Question 4.9: Determine the chemical formulas for the ionic compounds that......

a. In order to equalize the total positive and negative charge, we need two sodium ions (+1 charge) for each sulfate ion (-2 charge). We indicate the presence of two Na^{+} ions with the subscript 2 following the symbol of this ion. The chemical formula of the compound is Na_{2}SO_{4}. The convention that the positive ion is always written first in a chemical formula still holds when polyatomic ions are present.
b. Two nitrate ions (-1 charge) are required to balance the charge on one magnesium ion (+2 charge). Because more than one polyatomic ion is needed, the chemical formula contains parentheses, Mg(NO_{3})_{2}. The subscript 2 outside the parentheses indicates two of what is inside the parentheses. If parentheses were not used, the chemical formula would appear to be MgNO_{32}, which is not intended and conveys false information.
c. In this compound, both ions are polyatomic, which is a perfectly legal situation. Because the ions have equal but opposite charges, they combine in a one-to-one ratio. Thus the cheical formula is NH_{4}CN. No parentheses are necessary because we need only one polyatomic ion of each type in a formula unit. The appearance of the symbol for the element nitrogen (N) at two locations in the chemical formula could be prevented by combining the two nitrogens, resulting in N_{2}H_{4}C. But the chemical formula N_{2}H_{4}C does not convey the message that NH_{4}  ^{+} and CN^{-} ions are present. Thus, when writing chemical formulas that contain polyatomic ions, we always maintain the identities of these ions, even if it means having the same elemental symbol at more than one location in the formula.