![]() (In fact, it is ionic.) In contrast, the compound NO 2 contains two elements that are both nonmetals (nitrogen, from group 15, and oxygen, from group 16. Therefore, it is most likely an ionic compound. For example, CaBr 2 contains a metallic element (calcium, a group 2 metal) and a nonmetallic element (bromine, a group 17 nonmetal). First, compounds between metal and nonmetal elements are usually ionic. There are two ways to recognize ionic compounds. Write the symbol and charge of the cation followed by the symbol and charge of the anion. Writing a formula for ionic compounds containing polyatomic ions also involves the same steps as for a binary ionic compound. This is to show that the subscript applies to the entire polyatomic ion. If more than one of a particular polyatomic ion is needed to balance the charge, the entire formula for the polyatomic ion must be enclosed in parentheses, and the numerical subscript is placed outside the parentheses. The rule for constructing formulas for ionic compounds containing polyatomic ions is the same as for formulas containing monatomic (single-atom) ions: the positive and negative charges must balance. It is important to understand the relationship between the name, formula, and charge of polyatomic ions before beginning to use them within ionic compounds. Polyatomic ions were introduced in the previous subsection. Writing Formulas for Ionic Compounds Containing Polyatomic Ions ![]() In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Leave out all charges and all subscripts that are 1.Īn alternative way to writing a correct formula for an ionic compound is to use the crisscross method. Use the multipliers as subscript for each ion.Ĥ. Total charge of cations = total charge of anionsģ. Use a multiplier to make the total charge of the cations and anions equal to each other. ![]()
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