Chemistry / Unit 2 Chem 0309 worked example, 60 to 90 seconds
Writing Formulas for Ionic Compounds by Charge Balance
The idea
Writing an ionic compound's formula means choosing whole-number counts of each ion so the total positive charge exactly cancels the total negative charge.
In practice
Combining Al3+ with O2− requires two aluminum ions, for six positive charges, against three oxide ions, for six negative charges, giving Al2O3. Combining Ca2+ with the phosphate ion, PO4 3−, requires three calcium ions against two phosphate ions, giving Ca3(PO4)2, with parentheses marking the polyatomic ion as a repeated unit. Combining Na+ with S2− requires two sodium ions to balance one sulfide ion's two negative charges, giving Na2S with no parentheses needed since sulfide is not polyatomic.
Where people slip
Writing a polyatomic ion's subscript without parentheses when more than one is needed, as in "Ca3PO42," obscures which atoms belong to the repeated ion and which are separate calcium atoms.
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