# What is the analogue of “molecule” for ionic compounds?

In a system, if we have $\pu{18 g}$ of $\ce{H2O}$ ($M_\mathrm r = 18$), we can say we have a mole of water molecules or $6 \times 10^{23}$ molecules. But in another system, if we have $\pu{342 g}$ of $\ce{Al2(SO4)3}$ ($M_\mathrm r = 342$), what is the best way to say that there is one mole of "aluminum sulfates" or $6 \times 10^{23}$ "aluminum sulfates"?

Since it is a ionic compound, we can't say molecules, but I can't find an analogous form for ionic compounds. I thought about saying "a mole of aluminum sulfate ionic aggregates", but it still doesn't too good, since I've never seen a definition of ionic aggregate as a unit of the ionic compound formula. It only tells the way the ions organize themselves: as an aggregate.

I know it is a silly question, but in terms of language and symbolism, it is important to me as a chemistry teacher.

• Note that you might need to give amounts of each ion separately anyway, due to common ion effect. So having only one salt with equal parts of cat and an will be a special case. – JDługosz Sep 12 '17 at 4:48

$\pu{58.44 g}$ of $\ce{NaCl}$ contains $6.022 \times 10^{23}$ formula units of sodium chloride
• Depending on what is meant by a chemical formula, the terms chemical species, and molecular entity might also be what OP is after. In short, chemical species is a collection of $\ce{NaCl}$ formula units (e.g., as a big solid) whereas molecular entity is one particular unit (molecule, atom, radical, etc). Ion pair is also pertinent in this case. See DOI: 10.1351/pac199466051077 for more details. Feel free to add to your answer if you like. – Linear Christmas Sep 11 '17 at 20:13