the name of Al2(SO4)3 is..

Question

Aluminum sulfate, but my question is is why? At first, I thought the answer was Aluminum (III) Sulfate because of the 3 behind the parenthesis but apparently that isn't correct. Can you explain to me why the formula is named this way please. I know it has something to do with the charges, but I am still struggling with that portion of naming.

Answer 1

Al has a 3⁺ charge, and SO₄ has a 2^- charge.  So, it takes 3 of the SO₄ anions to balance 2 of the Al cations.  So, you get Al₂(SO₄)₃.  Now, for naming this compound, since Al is ONLY 3⁺ and does not have variable valence, you don't use the (III) in parentheses.  The Roman number in parentheses is ONLY used for transition metals that have variable valence, such as Cu(I) or Cu(II), or Fe(II) and Fe(III), etc. This is called the Stock system.  If this doesn't make sense, message me or comment on this answer.

Here are some examples:

FeCl₃ is iron(III) chloride

CuCl is copper(I) chloride

CuCl₂ is copper(II) chloride

ZnCl₂ is zinc chloride (you don't put a II in parentheses because Zn is ONLY 2+. It has no other valence.