A series of elements in the periodic table, generally considered to range in proton number from cerium (58) to lutetium (71) inclusive. The lanthanoids all have two outer s-electrons (a 6s2configuration), follow lanthanum, and are classified together because an increasing proton number corresponds to increase in number of 4f electrons. In fact, the 4f and 5d levels are close in energy and the filling is not smooth. The outer electron configurations are as follows:
Note that lanthanum itself does not have a 4f electron but it is generally classified with the lanthanoids because of its chemical similarities, as are yttrium (Yt) and scandium (Sc). Scandium, yttrium, and lanthanum are d-block elements; the lanthanoids and actinoids make up the f-block.
The lanthanoids are sometimes simply called the rare earths, although strictly the ‘earths’ are their oxides. Nor are they particularly rare: they occur widely, usually together. All are silvery very reactive metals. The f-electrons do not penetrate to the outer part of the atom and there is no f-orbital participation in bonding (unlike the d-orbitals of the main transition elements) and the elements form few coordination compounds. The main compounds contain M3+ ions. Cerium also has the highly oxidizing Ce4+ state and europium and ytterbium have a M2+ state.
The 4f orbitals in the atoms are not very effective in shielding the outer electrons from the nuclear charge. In going across the series the increasing nuclear charge causes a contraction in the radius of the M3+ ion – from 0.1061 nm in lanthanum to 0.0848 nm in lutetium. This effect, the lanthanoid contraction (or lanthanide contraction), accounts for the similarity between the transition elements zirconium and hafnium. Calculations using the Hartree–Fock procedure indicate that the peak of the charge density for 4f electrons is about half the distance from the corresponding peaks for 5s and 5p electrons. This gives a quantitative explanation for why all the lanthanoids have very similar chemical properties and why it took so long for them to be discovered: the 4f sub-shell being filled makes little difference to the outer electrons that determine chemical behaviour. These calculations also explain why an external crystal field has a much smaller effect on lanthanoids than it does on transition elements: because lanthanoids have a high atomic number, the effect of spin-orbit coupling on atomic energy levels is stronger than the effect of external crystal fields.