An ionic compound is represented by a formula that lists the symbol for each element in the compound, with subscript numbers indicating the number of each atom present in a single molecule. For example, the ionic formula for combing the Ca^+2 ion with a Cl^-1 ion is CaCl2.
In formulas for ionic compounds, standard notation lists the cation before the anion. The cation is the ion with a positive charge, while the anion is the ion with a negative charge. In the formula CaCl2, Ca^+2 is the cation and Cl^-1 is the anion. The number of atoms of each element is determined by balancing out the charges.