Predicting ionic compound formula units: Ionic substances exist with separate anions and cations arrayed in a regular pattern. The negative and positive charges cancel in the array. The formula unit for an ionic compound can be predicted using the charges on the respective ions as subscripts. The illustration shows how this works for the combination of calcium ions and chloride ions.

The formula unit for the compound is shown here. The formula says that the 2+ charge on the calcium ion can be cancelled out by two chloride ions.

This illustration shows how to form a formula unit for the combination of aluminum ions and oxide ions. The formula says that the 3+ charge on two aluminum ions can be cancelled out by three oxide ions.

Polyatomic ions The trick that works for single atom ions also works for polyatomic ions. The difference here is that a pair of parenthesis must be used to separate the polyatomic ion and the subscript for it. The subscript on the polyatomic ion is written after the parenthesis. The subscript is not multiplied through to alter the subscript on the polyatomic ion.

Exercise: What is the predicted formula for the combination of magnesium ion, Mg2+, and carbonate ion, CO32-? click here for answer

 

Exercise: What is the predicted formula for the combination of ammonium ion, NH41+, and phosphate ion, PO43-? click here for answer