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Lesson 2.8: Naming Inorganic Compounds that Include Polyatomic Ions

← Lesson 2.7: Lesson 2.9: →

When a neutral molecule like an acid loses one or more protons the species which remains is still covalently bonded but now has a negative charge. It is called a negative polyatomic ion, where poly- means many. The picture below shows the formation of the sulfate ion, SO42-. Polyatomic ions interact with ions of opposite charge just like monatomic ioins to form neutral compounds with no net charge.

Common Polyatomic Ions and Acids

Example 1: What is the formula for calcium hydroxide.

The charge on the calcium ion is 2+, and on the hydroxide 1-. To show a neutral compound we need two hydroxide ions for every one calcium ion.

Name Cation Anion Formula Unit Formula
calcium hydroxide Ca(OH)2

Example 2: What is the formula for potassium hydroxide.

The charge on the potassium ion is 1+, and on the hydroxide 1-. To show a neutral compound we need one hydroxide ions for every one potassium ion. Note that since only one hydroxide ion is needed we do not surround it with parentheses.

Name Cation Anion Formula Unit Formula
potassium hydroxide KOH

Example 3: What is the formula for potassium carbonate.

The charge on the potassium ion is 1+, and on the carbonate 2-. To show a neutral compound we need one carbonate ion for every two potassium ions. Note that since only one carbonate ion is needed we do not surround it with parentheses.

Name Cation Anion Formula Unit Formula
potassium carbonate K2CO3

Example 4: What is the formula for aluminum nitrate.

The charge on the aluminum ion is 3+, and on the nitrate 1-. To show a neutral compound we need one aluminum ion for every three nitrate ions. Note that since three nitrate ions are needed we surround it with parentheses.

Name Cation Anion Formula Unit Formula
potassium hydroxide Al(NO3)3

Example 5: What is the formula for aluminum sulfate.

The charge on the aluminum ion is 3+, and on the sulfate 2-. To show a neutral compound we need two aluminum ions for every three sulfate ions. Note that since three sulfate ions are needed we surround it with parentheses.

Name Cation Anion Formula Unit Formula
aluminum sulfate Al2(SO4)3

Example 6: What is the formula for calcium phosphate.

The charge on the calcium ion is 2+, and on the phosphate 3-. To show a neutral compound we need three calcium ions for every two phosphate ions. Note that since two phosphate ions are needed we surround it with parentheses.

Name Cation Anion Formula Unit Formula
calcium phosphate Ca3(PO4)2