Atoms, Molecules, and Ions. Search for:. Formulas of Ionic Compounds. Learning Objective Generate the empirical formula of an ionic compound given its molecular constituents. Key Points The overall ionic formula for a compound must be electrically neutral, meaning it has no charge.
When writing the formula for the ionic compound, the cation comes first, followed by the anion, both with numeric subscripts to indicate the number of atoms of each. Polyatomic ions form ionic bonds in the typical way, balancing so that the overall compound is electrically neutral. 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.
This is to show that the subscript applies to the entire polyatomic ion. An example is Ba NO 3 2. Writing a formula for ionic compounds containing polyatomic ions also involves the same steps as for a binary ionic compound.
Write the symbol and charge of the cation followed by the symbol and charge of the anion. Write the chemical formula for an ionic compound composed of the potassium ion and the sulfate ion.
There are two ways to recognize ionic compounds. First, compounds between metal and nonmetal elements are usually ionic. For example, CaBr 2 contains a metallic element calcium, a group 2 [or 2A] metal and a nonmetallic element bromine, a group 17 [or 7A] nonmetal.
Therefore, it is most likely an ionic compound. In fact, it is ionic. In contrast, the compound NO 2 contains two elements that are both nonmetals nitrogen, from group 15 [or 5A] , and oxygen, from group 16 [or 6A].
It is not an ionic compound; it belongs to the category of covalent compounds discussed elsewhere. Also note that this combination of nitrogen and oxygen has no electric charge specified, so it is not the nitrite ion. Second, if you recognize the formula of a polyatomic ion in a compound, the compound is ionic.
It becomes easier with practice! Many familiar chemicals are ionic compounds. A metal bonded to a nonmetal is a dead giveaway that you're dealing with an ionic compound.
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