Lanthanides

Lanthanides in the periodic table of the elements (rode by a black edge).

The lanthanoids (name recommended by IUPAC) or lanthanides are a group of elements that are part of period 6 of the periodic table of elements. These elements are called “rare earths” because they are in the form of oxides, and also, together with the actinides, they form the “internal transition elements”.

The name comes from the chemical element lanthanum, which is usually included within this group, giving a total of 15 elements, from atomic number 57 (lanthanum) to 71 (lutetium). Although it is usually included in this group, lanthanum has no electrons occupying any f orbital, while the next fourteen elements have this 4f orbital partially or fully filled (see electron configuration).

These elements are chemically quite similar to each other since the electrons located in f orbitals are unimportant in the bonds they form, compared to those in p and d. The elements yttrium and scandium are also quite similar to the lanthanides, because they have a similar radius and, like the lanthanides, their most important oxidation state is +3. This is the most important oxidation state of the lanthanides, but they also have the +2 and +4 oxidation state.

The abundance of these elements in the earth's crust is relatively high, in minerals such as monazite, in which different lanthanides and yttrium are found.

In the periodic table, these elements are usually placed below the rest, together with the actinides, giving a more compact table than if they were placed between the elements of the s block and those of the d, although in some periodic tables they can be seen located between these blocks, giving a much wider table.

The radius of the lanthanides decreases as the atomic number increases; They are not large variations, but they accumulate. This causes the d-block elements of the second and third transition series to have similar radii within a group: they should increase going down a group, but since the lanthanides have been intercalated, this increase in radius going down within a group it is counteracted by the reduction of the radius due to the presence of the lanthanides. This is known as lanthanide contraction.

Since the publication of the Red Book (1985, p. 45), the IUPAC recommends the use of “lanthanoid” instead of “lanthanide”. The suffix -ido generally indicates a negative ion; however, due to current wide usage, the term lanthanide is still permitted. Since lanthanoid means 'lanthanum-like' (cf. humanoid, android), it has been argued for semantic reasons that lanthanum cannot logically be a lanthanide, but IUPAC acknowledge their inclusion based on common usage.

Magnetic and spectral properties

Several aspects of the magnetic and spectral behavior of the lanthanides differ fundamentally from those of the block corresponding to the transition elements. The basic reason for these differences is that the electrons that are responsible for the properties of the lanthanide ions are 4f electrons, and that the 4f orbitals are very effectively shielded from the influence of external forces in the outer 5s^{2 and 5p6. That is why the states that originate from the various 4fn configurations are only slightly affected by the environment that surrounds the ions and remain practically invariable for a given ion in all its compounds.}

The spin-orbit coupling constants are quite large. This has the consequence that, with few exceptions, the lanthanide ions possess ground states with a single and well-defined value of total angular momentum J, with the next lower state of J, and with energies many times greater than the value of KT, and therefore the upper state is virtually unpopulated.

The colors and fundamental electronic states of M³⁺ ions are given in the table below; the consequence of the colors in the series from lanthanum to gadolinium are accidentally repeated in the series from lutetium to gadolinium. As implicit in the previous explanations, the colors are due to f-f transitions which are virtually independent of the external ion environment.