Mendelevium Md s Nobelium No s Lawrencium Lr d s Fig . : Chemical reactions of the lanthanoids. The actinoids are radioactive elements and the earlier members have relatively long half-lives, the latter ones have half-life values ranging from a day to minutes for lawrencium ( Z = ). The latter members could be prepared only in nanogram quantities.
These facts render their study more difficult. All the actinoids are believed to have the electronic configuration of 7s and variable occupancy of the 5f and 6d subshells. The fourteen electrons are formally added to f , though not in thorium (Z = ) but from Pa onwards the f orbitals are complete at element . The irregularities in the electronic configurations of the actinoids, like those in the lanthanoids are related to the stabilities of the f , f and f occupancies of the 5f orbitals.
Thus, the configurations of Am and Cm are [Rn] f s and [Rn] f d s . Although the f orbitals resemble the f orbitals in their angular part of the wave-function, they are not as buried as f orbitals and hence f electrons can participate in bonding to a far greater extent. The general trend in lanthanoids is observable in the actinoids as well. There is a gradual decrease in the size of atoms or M + ions across the series.
This may be referred to as the actinoid contraction (like lanthanoid contraction). The contraction is, however, greater from element to element in this series resulting from poor shielding by f electrons. There is a greater range of oxidation states, which is in part attributed to the fact that the f , d and s levels are of comparable energies. The known oxidation states of actinoids are listed in Table .
. The actinoids show in general + oxidation state. The elements, in the first