Saturday, November 13, 2010

Is Scandium A Rare Earth Element?

Currently being grouped with titanium and zirconium in the First Transition Metals part of the periodic table, does the element scandium rightfully belong in the rare earth metals section of the Periodic Table?

By: Ringo Bones

The last time scandium gained widespread press coverage was back in the mid 1990s when this “space age” first transition metal was used in the manufacture of light weight but strong revolvers and related handguns due to the fact that scandium is as light as aluminum but with a much higher melting point. Progress then for such an under-utilized “space age” metal since chemically pure scandium was only produced in one pound quantities only as recently in 1960. But the question now is, does scandium belong to in the rare earth “leg” of the Periodic Table rather than in the first transition metals group?

Even though the International Union of Pure and Applied Chemistry or IUPAC has since placed scandium in the Group III B of the first transition metals, it does have chemical properties and an atomic structure that eerily mimics that of the lanthanide or rare earth elements. During the “space age” science boom of the 1960s, the element scandium, Sc, is usually associated chemically with the lanthanide or rare earth elements of atomic numbers 57 to 71, although its electron structure does not conform to this long series system. Scandium’s differentiating electron, compared to its predecessor, calcium, is in a (n-1) state rather than a (n-2) level. Scandium is found to be a slight fraction of the total in rare-earth minerals – such as monazite, gadolinite, etc. The most abundant source of scandium is the mineral thortveitite.

To separate the element, the ore is first digested with strong sulfuric acid or fused with potassium hydrogensulfate. Water treatment removes the soluble sulphate salts. Unwanted heavy metals are removed by precipitation as sulfides or by careful adjustment of the hydroxide-ion concentration. All of the rare earths, including scandium, are precipitated by addition of oxalic acid. Finally, scandium is separated from the other elements – all of which form soluble oxalates – by forming a sulfate called scandium sulfate pentahydrate which is more soluble than the rare earth sulfates.

Scandium has also been prepared by the electrolysis of its fused chloride on a zinc cathode. The zinc metal is removed from the deposited scandium metal by volatilizing the former. Scandium differs from the other members of the Group III B elements in forming less-basic oxides, though the oxide is still not amphoteric – i.e. soluble in excess base. The chloride is more volatile and the nitrate more easily decomposed. The complexes are somewhat more stable, and surprisingly is the fact that it will form a normal carbonate in view of the relatively small size and the high charge exhibited by the trivalent ion of scandium.

For sometime now, the Union of Pure and Applied Chemistry (IUPAC) has placed scandium in the Group III B of the Periodic Table of the Elements even though chemically, it does possess some very intriguing chemical properties that make it behave as if it is a rare earth element. Given that Mendeleyev’s “construction” of the Periodic Table of the Elements primarily sorts the individual elements by their chemical properties while atomic structure is only a notch below this in terms of importance, shouldn’t scandium be a rare earth element because it has rare earth chemical properties?

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