Lesson 1: Ionic Compounds

Part c: Binary Ionic Compounds

Part 1a: Properties of Ionic Compounds
Part 1b: Combining Ions to Form Ionic Compounds
Part 1c: Binary Ionic Compounds
Part 1d: Compounds Containing Polyatomic Ions

 
 

What is a Binary Ionic Compound?

A binary ionic compound is an ionic compound that contains only two elements. One of the elements is a metal and serves as the cation (+). The other element is a nonmetal and serves as the anion (-). Here are a few examples of binary ionic compounds, written in formula form.
   
Binary ionic compounds typically do not contain polyatomic ions. Most polyatomic ions consist of two or more elements. Once the ion forms a compound with an ion of opposite charge, the number of elements exceeds two. By definition, it is not a binary ionic compound. As an example, if sulfate (SO₄^2-) or carbonate (CO₃^2-) or nitrate (NO₃^-), all polyatomic ions, forms an ionic compound with a cation like Na⁺, the result is an ionic compound with three elements. This is shown at the right. There are a small number of exceptions of polyatomic ions (contain two or more atoms) that contain only one element. The peroxide ion (O₂^2-) and the azide ion (N₃^-) are examples. When these polyatomic ions combine with another element to form ionic compounds, the result is a binary ionic compound.
   
 
 
 

Two Types of Binary Ionic Compounds

We distinguish between two types of binary ionic compounds – Type I and Type II. In Type I binary ionic compounds, the two elements are main group elements. The charge of the ions can be determined by their location within the periodic table. Ions of main group elements were discussed thoroughly in Chapter 3. (Review ions of main group elements.) Type II binary ionic compounds include a transition metal ion and a nonmetal ion. Most transition metal ions are multivalent; that is, they form ions that have more than one charge. For instance, iron can be Fe²⁺ or Fe³⁺. When naming these two ions of iron, we distinguish between them by using a Roman numeral in parenthesis – iron(II) and iron(III). Transition metal ions were discussed thoroughly in Chapter 3. (Review ions of transition metal elements.)
 
Of course there are always exceptions. It’s the exceptions to the rule that makes it difficult in Chemistry to keep all things straight. One notable exception is that lead (Pb) and tin (Sn) are two main group elements that are also multivalent. While they form the expected 4+ ion, they also form a 2+ ion. As such, Roman numerals are used to distinguish between the two ions of these elements.
   
The reason we distinguish between Type 1 and Type II binary ionic compounds is because the methods of naming them and writing chemical formulas for them will vary slightly.
     

Nomenclature

Nomenclature is a collection of rules for naming the parts of a system. All fields of science and non-science typically include such rules. In Chemistry, the rules are essential as they allow chemists to communicate clearly about a vast array of chemicals. Slight alterations in the naming of a chemical can be the difference between life and death. Carbon dioxide is a component in the air exhaled by the living. Carbon monoxide is a poisonous gas. The prefixes di- and mon- on the second word are the difference between life and death.
 
The goal of nomenclature is to get the world to speak the same language such that the words we use to refer to chemical substances leaves no ambiguity. The names of binary ionic compounds always include two separate words. The first word is the name of the metal ion. The second word is the name of the nonmetal ion. Names of ions were discussed in Lesson 3 of Chapter 3.
 
For Type II binary ionic compounds: An ion of a transition metal element (and also of lead and tin) can have multiple charges.  A roman numeral enclosed in parentheses must follow the name of the metal so as to leave no ambiguity regarding what ion is in the compound.
 
 
 

From Formula to Name

A common task for Chemistry students is to identify the name of an ionic compound if given its formula. Identifying CaCl₂ as calcium chloride and Fe₂O₃ as iron(III) oxide is a skill used throughout the course. To do so, you must first identify the ions. The metal ion comes first in the formula. Use a periodic table to determine the name associated with the symbol. You must determine the charge for transition metals, lead, and tin. The charge can be determined by using what we learned about ion ratios on the previous page. The nonmetal ion comes second. Remember the -ide ending for main group nonmetals. Once you have identified the ions, use the two steps (above) to write the name of the ionic compound. Here are some examples.  
The Roman numeral is included for transition metals, lead, and tin since they can have more than one ion charge. This is the case in rows e through n. Give attention to the charge on the anion and the subscripts in the formula to determine the ion charge. Be guided by the principle that the net charge of an ionic compound is 0. The sum of all negative charge from the anion balances the sum of all positive charge from the cation. Here’s how the principle is applied for row k through n.
   
 
 

Formula Writing

Chemistry students need to learn how to write the formula of an ionic compound if given the name of the ionic compound. The name consists of two words. The first word is the cation (+ ion). The second word is the anion (- ion). Use the periodic table to determine the symbol and the charge of the cation and the anion. (Review Lesson 3 of Chapter 3 if necessary.) If the cation is a transition metal, lead, or tin, then the charge will be indicated in the name with a Roman numeral inside parenthesis. Write down the symbols with the charge for the two ions. Then determine the ion ratio knowing that the ionic compound is overall neutral. (Review Lesson 1b of this chapter or use our Study Card.) The ion ratio can be used to determine the subscripts in the formula. If a subscript is 1, then it is usually not written. The table provides several examples.
   
