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Lanthanide and Actinide Chemistry

Simon Cotton(Autor*in)
Wiley (Verlag)
2. Auflage
Erschienen am 12. Januar 2024
336 Seiten
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978-1-118-87346-5 (ISBN)
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LANTHANIDE AND ACTINIDE CHEMISTRY

Lanthanides and actinides, also known as "f elements," are a group of metals which share certain important properties and aspects of electronic structure. They have a huge range of applications in the production of electronic devices, magnets, superconductors, fuel cells, sensors, and more. The cursory treatment of these important metals in most inorganic chemistry textbooks makes a book-length treatment essential.

Since 2006, Lanthanide and Actinide Chemistry has met this need with a thorough, accessible overview. With in-depth accounts of the lanthanides, actinides, and transactinides, this book is ideal for both undergraduate and postgraduate students in inorganic chemistry or chemical engineering courses. Now updated to reflect groundbreaking recent research, this promises to continue as the essential introductory volume on the subject.

Readers of the second edition of Lanthanide and Actinide Chemistry will also find:

* New and expanded subject areas including lanthanide enzymes, single-molecule magnets, luminescence and upconversion, organometallic and coordination chemistry; and many more.

* Up-to-date information on the myriad modern applications of f-elements

* Lists of objectives and learning goals at the start of each chapter

Lanthanide and Actinide Chemistry is ideal for advanced undergraduates and graduate students in f-element chemistry, inorganic chemistry, or any related field.

INORGANIC CHEMISTRY ADVANCED TEXTBOOK

This series reflects the pivotal role of modern inorganic and physical chemistry in a whole range of emerging areas, such as materials chemistry, green chemistry and bioinorganic chemistry, as well as providing a solid grounding in established areas such as solid state chemistry, coordination chemistry, main group chemistry and physical inorganic chemistry.
SIMON COTTON, PHD, is a retired Honorary Senior Lecturer in Chemistry at the University of Birmingham, UK. He has decades of teaching and publishing experience in inorganic chemistry and related fields, and first worked with uranium and the lanthanides over fifty years ago; his 'Soundbite Molecules' column regularly appeared In the magazine Education in Chemistry for fifteen years, whilst he has written over 100 'Molecules of the Month' at https://www.chm.bris.ac.uk/motm/motm.htm

1
Introduction to the Lanthanides


By the end of this chapter you should be able to:

  • understand that lanthanides differ in their properties from the s- and d-block metals;
  • recall characteristic properties of these elements;
  • appreciate reasons for their positioning in the Periodic Table;
  • understand how the size of the lanthanide ions affects certain properties and how this can be used in the extraction and separation of the elements;
  • understand how to obtain pure samples of individual Ln3+ ions.

1.1 Introduction


Lanthanide chemistry started in Scandinavia. In 1794, Johann Gadolin succeeded in obtaining an 'earth' (oxide) from a black mineral subsequently known as gadolinite; he called the earth yttria. Soon afterwards, M.H. Klaproth, J.J. Berzelius and W. Hisinger obtained ceria, another earth, from cerite. However, it was not until 1839-1843 that the Swede C.G. Mosander first separated these earths into their component oxides; thus, ceria was resolved into the oxides of cerium and lanthanum and a mixed oxide 'didymia' (a mixture of the oxides of the metals from Pr through Gd). The original yttria was similarly separated into substances called erbia, terbia, and yttria (though some 40 years later, the first two names were to be reversed!). This kind of confusion was made worse by the fact that the newly discovered means of spectroscopic analysis permitted misidentifications, so that around 70 'new' elements were erroneously claimed in the course of the century.

Nor was Mendeleev's revolutionary Periodic Table a help. When he first published his Periodic Table in 1869, he was able to include only lanthanum, cerium, didymium (now known to have been a mixture of Pr and Nd), another mixture in the form of erbia, and yttrium; unreliable information about atomic mass made correct positioning of these elements in the table difficult. Some had not yet been isolated as elements. There was no way of predicting how many of these elements there would be until Henry Moseley (1887-1915) analysed the X-ray spectra of elements and gave meaning to the concept of atomic number. He showed that there were 15 elements from lanthanum to lutetium (which had only been identified in 1907). The discovery of radioactive promethium had to wait until after World War II.

It was the pronounced similarity of the lanthanides to each other, especially each to its neighbours (a consequence of their general adoption of the +3 oxidation state in aqueous solution), that caused their classification and eventual separation to be an extremely difficult undertaking.

Subsequently, it was not until the work of Bohr and of Moseley that it was known precisely how many of these elements there were. Most current versions of the Periodic Table place lanthanum under scandium and yttrium.

