A series of elements in the periodic table, generally considered to range in atomic number from thorium (90) to lawrencium (103) inclusive. The actinoids all have two outer s-electrons (a 7s2 configuration), follow actinium, and are classified together by the fact that increasing proton number corresponds to filling of the 5f level. In fact, because the 5f and 6d levels are close in energy the filling of the 5f orbitals is not smooth. The outer electron configurations are as follows:
The first four members (Ac to Ur) occur naturally. All are radioactive and this makes investigation difficult because of self-heating, short lifetimes, safety precautions, etc. Like the lanthanoids, the actinoids show a smooth decrease in atomic and ionic radius with increasing proton number. The lighter members of the series (up to americium) have f-electrons that can participate in bonding, unlike the lanthanoids. Consequently, these elements resemble the transition metals in forming coordination complexes and displaying variable valency. As a result of increased nuclear charge, the heavier members (curium to lawrencium) tend not to use their inner f-electrons in forming bonds and resemble the lanthanoids in forming compounds containing the M3+ ion. The reason for this is pulling of these inner electrons towards the centre of the atom by the increased nuclear charge. Note that actinium itself does not have a 5f electron, but it is usually classified with the actinoids because of its chemical similarities. Because the actinoids have higher atomic numbers than the lanthanoids, the effect of spin–orbit coupling on atomic energy levels is even greater—and greater than the effects of external crystal fields. See also transition elements.