Actinides
The elements with atomic numbers 90 to 103 i.e. thorium to lawrencium (which come immediately after actinium, Z = 89) are called actinides or actinones. These elements involve the filling of 5 f-orbitals. Their general electronic configuration is, [Rn] 5f1–14 6d0–1 7s2.
They include three naturally occuring elements thorium, protactinium and uranium and eleven transuranium elements or transuranics which are produced artificially by nuclear reactions. They are synthetic or man made elements. All actinides are radioactive.
Properties of actinides
(i) Oxidation state : The dominant oxidation state of actinides is +3 which shows increasing stability for the heavier elements. Np shows +7 oxidation state but this is oxidising and is reduced to the most stable state +5. Pu also shows states upto +7 and Am upto +6 but the most stable state drops to Pu (+4) and Am (+3). Bk in +4 state is strongly oxidising but is more stable than Cm and Am in 4 state due to f 7 configuration. Similarly, No is markedly stable in +2 state due to its f14 configuration. When the oxidation number increases to + 6, the actinide ions are no longer simple. The high charge density causes the formation of oxygenated ions e.g., UO22+, NpO22+ etc. The exhibition of large number of oxidation states of actinides is due to the fact that there is a very small energy gap between 5f, 6d and 7s subshells and thus all their electrons can take part in bond formation.
(ii) Actinide contraction : There is a regular decrease in ionic radii with increase in atomic number from Th to Lr. This is called actinide contraction analogous to the lanthanide contraction. It is caused due to imperfect shielding of one 5f electron by another in the same shell. This results in increase in the effective nuclear charge which causes contraction in size of the electron cloud.
(iii) Colour of the ions : Ions of actinides are generally coloured which is due to f – f transitions. It depends upon the number of electrons in 5 f orbitals.
(iv) Magnetic properties : Like lanthanides, actinide elements are strongly paramagnetic. The magnetic moments are lesser than the theoretically predicted values. This is due to the fact that 5 f electrons of actinides are less effectively shielded which results in quenching of orbital contribution.
(v) Complex formation : Actinides have a greater tendency to form complexes because of higher nuclear charge and smaller size of their atoms. They form complexes even with p-bonding ligands such as alkyl phosphines, thioethers etc, besides EDTA, b-diketones, oxine etc. The degree of complex formation decreases in the order.
M4+ > MO22+ > M3+ > MO2+
where M is element of actinide series. There is a high concentration of charge on the metal atom in MO22+ which imparts to it relatively high tendency towards complex formation.
