f block

The f-block elements include the lanthanons, whose electrons fill the positions of 4f subshell, and the actinons, whose electrons fill the 5f subshell. The rare earths cerium, gadolinium, and lutetium, all members of the f-block group, each have a single d electron, as do the actinons protactinium, uranium, neptunium, curium, and lawrencium. The actinon thorium has two d electrons and no f electrons. Despite these unusual configurations, all of the foregoing elements are classified as f-block member. In the case of cerium, the sudden contraction and reduction in energy of the 4f orbitals immediately after lanthanum is not yet sufficient to avoid occupancy of the d-shell. Similar conditions apply to thorium, protactinium, and uranium. Gadoliniumís single d-electron reflects the stability of the half-filled f-shell, as does its actinon homologue, curium. Lutetium and lawrencium, at the ends of their respective series, represent the filling of the f-shell. Some versions of the periodic table include in this block the rare earth lanthanum, which has chemical properties similar to the lanthanons, although none of its electrons fill the 4f subshell. Actinium is often included in the f-block for similar reasons.