1. Oxidation states:
Exhibition of large number of oxidation states of actinoids 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.
Though 4f and 5f orbitals have similar shapes but 5f is less deeply buried than 4f. Hence, 5f electrons can participate in bonding to a far greater extent.
2. Actinoid contraction:
There is a regular decrease in ionic radii with increase in atomic number from Th to Lr. This is called actinoid contraction analogous to the lanthanoid 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.
This contraction is greater from element to element in this series due to poor shielding by 5f electrons. This is because 5f orbitals extend in space beyond 6s and 6p orbitals whereas 4f orbitals are buried deep inside the atom.
3. Colour of the ions:
Ions of actinoids are generally coloured which is due to f-transitions. It depends upon the number of electrons in 5f orbitals. The cations containing 5f0 and 5f7 (exactly half filled f subshell) electrons are colourless. The cations containing 5f2-6 electrons are coloured both in the crystalline state as well as in aqueous solution. For example, Ac3+ (5f0), Cm3+ (5f7), Th4+ (5f0) are colourless while U3+ (5f3) is red, Np3+ (5f4) is blue, Pu3+ (5f3) is violet, Am3+ (5f6) is pink.
5. Complex formation.
Actinoids 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 alkylphosphines, thioethers etc. besides EDTA, β-diketones, oxine etc. The degree of complex formation decreases in the order.
M4+ > MO22+ > M3+ > MO2+
Where M is element of actinoid series. There is a high concentration of charge on the metal atom in MO22+ which imparts to it relatively high tendency towards complex formation.
