Lanthanide

The lanthanides are a set of 15 metallic elements, spanning atomic numbers 57 through 71, from lanthanum to lutetium. In the periodic table, the first fourteen of these (up to ytterbium) fill the 4f orbitals. Lutetium, element 71, is a d-block element and a transition metal, but it is still generally counted as a lanthanide because its behavior closely matches the others. The International Union of Pure and Applied Chemistry (IUPAC) officially recommends the name "lanthanoid" for this series (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), as the suffix "-oid" better indicates similarity to lanthanum, while "-ide" is usually reserved for negative ions. Despite this, "lanthanide" remains the common term. In discussions of lanthanide chemistry, the informal symbol Ln is used to refer to any element in the group. All lanthanides except lutetium are f-block elements, corresponding to the filling of the 4f electron shell. The term "rare-earth element" or "rare-earth metal" often includes the stable group 3 elements scandium, yttrium, and lutetium along with the 4f elements. Every lanthanide forms a trivalent cation, Ln³⁺, and their chemistry is largely governed by a steady decrease in ionic radius from lanthanum to lutetium. Several can also take on +2 or +4 oxidation states.

In periodic table layouts, the f-block elements are usually placed in two separate rows below the main table—a convention driven by aesthetics and formatting convenience. A wide-formatted table, though rarely used, inserts the 4f and 5f series into their proper positions in the sixth and seventh periods.

The term "lanthanide" was introduced by Victor Goldschmidt in 1925. Despite their actual abundance, the name reflects a sense of elusiveness, coming from the Greek *lanthanein*, meaning "to lie hidden." This refers to how these elements "hide" behind each other in minerals. The name derives from lanthanum, first discovered in 1838, which was itself a new rare-earth element "lying hidden" in a cerium mineral. Ironically, lanthanum later became the first in a whole series of chemically similar elements and gave its name to the group. Because "lanthanide" means "like lanthanum," some argue that lanthanum itself cannot logically be a lanthanide, but IUPAC includes it based on common usage.

The properties of the lanthanides come from the way their electron shells fill: the outermost 6s shell has the same configuration for all, while the deeper 4f shell fills progressively from atomic number 57 to 71. For many years, mixtures of rare earths were mistaken for single elements—for instance, neodymium and praseodymium were thought to be the single element didymium. Purifying these metals requires repeated application of solvent and ion-exchange methods, exploiting very small differences in solubility. The wide range of applications for refined lanthanides and their compounds stems from subtle variations in their electronic, electrical, optical, and magnetic properties.

The melting point generally increases across the series, from lanthanum (920 °C) to lutetium (1622 °C), with cerium having the lowest at 795 °C. This trend is attributed to the degree of hybridization of the 6s, 5d, and 4f orbitals. The lanthanide metals are soft, and their hardness increases across the series. Europium stands out with the lowest density (5.24 g/cm³) and the largest metallic radius (208.4 pm) in the series—comparable to barium's 222 pm. It is thought that europium metal contains the larger Eu²⁺ ion, with only two electrons in the conduction band. Ytterbium also has a large metallic radius, likely for a similar reason. The resistivities of these metals are relatively high, ranging from 29 to 134 μΩ·cm, compared to a good conductor like aluminum at 2.655 μΩ·cm. Except for lanthanum, ytterbium, and lutetium (which have no unpaired f electrons), the lanthanides are strongly paramagnetic. Gadolinium becomes ferromagnetic below 16 °C (its Curie point), while the heavier lanthanides—terbium, dysprosium, holmium, erbium, thulium, and ytterbium—become ferromagnetic at much lower temperatures.

The f → f transitions in lanthanides are symmetry forbidden (Laporte-forbidden), similar to transition metals. However, transition metals can use vibronic coupling to break this rule. In lanthanides, the valence orbitals are almost entirely non-bonding, so little effective vibronic coupling occurs. As a result, the spectra from f → f transitions are much weaker and narrower than those from d → d transitions, making the colors of lanthanide complexes generally far fainter.