Naming Covalent Compounds

The purpose of this section is to specifically address the issues of chemical nomenclature. Several methods or systems for naming chemicals have been developed over the years. Initially, each chemical got its own name based on whatever feature (or whim) impressed people at the time. In the late 1700s, Antoine Laurent Lavoisier proposed a systematic nomenclature in which compounds were named according to the elements contained in them. That basic approach has been kept over the years even though different methods have been developed to achieve it.

First we'll have an overview of nomenclature, including a little review of some of the things you learned about naming ionic compounds. Then we'll learn the common, systematic method for naming covalently bonded compounds: the system of prefixes.

Nomenclature Review

In Lesson 7 you learned how to name ionic compounds; we'll round out your nomenclature skills in this lesson with naming covalent compounds.

The naming of ionic compounds is often deceptively simple. As an example, the compound Al₂(CO₃)₃ which has a big, unwieldy formula, has a simple name: aluminum carbonate. The reasons for this are twofold. First, polyatomic ions like carbonate have well-known names. Saying "carbonate" communicates what ion you are dealing with. Second, and more importantly, the subscripts in the formula can be derived just from knowing what ions are involved. The aluminum ion can only ever take a +3 charge, and carbonate is defined to have a -2 charge. In order for the charges to balance, Al₂(CO₃)₃ is the only formula possible from the name "aluminum carbonate" - so that name is sufficient.

We saw that, sometimes, ionic compound names needed a little extra info: a roman numeral to indicate charge. So we saw things like vanadium (II) chloride (VCl₂) and vanadium (IV) chloride (VCl₄). We will not use roman numerals to name covalent compounds. However, we will borrow somewhat from this format.

Prefix Names of Covalent Compounds

So, if I want the name of the compound N₂O₄, all I need to do is remember that the prefix for 2 is "di-" and 4 is "tetra-", as well as changing "oxygen" to "oxide." So this compound's name would be dinitrogen tetroxide. (We leave out the "a" or "o" of a prefix if it comes before a vowel.)

The important exception you need to know is that, if there is only one of the first element, we do not use the "mono-" prefix. So while N₂O₄ is dinitrogen tetroxide, CCl₄ would simply be carbon tetrachloride. The "mono" prefix is used for the second element, so CO is carbon monoxide and IBr is iodine monobromide.

Students sometimes stumble over compounds ending in hydrogen. These compounds' names end in "hydride." For example, B₂H₆ is named "diboron hexahydride." Take care not to confuse either "hydride" or "oxide" with the polyatomic ion "hydroxide" (OH^-) that you learned about in Lesson 7.

Practice

Practice naming the following compounds.

1. PH₃

2. N₂O₃

3. BrF

4. Cl₂O

5. SCl₆

Answers

1. phosphorus trihydride

2. dinitrogen trioxide

3. bromine monofluoride

4. dichlorine monoxide

5. sulfur hexachloride