The actinide or actinoid (IUPAC nomenclature) series encompasses the 14 chemical elements with atomic numbers from 90 to 103, thorium to lawrencium. 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.
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. 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. In 1945, he went against the advice of colleagues and proposed 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 lanthanoid 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.
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 loose 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 UO3, 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 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.
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.
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.
All actinides are radioactive. protoactinium 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 einstinium 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 oxidising agents. For example,
|244Cm||241Pu f||250Cf||243Cmf||10–30 y||137Cs||90Sr||85Kr|
|232U f||238Pu||f is for
|69–90 y||151Sm nc➔|
|4n||249Cf f||242Amf||141–351||No fission product
has half-life 102
to 2×105 years
|4n||245Cmf||250Cm||239Pu f||8–24 ky|
|248Cm||242Pu||340–373||Long-lived fission products|
|237Np||4n+2||1–2 my||93Zr||135Cs nc➔|
|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.