Unit B Lesson 9: Multivalent  and Polyatomic Ions

 Learning Targets

Big Question: How do I name and write formulas for ionic compounds that contain multivalent ions?
 Some elements form multivalent ions. This means that there are two (or more) possible ions that can be used when forming an ionic compound.

At the end of this inquiry, you should be able to answer the following questions:

  • What is a multivalent ion?
  • How is an ionic compound formula written when it contains a multivalent metal ion?
  • What is a polyatomic (complex) ion?
  • How is an ionic compound formula written when it contains a polyatomic ion?
Page 146 to 147  in your text will help you answer these questions about multivalent and polyatomic ions.

Periodic Table links:
Online ADLC Periodic Table
Online Interactive Periodic Table
Printable ADLC Periodic Table
 




Introduction


Figure 1 – Copper oxides
Multivalent Ions

Both of the  solids in Figure 1 are copper oxide, but they are completely different compounds. How is this possible? How do we tell them apart?

Certain elements can form more than one type of ion. You may have noticed a few elements on the periodic table that have more than one ion charge. Look at copper, for example, atomic number 29. It shows charges of 1+ or 2+. This means that if copper is involved in a chemical reaction, either a Cu1+ ion or a Cu2+ ion can form a new compound.

The term multivalent is used to describe these ions. Multivalent means "multiple ways to gain or lose electrons".


Figure 2 – Copper

Watch

Multiple Ion Charges

Watch the following video to learn how to deal with ions that have multiple charges.

What are multivalent ions?

Multivalent ions are metal ions that have more than one possible charge. 
These metals are listed in the middle of the periodic table.

Some examples of multivalent ions:

  • Fe2+ and Fe3+
  • Cu+ and Cu2+
  • Pb2+   and Pb4+
Roman numerals are used to differentiate multivalent ions.  The Roman numeral indicates the charge of the ion.

  • One (I) Two (II) Three (III) Four (IV) Five (V) Six (VI) Seven (VII)
  • For example: Cu+ is written as copper(I), pronounced “copper one”.
  • Cu2+ is written as copper(II), pronounced “copper two”.

How do I write the formula for an ionic compound that contains a multivalent metal ion?

Writing formulas for ionic compounds that contain a multivalent metal ion (such as copper(I) oxide) is the same as writing the formula for a compound that does not contain a multivalent ion (such as lithium oxide).  The only difference is that the charge of the metal ion is identified when the ionic compound contains a multivalent compound.

Write the formula for copper(I) oxide.

Step 1:  Write symbols.

Cu        O
Step 2:  Write charges.

Cu+       O2–
Remember that the Roman numeral gives the ion’s charge,
You do not need to look up the metal ion’s charge on the periodic table.


Figure 3 – Cu2O, Copper (I) oxide
Step 3:  Balance ions so the net charge is zero.

Cu+       O2–
Cu+      
Step 4:  Write the formula, removing all charges.

Cu2O
Step 5:  Check
(+1) + (+1) + (–2) = 0
Write the formula for copper(II) oxide.

Step 1:  Write symbols.

Cu        O
Step 2:  Write charges.

Cu2+        O2–
Remember that the Roman numeral gives the ion’s charge.
You do not need to look up the metal ion’s charge on the periodic table.


Figure 4 – CuO, Copper (I) oxide
Step 3:  Balance ions so net charge is zero.

Cu2+        O2–

Step 4:  Write the formula removing all charges.

CuO
Step 5:  Check
(+2) + (–2) = 0

Try It!

Practice Worksheet: Writing Ionic Formulas Part 2
(Part 1 appears in the previous lesson)
  1. DOWNLOAD the practice worksheet (S9_UB_S3_L9a_ionic_formulas2). If you prefer to use a Google Drive or PDF version of the worksheet, click here.

  2. Write the formulas of the ionic compounds. If you need help, click the links on the worksheet for video explanations for each compound.

  3. When you are satisfied with your responses you can check your work by clicking on the "SUGGESTED ANSWERS" button below.

    Wait! Don't view the suggested answers first. This practice work is not for marks, it is meant to help you check your understanding. Check the answers AFTER doing the questions! Keep the practice worksheet for study purposes. If you don't understand something, contact your teacher!

Naming Ionic Compounds that contain a Multivalent Metal (metals that have more than one possible charge)

Step 1
Name the metal.

Step 2
Write the name of the non-metal second (change the ending to “ide”).

Step 3
If the ion has more than one possible charge, identify the ion with roman numerals:
One (I) Two (II) Three (III) Four (IV) Five (V) Six (VI) Seven (VII)
Example

Name the compound, NiCl2

Step 1:   Name the metal

  • nickel

Step 2:  Write the name of the non-metal second (change the ending to “ide”).

  • chloride

Step 3:   If the ion has more than one possible charge, identify the ion with Roman numerals.

  • There is one nickel ion and two chloride ions (Cl–, Cl–). 
  • Because the total negative charge for two chloride ions is –2, the total positive charge is +2.
  • The nickel ion must have a +2 charge.
  • The name of NiCl2 is nickel(II) chloride.

Example

Name the compound Fe2O3

Step 1:  Name the metal.

  • iron

Step 2:   Write the name of the non-metal second (change the ending to “ide”).

  • oxide

Step 3:   If the ion has more than one possible charge, identify the ion with Roman numerals.

  • There are two iron ions and three oxide ions (O2–, O2–, O2–).
  • Because the total negative charge for three oxide ions is –6, the total positive charge has to be +6.
  • The two iron ions must divide the +6 charge equally.  Therefore, each iron ion must have a +3 charge (Fe3+, Fe3+).
  • The name of Fe2O3 is iron(III) oxide.

 Watch

Watch how to name NiCl2

 
 
Watch how to name Fe2O3


Try It!

Practice Worksheet: Naming Ionic Compounds Part 2

  1. DOWNLOAD this practice worksheet (S9_UB_S3_L9b_ionic_compounds2) worksheet.  If you prefer to use a Google Drive or PDF version of the worksheet, click here.

  2. Write the formulas of the ionic compounds. If you need help there are links on the worksheet for video explanations for the first three compounds. 

  3. When you are satisfied with your responses you can check your work by clicking on the "SUGGESTED ANSWERS" button below.

    Wait! Don't view the suggested answers first. This practice work is not for marks, it is meant to help you check your understanding. Check the answers AFTER doing the questions! Keep the practice worksheet for study purposes. If you don't understand something, contact your teacher!


Watch

Polyatomic Ions

As you study chemistry more, you may run across ionic compounds that involve polyatomic ions. Polyatomic ions (sometimes called complex ions) are groups of ions that act like a single ion. An example is OH–, also known as the hydroxide ion. Notice that there are two different atoms in the ion; oxygen and hydrogen. The two act like a single ion, and join with a positive cation like sodium (Na+) to form  the compound NaOH.

Don't worry, you won't have any of these special cases showing up on any of your quizzes or exams in this course. However, they are very interesting and understanding them will you advance your knowledge of how ionic compounds form. So please watch and enjoy the following video on polyatomic ions.