COMPOUNDS

5.8 - Chemical Formulas for Ionic compounds

5.8.3 - Using parentheses in formula writing

In section 5.8.2 when we wrote the chemical formula for strontium nitrate we needed to put in a bracket around the NO₃^- ions to represent that in strontium nitrate, there are two NO₃^- ions.

In general,

When writing the chemical formula of a compound that contains a polyatomic ion, in order to indicate more than one polyatomic ion in the formula, put parentheses around the polyatomic ion before writing the subscript.

This does not apply if the subscript is '1'. When the subscript is '1', there is no need for any parentheses as it is usually not written.

For example,

Chemical Name	Correct Chemical Formula	Incorrect Chemical Formula
ammonium sulfate Ions: NH4+, SO42-	(NH4)2SO4 Not sure how to figure out this formula? Click to review section 5.8.2.	(NH4)2(SO4) There is an extra bracket around the 'SO4' unit. (NH4)2(SO4)1 Subscript of '1' is usually not written.
aluminum sulfate Ions: Al3+, SO42-	Al2(SO4)3 Not sure how to figure out this formula? Click to review section 5.8.2.	(Al)2(SO4)3 There is an extra bracket around 'Al'.
calcium chloride Ions: Ca2+, Cl-	CaCl2 Not sure how to figure out this formula? Click to review section 5.8.1.	Ca(Cl)2 There is an extra bracket around 'Cl'.

Section 6.9
Writing Formulas ..p147

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