Both spellings, aluminum, and aluminium, are technically correct, although my spell checker doesn’t like aluminum. The history of the spelling of the word is longer than most words, usually there’s nothing to tell. Humphry Davy a chemist and physicist from the late 18th and early 19 centuries was the start of these spelling confusions.
In 1808 Davy was trying to isolate the metal aluminium from alumina, which is the element of aluminium and oxygen combined. He originally suggested the word be spelled alumium, but in 1812 he changed it to aluminum, which better suited the Latin root. Also in 1812 an anonymous contributor suggested the word be spelled aluminium, this suggestion was placed in a British political-literary journal, a good sign of how much respect scientists, and I might add technical support people, get. This anonymous person suggested the new spelling because it had a more ‘classical’ sound.
It didn’t just stick because of some unknown character though, the ium ending fit with the names of other elements like: potassium, sodium, calcium, and magnesium, although I should add that platinum and a couple other elements were known and named at the time.
In 1926 the American Chemical Society officially decided on the spelling aluminum, but American dictionaries use the British variant, aluminium.
If I were you I’d use aluminium when referring to aluminium; however if you happen to be writing for a scientist you should use the spelling aluminum for aluminum.
