Aluminum is the older term, while
aluminium was created later by the British to make it sound more
like the other elements. Here is a timeline:
1808: Sir Humphrey Davy isolates the metal for the first time. He
calls it alumium
1812: Sir Humphrey decides to change the spelling of his element: he
renames it to aluminum (the term adopted in the United States)
1812: British scientists dislike the new name and change it to
aluminium to match the other classic sounding elements, such as
Magnesium, Helium, Potassium, etc.
That's my symposium on aluminum. - Incidentally, the Greek
symposium was originally a drinking party and forum for men of
good family to debate, plot, boast, or simply to revel with
others.