Lanthanoids

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Lanthanoide [ lantanoˈiːdə ] ("lanthanum-like"; Greek : ending -ειδἠς ( -eides ) "similar") is a group name for similar elements . Attributed to it are the lanthanum and 14 in the periodic table following elements cerium , praseodymium , neodymium , promethium , samarium , europium , gadolinium , terbium , dysprosium , holmium , erbium , thulium , ytterbium and lutetium . In the sense of the term, lanthanum does not belong to the group of lanthanum-like. Here, however, the IUPAC nomenclature follows practical use. The use of the old name lanthanide is still allowed. All lanthanides are metals and are also known as elements of the lanthanum series. They are part of the group of rare earth metals .

57
La 		58
Ce 		59
Pr 		60
Nd 		61
pm 		62
Sm 		63
Eu 		64
Gd 		65
p 		66
Dy 		67
Ho 		68
he 		69
Tm 		70
yb 		71
Lu
All stable lanthanides at a glance

Occurrence

The lanthanoids are also known as rare earth metals. This name is confusing because the elements of this group, with the exception of the unstable promethium, are by no means as rare as it is suggested. For example, cerium is more common in nature than the elements arsenic or lead . They are involved in the structure of the earth's crust to a mass fraction of 0.02%. There are a total of 15 elements of the 6th period , of which the 14 elements following lanthanum can be understood as a subgroup of the 3rd subgroup .

Due to their chemical similarity, the lanthanides are usually associated in nature. Since the separation of the individual lanthanides is difficult and their chemical properties are very similar, these elements are often grouped under the (unofficial) chemical symbol Ln (not to be confused with La for lanthanum). Many of them can be obtained from monazite (also called secondary deposits - monazite sands). The most common and economically most important lanthanoid-bearing minerals are:

In the formulas, Ln denotes all elements from lanthanum to lutetium and the very similar yttrium (Y).

In almost all minerals there is an accumulation of either the light (Ce) or the heavy lanthanoids (Y behaves in terms of mineral chemistry like a heavy lanthanoid). For example, monazite contains predominantly Ce and La, while the content of the following lanthanides decreases with the atomic number (therefore the monazite formula is always given as CePO 4 ). In Xenotim one finds exactly the opposite case (hence also YPO 4 ). This mostly very effective fractionation is caused by the lanthanide contraction and the crystal lattice sites available, which vary in size from mineral to mineral. Other mineral groups can also incorporate high proportions of lanthanides into their structure (e.g. zircon , garnet ). Furthermore, the lanthanides occur on the moon in the form of the so-called KREEP ores .

properties

Crystal structure of the lanthanides except for cerium, samarium, europium and ytterbium.

Physical Properties

The lanthanoids are silvery, shiny, relatively soft and reactive metals. Almost all of them have the closest packing of spheres , which is typical for metals . The hardness increases with increasing atomic number.

Like actinides, lanthanoids belong to the inner transition elements or f-block elements , since the f orbitals in these rows are not completely filled with electrons.

The promethium isotopes are all unstable, i.e. radioactive.

Chemical properties

Due to the similar structure of the valence shell , the lanthanoids behave chemically like the elements of the 3rd group of the periodic table scandium and yttrium and together with these form the group of rare earths . They oxidize quickly in air and become dull. They react more or less quickly with water to form hydrogen .

Starting with Cer, the 4f orbital is gradually filled up. In the case of lutetium, it is completely occupied with 14 electrons. Since the 4f orbitals lie deep inside the atoms , they have little influence on the chemical behavior in contrast to the d orbitals of the other subgroup elements. The lanthanide elements are therefore relatively similar in terms of their chemical properties. They are so similar that when ytter earth was discovered in 1794, they were even thought to be the oxide of one and the same element. The same applies to the numerous components of the cerite earth. What they have in common is the oxidation number +3. In addition, the oxidation numbers +2 and +4 occur with some elements.

A discontinuity in the course of the ionic radii between gadolinium and terbium is called a gadolinium break in lanthanides. This explains why, despite the similarity of the lanthanoids, the chemical behavior of the elements changes after the gadolinium. The chemical behavior can easily be influenced on the Gadolinium Break. Traces of americium are sufficient for a terbium complex to adopt the structure type of the lighter lanthanoids.

Colors of the lanthanoid ions in an aqueous solution

Oxidation
number 		57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
+2 Sm 2+
blood red 		Eu 2+
colorless 		Tm 2+
purple red 		Yb 2+
yellow green
+3 La 3+
colorless 		Ce 3+
colorless 		Pr 3+
yellow-green 		Nd 3+
violet 		Pm 3+
purple pink 		Sm 3+
deep yellow 		Eu 3+
colorless 		Gd 3+
colorless 		Tb 3+
colorless 		Dy 3+
yellow green 		Ho 3+
yellow 		He 3+
deep pink 		Tm 3+
pale green 		Yb 3+
colorless 		Lu 3+
colorless
+4 Ce 4+
orange-yellow 		Pr 4+
yellow 		Nd 4+
blue-violet 		Tb 4+
red-brown 		Dy 4+
orange-yellow

Lanthanide contraction

Due to the lanthanide contraction, the atomic radius decreases  almost steadily within the range from cerium  (183 pm) to lutetium (172 pm) (exceptions are europium and ytterbium). This is due to the fact that the elements that - based on the atomic number - are in front of the lanthanides have already filled the 6s and 5p shells with electrons, but not the 4f shell. The lanthanoids now fill the 4f shell with electrons. In a simplified representation of the atom as consisting of spatially separated electron shells, an electron shell that is spatially closer to the nucleus is now filled with charge carriers. Incidentally, the nucleus naturally fills with the same number of protons as electrons are added to the 4f shell. The resulting stronger attraction between electrons and protons reduces the atomic radius while the atomic number increases.

This effect is actually not exceptional, as the radius always decreases when a bowl is filled within a period. However, this property has some consequences:

  • Due to the decreasing size, separation by means of ion exchangers is easily possible.
  • In the holmium, the radius of Ln 3+ is so small that it almost corresponds to that of Y 3+ ; therefore yttrium is usually found together with the "heavy earths"
  • Within a group, the transition elements in the 2nd and 3rd positions have very similar properties.

use

There are numerous examples of the uses of the lanthanides:

See also

Remarks

  1. ↑ Before the introduction of ion exchangers, different valence levels could only be used for cerium (trivalent and tetravalent) or europium (bivalent and trivalent); the other elements had to be recrystallized hundreds of times to separate them .

literature

Web links

Individual evidence

  1. ↑ Wolfgang Liebscher, Ekkehard Fluck: The systematic nomenclature of inorganic chemistry . Springer-Verlag, Berlin 1999, ISBN 3-540-63097-X .
  2. ^ Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 .
  3. ^ AM Mariano: Economic geology of rare earth minerals . In: BR Lipin, GA McKay (Ed.): Reviews in Mineralogy , Vol. 21 - Geochemistry and mineralogy of rare earth elements (1989). Published by the Mineralogical Society of America, ISBN 0-939950-25-1 , pp. 309-337.
  4. ↑ Georg Steinhauser: Structural Chemistry - A Touch of Nothing. In: News from chemistry. 66, 2018, p. 118, doi : 10.1002 / nadc.20184067855 .
  5. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1937.
  6. ↑ dtv atlas on chemistry . 1981 , part 1, p. 220.
  7. ↑ a b Lanthanide lecture script from the University of Bayreuth.