1.2 Characteristics of the Lanthanides


The lanthanides exhibit a number of features in their chemistry that differentiate them from the d-block metals. The reactivity of the elements is greater than that of the transition metals, akin to the Group II metals:

  1. A very wide range of coordination numbers (generally 6-12, but numbers of 2, 3, or 4 are known).
  2. Coordination geometries are determined by ligand steric factors rather than crystal field effects.
  3. They form labile 'ionic' complexes that undergo facile exchange of ligand.
  4. The 4f orbitals in the Ln3+ ion do not participate directly in bonding, being well shielded by the 5s2 and 5p6 orbitals. Their spectroscopic and magnetic properties are thus largely uninfluenced by the ligand.
  5. Small crystal-field splittings and very sharp electronic spectra in comparison with the d-block metals.
  6. They prefer anionic ligands with donor atoms of rather high electronegativity (e.g. O, F).
  7. They readily form hydrated complexes (on account of the high hydration energy of the small Ln3+ ion), and this can cause uncertainty in assigning coordination numbers.
  8. Insoluble hydroxides precipitate at neutral pH unless complexing agents are present.
  9. The chemistry is largely that of one (3+) oxidation state (certainly in aqueous solution).
  10. They do not form Ln = O or Ln = N multiple bonds of the type known for many transition metals and certain actinides.
  11. Unlike the transition metals, they do not form stable carbonyls and have (virtually) no chemistry in the 0 oxidation state.

1.3 Occurrence and Abundance of the Lanthanides


Table 1.1 presents the abundance of the lanthanides in Earth's crust and in the solar system as a whole. (Although not in the same units, the values in each list are internally consistent.)

Table 1.1 Abundance of the lanthanides.

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Crust (ppm) 35 66 9.1 40 0.0 7 2.1 6.1 1.2 4.5 1.3 3.5 0.5 3.1 0.8 31 Solar system (with respect to 107 atoms Si) 4.5 1.2 1.7 8.5 0.0 2.5 1.0 3.3 0.6 3.9 0.9 2.5 0.4 2.4 0.4 40.0

Two patterns emerge from these data:

  1. The lighter lanthanides are more abundant than the heavier ones.
  2. The elements with even atomic number are more abundant than those with odd atomic number.

Overall, cerium, the most abundant lanthanide on Earth, has a similar crustal concentration to the lighter Ni and Cu, whilst even Tm and Lu, the rarest lanthanides, are more abundant than Bi, Ag, or the platinum metals.

The abundances are a consequence of how the elements were synthesized by atomic fusion in the cores of stars, with heavy elements only made in supernovae. Synthesis of heavier nuclei requires higher temperature and pressures and so gets progressively harder as the atomic number increases. The odd/even alternation (often referred to as the Oddo-Harkins rule) is again general. It reflects the facts that elements with odd mass numbers have larger nuclear capture cross sections and are more likely to take up another neutron, so elements with odd atomic number (and hence odd mass number) are less common than those with even mass number. Even-atomic-number nuclei are more stable when formed.

1.4 Lanthanide Ores


Traditionally, there are three principal sources of the lanthanides (Table 1.2): Bastnäsite LnFCO3; Monazite (Ln,?Th)PO4 (richer in earlier lanthanides); and Xenotime (Y,?Ln)PO4 (richer in later lanthanides). Since the 1980s, when China became the leading source of these elements, mines have opened in several parts of China - Bayan Obo in Inner Mongolia; Sichuan in the southwest; Weishan in Shandong province in the east; and finally southern China, notably Jianxi province in the southeast. The ores in the Jianxi province are unique to China - ion-absorption ores, weathered granites with lanthanides adsorbed onto the surface of aluminium silicates, which fall into two types.

There is concern that the ion-absorption ores in particular may be depleted by 2025. Among these ion-absorption ones, the Longnan deposits are an example that is low in cerium and early lanthanides but rich in the 'heavier metals' (including yttrium, because the Y3+ ion is similar in size to the later lanthanide ions); these tend to be more sought-after on account of their rarity. The Xunwu deposits, also ion-absorption ores, are rich in the lighter metals, notably neodymium, in demand because of its widespread use in Nd2Fe14B magnetic alloys.

Table 1.2 Typical abundance of the lanthanides in oresa.

(%) La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Monazite 20 43 4.5 16 0 3 0.1 1.5 0.05 0.6 0.05 0.2 0.02 0.1 0.02 2.5 ... Bastnasite 33.2 49.1 4.3 12 0 0.8 0.12 0.17 160 310 50 35 8 6 1 0.1

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