General trends

Key point: The early actinoids do not exhibit the chemical uniformity of the lanthanoids; they exist in diverse oxidation states with An (III) progressively more stable across the series. The 14 elements from thorium (Th, Z=90, 5f¹) to lawrencium (Lr, Z=103, 5f¹⁴) involve the progressive completion of the 5f subshell, and in this sense are analogues of the lanthanoids. However, the actinoids do not exhibit the chemical uniformity of the lanthanoids. Whereas, like the lanthanoids, a common oxidation state of the actinoids is An (III), unlike the lanthanoids the early members of the series occur in a rich variety of other oxidation states. The Frost diagrams in Fig. 23.9 show that oxidation numbers higher than 3 are easily accessible and often preferred for the early elements of the block (Th, Pa, U, Np, Pu) whereas 3 becomes predominant in Am and beyond. Linear or nearly linear AnO₂ and AnO₂⁺² ions are the dominant aqua species for oxidation numbers +5 and +6 exhibited by U and Np. Unlike in the lanthanoids, the f orbitals of the early actinoids extend into the bonding region, so the spectra of their complexes are strongly affected by ligands. The 5f and 6d orbitals are less compact than the 4f and 5d orbitals and the electrons in them are more available for bonding. Thus, the outermost electron configuration of U is 5f³6d¹7s² with all six electrons available for bonding. At around Am and Cm the 5f and 6d orbital energies are lower due to their poor screening by the other electrons, so that the later actinoids behave like the lanthanoids with only the outermost three electrons being readily ionized.

The striking differences between the chemical properties of the lanthanoids and early actinoids led to controversy about the most appropriate placement of the actinoids in the periodic table. For example, before 1945, periodic tables usually showed U below W because both elements have a maximum oxidation number of +6. The emergence of oxidation states An (III) for the later actinoids was a key point in determining their current placement. The similarity of the heavy actinoids and the lanthanoids is illustrated by their similar elution behaviour in ion-exchange separation (compare Figs 23.8 and 23.10).

Like the lanthanoids, the actinoids have large atomic and ionic radii (the radius of an An³⁺ ion is typically about 5 pm larger than its Ln3 congener) and as a result often have high coordination numbers. For example, uranium in solid UCl₄ is eight-coordinate and in solid UBr₄ it is seven-coordinate in a pentagonal-bipyramidal array. Solid-state structures with coordination numbers up to 12 have been observed. Because of the small quantities of material available in most cases and their intense radioactivity, most of the chemical properties of the trans americium elements (the elements following americium, Z=95) have been established by experiments carried out on a microgram scale or even on just a few hundred atoms. For example, the trans americium ion complexes have been adsorbed on and eluted from a single bead of ion-exchange material of diameter 0.2 mm. For the heaviest and most unstable post-actinoids, such as hassium (Hs, Z=108), the lifetimes are too short for chemical separation and the identification of the element is based exclusively on the properties of the radiation it emits.

Figure 23.10 Elution of heavy actinoid ions from a cation exchange column using ammonium 2-hydroxyisobutyrate as the eluent. Note the similarity in elution sequence to Fig. 23.8: the heavy (smaller) An3 ions elute first.

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