Lesson 5-4

Naming Compounds

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     As first mentioned in an early lesson, there are two main types of compounds, ionic and molecular.  Some of the compounds that you will learn about this year will require special systems for naming, and we will learn about them at a later time.  For example, we will learn how to correctly name the various types of acids and bases when we study the chapters on acids and bases.  In this lesson you will learn enough to name most of the compounds that you will come in contact with in your laboratory activities this year.  References will be made to the tables from lesson 5-2, so it would be wise to have them handy as you go over this material.

I. Binary Compounds.  As you know, binary compounds consist of only two elements.  The formula for a binary compound may contain more than two letters, but it will contain only two capital letters.  When naming a binary compound, regardless of whether it is ionic or molecular, follow the following steps:

1.  Write the name of the element represented by the first symbol in the formula.

2.  Write the name of the element represented by the second symbol in the formula, but change the ending of the element's name to "ide".

3.  Check a reference table to determine the number of positive oxidation numbers that the first element forms.  If it only forms one then you are done.

4.  If the first element shows more than one oxidation number, than use the stock system.  Determine the oxidation number that the first element is showing and write that roman numeral in-between the two elemental names.

Example 1.  What is the correct name for the compound AlBr₃ ?

Step 1. Write the name of the element represented by the first symbol in the formula.

aluminum

Step 2.  Write the name of the element represented by the second symbol in the formula, but change the ending of the element's name to "ide".  In this case, bromine becomes bromide.

aluminum bromide

Step 3.  Check a reference table to determine the number of positive oxidation numbers that the first element forms.  If it only forms one then you are done.  Aluminum always has an oxidation number of +3, therefore there is no need for a roman numeral.  Our answer is;

aluminum bromide

 

Example 2.  What is the correct name for the element NiS ?

Step 1. Write the name of the element represented by the first symbol in the formula.

nickel

Step 2.  Write the name of the element represented by the second symbol in the formula, but change the ending of the element's name to "ide".  In this case, sulfur becomes sulfide.

nickel sulfide

Step 3.  Check a reference table to determine the number of positive oxidation numbers that the first element forms.  If it only forms one then you are done.  Nickel forms oxidation numbers of +2, +3 and +4, so we must go to the next step.

Step 4.  If the first element shows more than one oxidation number, than use the stock system.  Determine the oxidation number that the first element is showing and write that roman numeral in-between the two elemental names.  We check the oxidation number of sulfide and find that it is -2.  If one nickel is canceling out one sulfur than the apparent charge on the nickel must be +2.  (+2) + (-2) = 0.

nickel(II) sulfide

 

Example 3.  What is the correct name for the compound Fe₂O₃ ?

Step 1. Write the name of the element represented by the first symbol in the formula.

iron

Step 2.  Write the name of the element represented by the second symbol in the formula, but change the ending of the element's name to "ide".  So, oxygen becomes oxide.

iron oxide

Step 3.  Check a reference table to determine the number of positive oxidation numbers that the first element forms.  If it only forms one then you are done. Iron can be +2 or +3, so we must go on to step 4.

Step 4.  If the first element shows more than one oxidation number, than use the stock system.  Determine the oxidation number that the first element is showing and write that roman numeral in-between the two elemental names.  We know that oxygen is -2 in this case.   Since we have 3 atoms of oxygen, each with a charge of  -2, then the total negative charge is -6.  We must have +6 to balance out the -6.  Since there are two iron atoms to make up a total of +6, each must be +3. 
2(+3) + 3(-2) = 0.

iron(III) oxide

II. Ternary Compounds - Ternary compounds contain three elements.  The only type of ternary compounds that we will learn how to name in this chapter are those that consist of one polyatomic ion and one monatomic ion.  The vast majority of these types of compounds consist of a positive monatomic ion and a negative polyatomic ion.  For this type of compound  you follow the steps below:

Step 1.  Write the name of the element represented by the first symbol in the compound.

Step 2.  Write the name of the polyatomic ion, without changing the ending.

Step 3.  Check a reference table to determine the number of positive oxidation numbers that the first element forms.  If it only forms one then you are done.

Step 4.  If the first element shows more than one oxidation number, than use the stock system.  Determine the oxidation number that the first element is showing and write that roman numeral in-between the two names.

Example 1.  Name the compound Ca(CN)₂ ?

Step 1.  Write the name of the element represented by the first symbol in the compound.

Calcium

Step 2.  Write the name of the polyatomic ion, without changing the ending.

Calcium Cyanide

Step 3.  Check a reference table to determine the number of positive oxidation numbers that the first element forms.  If it only forms one then you are done. Calcium is always +2, so the final answer is as below:

Calcium Cyanide

 

Example 2.  What is the name of the compound Fe(NO₃)₂ ?

Step 1.  Write the name of the element represented by the first symbol in the compound.

iron

Step 2.  Write the name of the polyatomic ion, without changing the ending.

iron nitrate

Step 3.  Check a reference table to determine the number of positive oxidation numbers that the first element forms.  If it only forms one then you are done. Iron can be +2 or +3, so we must go on to step 4.

Step 4.  If the first element shows more than one oxidation number, than use the stock system.  Determine the oxidation number that the first element is showing and write that roman numeral in-between the two names.  Nitrate shows an oxidation number of -1.  Since there are two nitrate ions in the compound, the total negative charge is -2.  Therefore, the iron must be +2.  (+2) + 2(-1) = 0.

iron(II) nitrate

Special Exception:  The Ammonium ion (NH₄ ⁺) is a positive polyatomic ion.  When it combines with a negative monatomic ion, you change the ending of the negative ion to "ide".   When it combines with a negative polyatomic ion, you just name both ions.

(NH₄)₂S is called ammonium sulfide

NH₄OH is called ammonium hydroxide

Now use the links below to practice what you have learned.

Please forward all questions, comments and criticisms to Gregory L. Curran.
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Last Modified February 07, 2008