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Etymology/Nomenclature history
In 1808, Humphry Davy originally proposed the name alumium while trying to isolate the new metal electrolytically from the mineral alumina. In 1812, he changed the name to aluminum to match its Latin root. The same year, an anonymous contributor to the Quarterly Review, a British political-literary journal, objected to aluminum and proposed the name aluminium.

Aluminium, for so we shall take the liberty of writing the word, in preference to aluminum, which has a less classical sound. (Q. Review VIII. 72, 1812)

This had the advantage of conforming to the -ium suffix precedent set by other newly discovered elements of the period: potassium, sodium, magnesium, calcium, and strontium (all of which Davy had isolated himself). Nevertheless, -um spellings for elements were not unknown at the time. Platinum, which had been known to Europeans since the 16th century, molybdenum, which was discovered in 1778, and tantalum, which was discovered in 1802, all have spellings ending in -um.

The United States adopted the -ium for most of the 19th century with aluminium appearing in Webster's Dictionary of 1828. However, in 1892 Charles Martin Hall used the -um spelling in an advertising handbill for his new efficient electrolytic method for the production of aluminium, despite using the -ium spelling in all of his patents filed between 1886 and 1903. It has consequently been suggested that the spelling on the flier was a simple spelling mistake rather than a deliberate choice to use the -um spelling. Hall's domination of production of the metal ensured that the spelling aluminum became the standard in North America, even though the Webster Unabridged Dictionary of 1913 continued to use the -ium version.

In 1926, the American Chemical Society officially decided to use aluminum in its publications, and American dictionaries typically label the spelling aluminium as a British variant.

Present-day spelling
In English-speaking countries, the spellings (and associated pronunciations) aluminium and aluminum are both in common use in scientific and nonscientific contexts. In the United States and English-speaking Canada, the spelling aluminium is largely unknown, and the spelling aluminum predominates. Elsewhere in other English-speaking countries the spelling aluminium predominates, and the spelling aluminum is largely unknown.

Outside English, the "ium" spelling is widespread: the word is aluminium in French, Aluminium in German, and identical or similar forms are used in many other languages. Consequently it is the more common of the two spellings in global terms, even though there may be more users of aluminum in the English-speaking world.

The International Union of Pure and Applied Chemistry (IUPAC) adopted aluminium as the standard international name for the element in 1990, but three years later recognized aluminum as an acceptable variant. Hence their periodic table includes both, but places aluminium first [7]. IUPAC officially prefers the use of aluminium in its internal publications, although several IUPAC publications use the spelling aluminum.

[Quote from: Wikipedia, the free encyclopedia, http://en.wikipedia.org/wiki/Aluminium]