According to the IUPAC terminology, the lanthanoid (previously lanthanide) series comprises the fifteen elements with atomic numbers 57 through 71, from lanthanum to lutetium. All lanthanoids are f-block elements, corresponding to the filling of the 4f electron shell, except for lanthanum which is a d-block lanthanoid. The lanthanoid series (Ln) is named after lanthanum.
The trivial name "rare earths" is sometimes used to describe all the lanthanoids together with scandium and yttrium. The term "rare earths" arises from the minerals from which they were isolated, which were uncommon oxide-type minerals. The use of this name is deprecated by IUPAC, as they are neither rare in abundance nor "earths" (an obsolete term for water-insoluble strongly basic oxides of electropositive metals incapable of being smelted into metal using late 18th century technology). These elements are in fact fairly abundant in nature, although rare as compared to the "common" earths such as lime or magnesia. Cerium is the 26th most abundant element in the Earth's crust, neodymium is more abundant than gold and even thulium (the least common naturally-occurring lanthanoid) is more abundant than iodine. Despite their abundance, even the technical term "lanthanoids" could be interpreted to reflect a sense of elusiveness on the part of these elements, as it comes from the Greek λανθανειν (lanthanein), "to lie hidden". However, if not referring to their natural abundance, but rather to their property of "hiding" behind each other in minerals, this interpretation is in fact appropriate. The etymology of the term must be sought in the first discovery of lanthanum, at that time a so-called new rare earth element "lying hidden" in a cerium mineral, but we might call it a fortunate twist of irony that exactly lanthanum was later identified as the first in an entire series of chemically similar elements and could give name to the whole series.
IUPAC currently recommends the name lanthanoid rather than lanthanide, as the suffix "-ide" generally indicates negative ions whereas the suffix "-oid" indicates similarity to one of the members of the containing family of elements. In the older literature, the name "lanthanon" was often used.
There are alternative arrangements of the periodic table that exclude lanthanum or lutetium from appearing together with the other lanthanoids.
Lanthanoids are chemically similar to each other. Useful comparison can also be made with the actinoids, where the 5f shell is partially filled. The lanthanoids are typically placed below the main body of the periodic table in the manner of a footnote. The full-width version of the periodic table shows the position of the lanthanoids more clearly.
The ionic radii of the lanthanoids decrease through the period — the so-called lanthanide contraction. Except for cerium (III and IV) and europium (III and II), the lanthanoids occur as trivalent cations in nature. As a consequence, their geochemical behaviors are a regular function of ionic radius and, therefore, atomic number. This property results in variations in the abundances of lanthanoids that trace natural materials through physical and chemical processes. In addition, three of the lanthanoids have radioactive isotopes with long half-lives (138La, 147Sm and 176Lu) that date minerals and rocks from Earth, the Moon and meteorites. The lanthanoid contraction is responsible for the great geochemical divide that splits the lanthanoids into light and heavy-lanthanoid enriched minerals, the latter being almost inevitably associated with and dominated by yttrium. This divide is reflected in the first two "rare earths" that were discovered: yttria (1794) and ceria (1803). The divide is driven by the decrease in coordination number as the ionic radius shrinks, and is dramatically illustrated by the two anhydrous phosphate minerals, monazite (monoclinic) and xenotime (tetragonal). The geochemical divide has put more of the light lanthanoids in the Earth's crust, but more of the heavies in the Earth's mantle. The result is that although large rich orebodies are found that are enriched in the light lanthanoids, correspondingly large orebodies for the heavies are few. The lanthanoids obey the Oddo-Harkins rule - odd-numbered elements are less abundant than their even-numbered neighbors.
Due to their specific electronic configurations, lanthanoid atoms tend to lose three electrons, usually 5d1 and 6s2, to attain their most stable oxidation state as trivalent ions.
The lanthanoid trivalent cations feature a Xenon-core electronic configuration with the addition of n 4f electrons, with n varying from 0 [for La(III)] to 14 [for Lu(III)]. This 4fn sub-shell lies inside the ion, shielded by the 5s2 and 5p6 closed sub-shells. Thus, lanthanoid trivalent cations are sometimes referred to as “triple-positively charged noble gases”.
The contracted nature of the 4f orbitals and their small overlap with the ligand atom orbitals attaches a predominantly ionic character to lanthanoid-ligand atom bonds in complexes. Thus, the mainly electrostatic interactions between the lanthanoid trivalent cation and the atoms of the ligands result in irregular geometric arrangements and a handful of high coordination numbers. Indeed, this triple-positively charged closed shell inert gas electron density characteristic is the foundation of the lanthanide Sparkle Model, used in the computational chemistry of lanthanoid complexes.
Several properties, such as ionization energies, optical properties, magnetic moments and geometries of complexes, etc., serve as proof that the 4f orbitals are indeed wholly shielded from ligand effects.
All lanthanoids closely resemble lanthanum. They are electropositive trivalent metals. They are shiny and silvery-white, and tarnish easily when exposed to air. They react violently with most nonmetals. They are relatively soft but their hardness increases with their atomic number. Lanthanoids burn in air. They have high melting and boiling points.
Lanthanoids entering the human body due to exposure to various industrial processes can affect metabolic processes. Trivalent lanthanoid ions, especially La3+ and Gd3+, can interfere with calcium channels in human and animal cells. Lanthanoids can also alter or even inhibit the action of various enzymes. Lanthanoid ions found in neurons can regulate synaptic transmission, as well as block some receptors (for example, glutamate receptors).
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Most lanthanoids are widely used in lasers. These elements deflect ultraviolet and infrared radiation and are commonly used in the production of sunglass lenses. Other applications are summarized in the following table:
To remember the sequence of the lanthanoid elements, various mnemonic phrases have been used. One is:
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