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Rare Earths

"Lanthanons-these elements perplex us in our researches, baffle us in our speculations, and haunt us in our very dreams. They stretch like an unknown sea before us; mocking, mystifying and murmuring strange revelations and possibilities." Sir William Crookes spoke these well-quoted words in an address to the Royal Society in February 1887, when all but three of the lanthanoid elements had been isolated.

Lanthanoid (current IUPAC terminology... until recently called lanthanide) chemistry started in Scandinavia. The story of the lanthanoids begins in 1787 when a young Swedish artillery officer, Lieutenant Carl Axel Arrhenius, who was a keen amateur geologist, was exploring a quarry at a small town called Ytterby, near Stockholm. He found a new, very dense black mineral which he named ytterbite. At the time there was some speculation that the mineral might contain the recently discovered element tungsten, but the first serious chemical analysis was carried out in 1794 by Johan Gadolin, a Finnish chemist. Methods of chemical analysis were limited in the 18th century, but after a series of treatments with acids and alkalis, Gadolin was able to show that the new mineral contained oxides of iron, beryllium, and silicon and a new, previously unidentified 'earth' which he named 'yttria'. (At the time, the term 'earth' was applied rather loosely to insoluble metal oxides.) Yttria was later shown to be a mixture of the oxides of six rare earth elements.

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) analyzed 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 lanthanoids to each other, especially each to its neighbors (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.
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The actinoid series encompasses the 15 chemical elements that lie between actinium and lawrencium included on the periodic table, with atomic numbers 89 - 103. The actinoid series derives its name from the first element in the series, actinium, and ultimately from the Greek aktis, "ray," reflecting the elements' radioactivity.

The actinoid series (An) is included in some definitions of the rare earth elements. IUPAC is currently recommending the name actinoid rather than actinide, as the suffix "-ide" generally indicates ions (moreover, from Latin, the suffix -ide means "sons of actinium", while -oid means "similar to actinium"). There are alternative arrangements of the periodic table that exclude actinium or lawrencium from appearing together with the other actinoids.

The actinoids display less similarity in their chemical properties than the lanthanoid series (Ln), exhibiting a wider range of oxidation states, which initially led to confusion as to whether actinium, thorium, and uranium should be considered d-block elements. All actinoids are radioactive.

Only thorium and uranium occur naturally in the earth's crust in anything more than trace quantities. Neptunium and plutonium have been known to show up naturally in trace amounts in uranium ores as a result of decay or bombardment. The remaining actinides were discovered in nuclear fallout, or were synThesized in particle colliders. The latter half of the series possess exceedingly short half-lives. > >

The actinoids - in standard flat periodic tables - are typically placed below the main body of the periodic table (below the lanthanoid series), in the manner of a footnote. The full-width version of the periodic table shows the position of the actinoids more clearly, and in the Alexander Arrangement 3-D model, is fully integrated.

An organometallic compound of an actinoid is known as an organoactinoid.

From the earlier known chemical properties of actinium (89) up to uranium (92), indicating a relation to the transition metals, it was generally assumed that the transuraniums would have similar qualities. During his Manhattan Project research in 1944, Glenn T. Seaborg experienced unexpected difficulty isolating americium (95) and curium (96). He began wondering if these elements more properly belonged to a different series than the transition metals, which would explain why the expected chemical properties of the new elements were different. In 1945, he went against the advice of colleagues and proposed the most significant change to Mendeleev's periodic table to have been accepted universally by the scientific community: the actinide series.

In 1945, Seaborg published his actinide concept of heavy element electronic structure, predicting that the actinides would form a transition series analogous to the rare earth series of lanthanoid elements.


The Rare Earths - Lanthanoids and Actinoids

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