Lesson 3: Ions

Part c: Polyatomic Ions

Part 3a: Metals, Nonmetals, and Ions
Part 3b: Transition Metal Ions
Part 3c: Polyatomic Ions

 

What is a Polyatomic Ion?

Previously in Lesson 3 we have discussed monatomic ions formed from main group elements and transition metal elements. Monatomic ions are charged particles that include a single atom. Because the number of protons is unequal to the number of electrons, the particle has an overall charge.
   
A polyatomic ion includes multiple atoms packaged together as a single unit. The total number of protons contributed by all the atoms is not equal to the number of electrons. Thus, the particle is charged. Here are five of many examples of polyatomic ions:
   
 
 

Contents of a Polyatomic Ion

Polyatomic ions consist of a combination of two or more atoms packaged together as a single unit. Just as a molecule is different than an atom, so a polyatomic ion is different from a monatomic ion. A molecule is a neutral particle that contains two or more atoms; CO₂, NH₃, and Cl₂ are examples. In the same way, a polyatomic ion like NH₄⁺, CO₃^2-, and O₂^2- consist of two or more atoms bound together as a unit. But the difference between a molecule and a polyatomic ion has to do with the comparison of the number of protons and electrons within the particle. In the case of a molecule, the numbers of protons and electrons is equal or balanced and the particle is neutral. In the case of a polyatomic ion (or any ion), there are unequal numbers or protons and electrons and the particle has an overall charge.
   
The atoms within a polyatomic ion are held together by chemical bonds. The atoms form a single, charged unit. The particle diagrams below represent four common polyatomic ions.
   
The formula or symbolic representation of a polyatomic ion indicates what atoms and how many are present in the ion. The charge is represented as a superscript at the tail end of the formula. The polyatomic ion H₃O⁺ contains 3 atoms of hydrogen and one atom of oxygen. The overall charge of this bound collection of atoms is +1. The polyatomic ion SO₄^2- contains 1 atom of sulfur and four atoms of oxygen. The overall charge of this bound collection of atoms is -2. Here are three more examples:
   
The number of protons and electrons in a polyatomic ion can be determined by using the ion charge and the ion formula. The ion formula will allow you to determine the number of protons. Use the periodic table to determine the number of protons for each atom. Then sum the numbers to determine the total number of protons. Once done, the ion charge can be used to determine the number of electrons. Here are two examples for NH₄⁺ and CO₃^2-.
   
In the process demonstrated above, a 1+ charge indicates that there is one more proton than electrons. Thus, the number of electrons is determined by subtracting one from the number of protons. A 2- charge indicates that there are two more electrons than protons. Thus, the number of electrons is determined by adding two to the number of protons.

   
 
 

Names and Formulas

Monatomic ions are formed when atoms of elements gain or lose one or more electrons. Polyatomic ions are not necessarily formed in the same manner. For instance, the OH^- ion is not formed by adding an electron to an OH molecule. And an NH₄⁺ ion is not formed by removing an electron from an NH₄ molecule. The formation of polyatomic ions is a bit more complicated and varied compared to monatomic ions. The result is that polyatomic ion names and their resulting formulas are not as intuitive as they are for monatomic ions. Such names and formulas must either be memorized or determined by consulting a polyatomic ion list. Check with your instructor to determine the expectation for your own course.
 
The table below provides names and formulas for the several common polyatomic ions.
   
 
 
In the next chapter of our Chemistry Tutorial, we will discuss ionic compounds that are formed by the electrical attraction of oppositely charged ions. Some compounds include polyatomic ions. We will learn how to use the names and formulas of polyatomic ions to write the names and formulas of these ionic compounds.
 
   
 

Before You Leave

• Download our Table of Polyatomic Ions. Save it to a safe location and use it throughout the course. Download Table.
• The Check Your Understanding section below include questions and problems with answers and explanations and solutions. It provides a great chance to self-assess your understanding.

 
 
 

Check Your Understanding

Use the following questions to assess your understanding. Tap the Check Answer buttons when ready.
 
1. Use the particle diagrams to write the formulae for the three polyatomic ions.
   
 

2. Determine the number of protons and the number of electrons in the following polyatomic ions.

1. CN^-
2. H₃O⁺
3. SO₄^2-
4. C₂H₃O₂^-