Aluminium - aluminum
In the UK, the name of the element aluminium is written and pronounced with five syllables: aluminium 'al-you-MIN-ny-um', IPA: /æl juː ˈmɪn ɪ ə(or more pedantically ʌ)m/. (Note the British pronunciation of the first '-u-' as 'you'.)
In the US, both the spelling (aluminum) and the pronunciation have only four syllables: aluminum 'al-OO-min-um', IPA: /æl ˈuː mɪn əm/. (The first '-u-' is a typically American 'oo' pronunciation.)
In British academic English,
aluminum is a mistake, and the pronunciation 'al-OO-min-um' may cause raised eyebrows.