Concept 06 / 06The d and f Block Elements
Actinoids and Comparison with Lanthanoids; Applications of d- and f-Block Elements
Electronic configuration, oxidation states, actinoid contraction, radioactivity and chemical reactivity of actinoids, comparison of lanthanoids and actinoids, and important applications of d- and f-block elements and their compounds.
Actinoids (Th to Lr, following actinium) fill the orbitals. All are radioactive; those beyond uranium (transuranic) are synthetic and many have short half-lives, so their chemistry is less studied.
Electronic configuration. . The , and levels are very close in energy, especially for the early members, so configurations are irregular (Th , with no 5f electron; U ).
Oxidation states (see figure). is common, but the early actinoids show a much wider range than lanthanoids, because electrons are less tightly held and can take part in bonding: Th ; Pa ; U (as in , ); Np and Pu up to . In later actinoids (from Am), dominates again. Actinoids in and states form oxocations such as and .
Actinoid contraction. A decrease in size along the series similar to the lanthanoid contraction, but greater from element to element, because electrons shield even more poorly than .
Properties. Silvery metals with varied structures (because of irregular metallic radii). Highly reactive, especially when finely divided: they react with boiling water to give a mixture of oxide and hydride, and combine with most non-metals at moderate temperatures. Hydrochloric acid attacks them all, but concentrated nitric acid renders them passive by an oxide film. Alkalis have no effect. Magnetic behaviour is complex (5f orbitals interact more with the environment).
Comparison of lanthanoids and actinoids.
Similarities: both fill f orbitals; is common to both; both show a contraction; both are electropositive and reactive; many ions of both are coloured and paramagnetic.
Differences:
oxidation states: lanthanoids mostly (occasionally , ); actinoids up to ;
radioactivity: only promethium among lanthanoids; all actinoids;
complex formation and oxocations: much greater for actinoids (); lanthanoids form few oxocations;
contraction: larger from element to element in actinoids;
basicity: actinoid compounds are less basic;
magnetic properties: lanthanoids easier to explain; actinoids complex.
Applications of d- and f-block elements.
Iron and steel: construction, machinery; alloy steels with Cr, Ni, V, Mo, W, Mn.
Titanium and : aerospace alloys (light, strong, corrosion resistant); white pigment.
Batteries: in dry cells; Zn and Ni/Cd in batteries.
Coinage metals: Cu, Ag, Au (and alloys).
Catalysts: (contact process), with (Ziegler catalyst for polythene), Fe (Haber process), Ni (hydrogenation), (Wacker oxidation of ethene to ethanal), Ni complexes (polymerisation of alkynes).
Photography: AgBr (light-sensitive).
Nuclear energy: U and Pu as fuels; Th as a fertile material.
Key Takeaways:
- Actinoids: ; all radioactive.
- Wider range of oxidation states (up to for Np, Pu); U commonly .
- Actinoid contraction is larger per element than lanthanoid contraction.
- Oxocations () are typical of actinoids.
- Pitfall: thorium has no 5f electron in its ground state (), yet it is counted among the actinoids.
Brain-Teaser 1: Why do actinoids show a greater range of oxidation states than lanthanoids?
Solution: In actinoids, the , and orbitals are very close in energy, and orbitals extend further from the nucleus than orbitals, so electrons can be used in bonding. In lanthanoids, the electrons are buried inside and strongly held, so after losing three electrons, further ionisation is difficult.
Brain-Teaser 2: Why is the actinoid contraction greater from element to element than the lanthanoid contraction?
Solution: electrons provide even poorer shielding of the nuclear charge than electrons (5f orbitals are more diffuse), so each increase in nuclear charge produces a larger increase in effective nuclear charge on the outer electrons, and hence a bigger decrease in size.
Brain-Teaser 3: Name the transition metal catalysts used in (a) the contact process, (b) polymerisation of ethene to high-density polythene, (c) the Wacker process, and state the oxidation state of V in (a).
Solution: (a) , vanadium . (b) Ziegler-Natta catalyst, with (or ). (c) (with ) for converting ethene to ethanal.
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