heavier particles, such as α particles or heavier nuclei.

In 1961, Antoni Przybylski discovered a star, HD 101065, commonly called Przybylski's star, that contains unusually high amounts of actinides.
See also

* Actinides in the environment

References

1. ^ IUPAC Periodic Table
2. ^ IUPAC Periodic Table 2007 .pdf
3. ^ Connelly, Neil G.; et al. (2005). "Elements". Nomenclature of Inorganic Chemistry. London: Royal Society of Chemistry. pp. 52.
4. ^ Seaborg, Glenn T. (1946). "The Transuranium Elements". Science 104 (2704): 379–386. doi:10.1126/science.104.2704.379. http://www.jstor.org/stable/1675046.
5. ^ Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0080379419 , p 1263
6. ^ Arnold F. Holleman, Nils Wiberg: Lehrbuch der Anorganischen Chemie, 102. Auflage, de Gruyter, Berlin 2007, S. 1956; ISBN 978-3-11-017770-1.
7. ^ dtv-Atlas zur Chemie 1981, Teil 1, S. 224.