 
 

Criss-Cross Method for Formula Writing

When it comes to formula writing, many Chemistry students prefer the so-called crisscross method. The crisscross method bypasses the task of determining the ion ratio. The method goes like this:

1. Determine the symbol of the cation; include the charge.
2. Determine the symbol of the anion; include the charge.
3. Write the formula from the symbols. The charge on the cation is the subscript on the anion symbol. The charge on the anion is the subscript on the cation symbol.
4. If needed, reduce the subscripts to the lowest whole numbers. For instance, Ca₂O₂ can be reduced to CaO.

 
The examples below show the crisscross method being used to write formulae from names.
   
 
 
Before You Leave

• Download our Study Card on Binary Ionic Compounds. Save it to a safe location and use it as a review tool.
• It was discussed on a different page, but if you missed it, then you might want to visit our Ion Ratio page or even download the Study Card on Ion Ratios.
• Try our Concept Builder titled Names to Formula 1. All three activities in the Concept Builder would provide great interactive practice with identifying the formulas for Type I and Type II binary ionic compounds.
• The Check Your Understanding section below include questions with answers and explanations. 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. Use our Periodic Table if necessary.
 
1. Explain when a Roman numeral inside of parenthesis should be used when naming a compound.
 


 
2. What’s Wrong with This?
A teacher is grading a quiz on nomenclature. She sees the following formulas. Identify what is wrong with each formula.

1. Calcium chloride → Ca₂Cl
   
   Check Answer
   The charge on Ca is 2+. So the subscript 2 should be on the chloride ion. Two Cl^- ions are needed for every one Ca²⁺ ion. CaCl₂ is the appropriate formula.
   
   
    
   
    
2. Barium oxide → Ba₂O₂
   
   Check Answer
   The correct formula is BaO. Barium and oxide ions have opposite types and the same amount of charge. So they are present in the compound in a 1:1 ion ratio. The identical subscripts of 2 often show up when students use the crisscross method and fail to reduce the subscript to the lowest whole number ratios.
   
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3. Aluminum chloride → Al₁Cl₃
   
   Check Answer
   If there is only 1 of an ion present in the formula, then the 1 is not inserted as a subscript. The more appropriate way to write this is a AlCl₃ without the subscript on Al.
   
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4. Tin(IV) oxide → Sn₂O
   
   Check Answer
   In this ionic compound, Sn has a charge of 4+. That is twice the charge on the oxide ion. So there should be twice as many oxide ions as tin(IV) ions. The proper formula is SnO₂.
   
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5. Sodium selenide → Na_-2Se
   
   Check Answer
   Look at the subscript on Na. It is a negative value. All subscripts are positive whole numbers.
   
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6. Chromium(V) nitride → N₅Cr₃
   
   Check Answer
   Ouch! The order of the metal and the nonmetal are switched. Write the symbol of the metal first with its subscript. Then write the symbol of the nonmetal with its subscript.
   
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3. And What’s Wrong with This?
A teacher is grading a quiz on nomenclature. He sees the following names of ionic compounds. Identify what is wrong with each name.

1. Al₂O₃ → aluminum(III) oxide 
   
   Check Answer
   Aluminum is a main group metal. We know its charge is 3+. Roman numerals and parentheses are not required.
   
    
   
    
2. PbI₂ → lead iodide 
   
   Check Answer
   Lead can have more than one possible charge. It is an exception to a general rule that main group elements have a known charge based on the group they are in. Since this is an exception, a Roman numeral inside of parenthesis is required.
   
    
   
    
3. BaSe → selenium baride 
   
   Check Answer
   Ouch. Write the name of the metal (cation) first. Then write the name of the nonmetal (anion). Here the order is switched. Barium selenide would be the correct name.
   
    
   
    
4. TiO₂ → titanium(II) oxide 
   
   Check Answer
   The formula shows two oxide ions. Each oxide ion has a 2- charge. The total amount of negative charge is 4-. So the charge on the single titanium ion is 4+. The correct name is titanium(IV) oxide.
   
    
   
    

 
 
4. Determine the names associated with the following formulas.

1. ZrI₄ → 
   
   Check Answer zirconium(IV) iodide
    
   
    
2. AlBr₃ → 
   
   Check Answer aluminum bromide
    
   
    
3. CuS → 
   
   Check Answer copper(II) sulfide
    
   
    
4. Ti₃N₄ → 
   
   Check Answer titanium(IV) nitride
    
   
    
5. Rb₂O → 
   
   Check Answer rubidium oxide
    
   
    
6. MnO₂ → 
   
   Check Answer manganese(IV) oxide
    
   
    
7. NiSe → 
   
   Check Answer nickel(II) selenide
    
   
    
8. Ag₂S → 
   
   Check Answer silver(I) sulfide
    
   
    
9. GaCl₃ → 
   
   Check Answer gallium chloride
    
   
    

 
 
5. Determine the formulas associated with the following names.

1. strontium fluoride → 
   
   Check Answer SrF₂
    
   
    
2. copper(II) selenide → 
   
   Check Answer CuSe
    
   
    
3. calcium sulfide → 
   
   Check Answer CaS
    
    
    
4. vandium(V) oxide →
   
   Check Answer V₂O₅
    
    
    
5. chromium(V) nitride → 
   
   Check Answer Cr₃N₅
    
   
    
6. manganese(IV) fluoride → 
   
   Check Answer MnF₄
    
   
    
7. platinum(IV) oxide → 
   
   Check Answer PtO2
    
   
    
8. gold(I) chloride → 
   
   Check Answer AuCl