chemistry worksheet naming & formula writing (ionic)

Naming and writing formulas for ionic compounds

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Ahmed Hammad Madin

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5.5: Writing Formulas for Ionic Compounds

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Learning Objectives

• Write the correct formula for an ionic compound.
• Recognize polyatomic ions in chemical formulas.

Ionic compounds do not exist as molecules. In the solid state, ionic compounds are in crystal lattice containing many ions each of the cation and anion. An ionic formula, like \(\ce{NaCl}\), is an empirical formula. This formula merely indicates that sodium chloride is made of an equal number of sodium and chloride ions. Sodium sulfide, another ionic compound, has the formula \(\ce{Na_2S}\). This formula indicates that this compound is made up of twice as many sodium ions as sulfide ions. This section will teach you how to find the correct ratio of ions, so that you can write a correct formula.

If you know the name of a binary ionic compound, you can write its chemical formula . Start by writing the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral, decide how many of each ion is needed in order for the positive and negative charges to cancel each other out.

Example \(\PageIndex{1}\): Aluminum Nitride and Lithium Oxide

Write the formulas for aluminum nitride and lithium oxide.

An alternative way to writing a correct formula for an ionic compound is to use the crisscross method . In this method, the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are dropped.

Example \(\PageIndex{2}\): The Crisscross Method for Lead (IV) Oxide

Write the formula for lead (IV) oxide.

Exercise \(\PageIndex{2}\)

Write the chemical formula for an ionic compound composed of each pair of ions.

• the calcium ion and the oxygen ion
• the 2+ copper ion and the sulfur ion
• the 1+ copper ion and the sulfur ion

Be aware that ionic compounds are empirical formulas and so must be written as the lowest ratio of the ions.

Example \(\PageIndex{3}\): Sulfur Compound

Write the formula for sodium combined with sulfur.

Exercise \(\PageIndex{3}\)

Write the formula for each ionic compound.

• sodium bromide
• lithium chloride
• magnesium oxide

Polyatomic Ions

Some ions consist of groups of atoms bonded together and have an overall electric charge. Because these ions contain more than one atom, they are called polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For example, NO 3 − is the nitrate ion; it has one nitrogen atom and three oxygen atoms and an overall 1− charge. Table \(\PageIndex{1}\) lists the most common polyatomic ions.

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. 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 Formulas for Ionic Compounds Containing Polyatomic Ions

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.

Example \(\PageIndex{4}\): Calcium Nitrate

Write the formula for calcium nitrate.

Example \(\PageIndex{5}\)

Write the chemical formula for an ionic compound composed of the potassium ion and the sulfate ion.

Exercise \(\PageIndex{5}\)

• the magnesium ion and the carbonate ion
• the aluminum ion and the acetate ion

Recognizing Ionic Compounds

There are two ways to recognize ionic compounds.

Method 1

Compounds between metal and nonmetal elements are usually ionic. For example, \(\ce{CaBr2}\) 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 \(\ce{NO2}\) 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. For example, if you see the formula \(\ce{Ba(NO3)2}\), you may recognize the “\(\ce{NO3}\)” part as the nitrate ion, \(\ce{NO3^{-}}\). (Remember that the convention for writing formulas for ionic compounds is not to include the ionic charge.) This is a clue that the other part of the formula, \(\ce{Ba}\), is actually the \(\ce{Ba^{2+}}\) ion, with the 2+ charge balancing the overall 2− charge from the two nitrate ions. Thus, this compound is also ionic.

Example \(\PageIndex{6}\)

Identify each compound as ionic or not ionic.

• \(\ce{Na2O}\)
• \(\ce{PCl3}\)
• \(\ce{NH4Cl}\)
• \(\ce{OF2}\)

Exercise \(\PageIndex{6}\)

• \(\ce{N2O}\)
• \(\ce{FeCl3}\)
• \(\ce{(NH4)3PO4}\)
• \(\ce{SOCl2}\)

Formulas for ionic compounds contain the symbols and number of each atom present in a compound in the lowest whole number ratio.