Lanthanide

A lanthanide is any of the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium.
In the periodic table, the first fourteen fill the 4f orbitals. Lutetium is also often considered a lanthanide, despite being a d-block element and a transition metal. The IUPAC lists the 15 elements La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, under the collective name lanthanoid, which it recommends as chemically more correct for this series.
The informal chemical symbol Ln is used in general discussions of lanthanide chemistry to refer to any lanthanide. All but one of the lanthanides are f-block elements, corresponding to the filling of the 4f electron shell. Lutetium is a d-block element, and on this basis its inclusion has been questioned; however, like its congeners scandium and yttrium in group 3, it behaves similarly to the other 14. The term rare-earth element or rare-earth metal is often used to include the stable group 3 elements Sc, Y, and Lu in addition to the 4f elements. All lanthanide elements form trivalent cations, Ln³⁺, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum to lutetium.
In presentations of the periodic table, the f-block elements are customarily shown as two additional rows below the main body of the table. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the 4f and 5f series in their proper places, as parts of the table's sixth and seventh rows, respectively.

Etymology

The term "lanthanide" was introduced by Victor Goldschmidt in 1925. Despite their abundance, the technical term "lanthanides" is interpreted to reflect a sense of elusiveness on the part of these elements, as it comes from the Greek λανθανειν, "to lie hidden".
The word reflects their property of "hiding" behind each other in minerals. The term derives from lanthanum, first discovered in 1838, at that time a so-called new rare-earth element "lying hidden" or "escaping notice" in a cerium mineral, and it is an irony that lanthanum was later identified as the first in an entire series of chemically similar elements and gave its name to the whole series.
These elements are called lanthanides because the elements in the series are chemically similar to lanthanum. Because "lanthanide" means "like lanthanum", it has been argued that lanthanum cannot logically be a lanthanide, but the International Union of Pure and Applied Chemistry acknowledges its inclusion based on common usage. The current IUPAC recommendation is that the name lanthanoid be used rather than lanthanide, as the suffix "-ide" is preferred for negative ions, whereas the suffix "-oid" indicates similarity to one of the members of the containing family of elements. However, lanthanide is still commonly used.

Physical properties of the elements

The properties of the lanthanides arise from the order in which the electron shells of these elements are filled—the outermost has the same configuration for all of them, and a deeper shell is progressively filled with electrons as the atomic number increases from 57 towards 71. For many years, mixtures of more than one rare earth were considered to be single elements, such as neodymium and praseodymium being thought to be the single element didymium. Very small differences in solubility are used in solvent and ion-exchange purification methods for these elements, which require repeated application to obtain a purified metal. The diverse applications of refined metals and their compounds can be attributed to the subtle and pronounced variations in their electronic, electrical, optical, and magnetic properties.

Chemical element	La	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
Atomic number	57	58	59	60	61	62	63	64	65	66	67	68	69	70	71
Image
Density	6.162	6.770	6.77	7.01	7.26	7.52	5.244	7.90	8.23	8.540	8.79	9.066	9.32	6.90	9.841
Melting point	920	795	935	1024	1042	1072	826	1312	1356	1407	1461	1529	1545	824	1652
Boiling point	3464	3443	3520	3074	3000	1794	1529	3273	3230	2567	2720	2868	1950	1196	3402
Atomic electron configuration *	5d¹	4f¹5d¹	4f³	4f⁴	4f⁵	4f⁶	4f⁷	4f⁷5d¹	4f⁹	4f¹⁰	4f¹¹	4f¹²	4f¹³	4f¹⁴	4f¹⁴5d¹
Metal lattice	dhcp	fcc	dhcp	dhcp	dhcp	**	bcc	hcp	hcp	hcp	hcp	hcp	hcp	fcc	hcp
Metallic radius	162	181.8	182.4	181.4	183.4	180.4	208.4	180.4	177.3	178.1	176.2	176.1	175.9	193.3	173.8
Resistivity at 25 °C	57–80 20 °C	73	68	64		88	90	134	114	57	87	87	79	29	79
Magnetic susceptibility χ_mol /10⁻⁶	+95.9	+2500	+5530	+5930		+1278	+30900	+185000	+170000	+98000	+72900	+48000	+24700	+67	+183

* Between initial Xe and final 6s² electronic shells
** Sm has a close packed structure like most of the lanthanides but has an unusual 9 layer repeat
Gschneider and Daane attribute the trend in melting point which increases across the series, – lutetium ) to the extent of hybridization of the 6s, 5d, and 4f orbitals. The hybridization is believed to be at its greatest for cerium, which has the lowest melting point of all, 795 °C.
The lanthanide metals are soft; their hardness increases across the series. Europium stands out, as it has the lowest density in the series at 5.24 g/cm³ and the largest metallic radius in the series at 208.4 pm. It can be compared to barium, which has a metallic radius of 222 pm. It is believed that the metal contains the larger Eu²⁺ ion and that there are only two electrons in the conduction band. Ytterbium also has a large metallic radius, and a similar explanation is suggested.
The resistivities of the lanthanide metals are relatively high, ranging from 29 to 134 μΩ·cm. These values can be compared to a good conductor such as aluminium, which has a resistivity of 2.655 μΩ·cm.
With the exceptions of La, Yb, and Lu, the lanthanides are strongly paramagnetic, and this is reflected in their magnetic susceptibilities. Gadolinium becomes ferromagnetic at below 16 °C. The other heavier lanthanides – terbium, dysprosium, holmium, erbium, thulium, and ytterbium – become ferromagnetic at much lower temperatures.

Chemical element	La	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
Atomic number	57	58	59	60	61	62	63	64	65	66	67	68	69	70	71
Ln³⁺ electron configuration*	4f⁰	4f¹	4f²	4f³	4f⁴	4f⁵	4f⁶	4f⁷	4f⁸	4f⁹	4f¹⁰	4f¹¹	4f¹²	4f¹³	4f¹⁴
Ln³⁺ radius	103	102	99	98.3	97	95.8	94.7	93.8	92.3	91.2	90.1	89	88	86.8	86.1
Ln⁴⁺ ion color in aqueous solution	—	Orange-yellow	Yellow	Blue-violet	—	—	—	—	Red-brown	Orange-yellow	—	—	—	—	—
Ln³⁺ ion color in aqueous solution	Colorless	Colorless	Green	Violet	Pink	Pale yellow	Colorless	Colorless	V. pale pink	Pale yellow	Yellow	Rose	Pale green	Colorless	Colorless
Ln²⁺ ion color in aqueous solution	—	—	—	—	—	Blood red	Colorless	—	—	—	—	—	Violet-red	Yellow-green	—

* Not including initial core
f → f transitions are symmetry forbidden, which is also true of transition metals. However, transition metals are able to use vibronic coupling to break this rule. The valence orbitals in lanthanides are almost entirely non-bonding and as such little effective vibronic coupling takes, hence the spectra from f → f transitions are much weaker and narrower than those from d → d transitions. In general this makes the colors of lanthanide complexes far fainter than those of transition metal complexes.

Oxidation state

Sm²⁺

Eu²⁺

Tm²⁺

Yb²⁺

La³⁺

Ce³⁺

Pr³⁺

Nd³⁺

Pm³⁺

Sm³⁺

Eu³⁺

Gd³⁺

Tb³⁺

Dy³⁺

Ho³⁺

Er³⁺

Tm³⁺

Yb³⁺

Lu³⁺

Ce⁴⁺

Pr⁴⁺

Nd⁴⁺

Tb⁴⁺

Dy⁴⁺