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 6s2 configuration), 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:
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.