Clackamas Community College

CH 104: INTRODUCTORY CHEMISTRY

 

 

Contact instructor:

Eden Francis

Physical Science
19600 Molalla Avenue
Oregon City, OR 97045
(503) 594-3352
TDD (503) 650-6649

Lesson 8: 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 a little overview of nomenclature, including a little review of some of the things you learned about naming ionic compounds. Then we'll examine two ways to name covalent compounds: using the prefix system and the Stock naming system.

Nomenclature Overview (and Review) | Prefix Names | Stock Names

Nomenclature Overview (and Review)

By the end of this lesson you will be responsible for using several types of names: simple, Latin, Stock  and prefix. You will also need to know when to use these different types of names. In Lesson 7 you learned how to name ionic compounds; we'll round out your nomenclature skills in this lesson with naming covalent compounds.

Simple names are used for ionic compounds (and some covalent compounds) where only one compound is made from those elements.

Latin names are used for ionic compounds (and some covalent compounds) where more than one compound is made from those elements.

Stock names are used for ionic compounds and covalent compounds where more than one compound is made from those elements. (Actually, Stock names can also be used for ionic compounds and covalent compounds where only one compound is made from those elements, but the Stock names are the same as simple names in those cases.)

Prefix names are used for covalent compounds. (There are a few cases where prefix names can be used for ionic compounds, but you have to know the specific cases where that is permissible if you want to do that.)

When you name chemicals, you will have to pay attention to the type of compound in order to determine the proper type (or types) of name to use for each compound.

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Prefix Names of Covalent Compounds

Covalent compounds are named in different ways than are ionic compounds (although there is some overlap). Many of these compounds have common names such as "methane", "ammonia" and "water". However, simple covalent compounds are generally named by using prefixes to indicate how many atoms of each element are shown in the formula. Also, the ending of the last (most negative) element is changed to -ide.

The prefixes used are mono-, di-, tri-, tetra-, penta-, hexa-, and so forth. The mono- prefix is usually not used for the first element in the formula. The "o" and "a" endings of these prefixes commonly are dropped when they are attached to "oxide." You should memorize the prefixes from 1-10; the complete list is found in Ex. 8 of your workbook.

1 mono-
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-

 

You also need to know which element to put first in the formulas and names of these compounds. Generally, they are in the same left-to-right order that they are on the periodic table, except that you would have to squeeze hydrogen in between nitrogen and oxygen.

Nonmetal portion of periodic table.
 

At this time, try the following practice problems (also found in exercise 9 in the workbook).  

Name the following compounds.

PH3
N2O3
CO
SO2
HI
SCl6

 

Answers:

phosphorus trihydride
dinitrogen trioxide
carbon monoxide
sulfur dioxide
hydrogen monoiodide*
sulfur hexachloride

*(This compound also is often called by its simple name, hydrogen iodide, since there is only one combination of just hydrogen and iodide.)

Let’s look at how to name the compounds we worked with in exercise 2.

CH4 can be named carbon tetrahydride to indicate that it contains one carbon atom and four hydrogen atoms bonded together. However, this compound was named long before its formula was figured out and it is called methane.

NH3 could have been named nitrogen trihydride to show that it contains nitrogen and three hydrogen atoms bonded together. This compound also was known and named long before its fomula was figured out. It is called ammonia.

H2O could have been called dihydrogen oxide or even dihydrogen monoxide to show that it contains two hydrogen atoms and one oxygen atom. But of course its common name as well as its chemical name is water.

The name for HCl is hydrogen chloride. Note that in this case there are no prefixes. When there is only one atom of an element in a molecule, the prefix mono- might or might not be used. Usually it is left off. When the elements can combine to form more than one compound and one of those compounds has just one atom of that element, the prefix mono- is generally used.

Formula Common Name Prefix Name
CH4 methane carbon tetrahydride
NH3 ammonia nitrogen trihydride
H2O water dihydrogen monoxide
HCl   hydrogen chloride

 

Practice

Now, name the compounds for which you determined the formulas (in exercise 2-e) using the prefix method. The correct answers follow.

Answers

The prefix name for SiH4 is silicon tetrahydride. (Its common name is silane.) The prefix name for PH3 is phosphorus trihydride. (Its common name is phosphine.) The prefix name for H2S dihydrogen sulfide. Since this is the simplest, most common, most expected and most reasonable of the sulfur-hydrogen compounds it is quite often simply called hydrogen sulfide. The lack of prefixes in this name leaves it to you (and your understanding of bonding and electron structure) to figure out that there are two hydrogens and one sulfur. HBr is called hydrogen bromide.

In general, you will not be expected to memorize the common names of covalent compounds, except for water. You may find it useful to also learn methane and ammonia, since they are fairly common chemicals, even outside the chemistry classroom.

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Stock Names of Covalent Compounds

Sometimes covalent compounds are named using Stock names. This system works similarly to naming ionic compounds with Stock names.  Think about why we use Stock names with ionic compounds; we use Stock names when more than one compound can be formed with the same elements. That occurs when the metal is a transition metal that forms more than one ion.  

Nonmetals often can combine to make more than one compound also - for example, carbon and oxygen can combine in a 1-to-1 ratio in CO or in a 1-to-2 ratio in CO2.  Using Stock names is another way to distinguish between the possible compounds.  Remember that with ionic compounds, the Roman numerals in a Stock name indicated the charge on the metal ion.  Covalent compounds do not contain ions, so how can we use a Stock name?  Since different nonmetals have different electronegativities (the ability to attract shared electrons), the atoms are not sharing their electrons equally; one of the atoms in a bond will have a stronger pull for those shared electrons.  One method for keeping track of the electrons in a compound is to assign an oxidation number to each element.  We are not going to be concerned in this lesson about the specific differences between oxidation numbers and ion charges (we will revisit oxidation numbers in CH 105).  For now, you can simplify the concept by thinking about oxidation numbers as "pseudo-charges".  When we figure out the correct ratios for the elements we will deal with the oxidation numbers as if they are actual charges (even though we know that is not completely correct).

Let's look at an example of an ionic compound that has a Stock name.

Stock name: Ions present:
iron(II) oxide Fe2+ and O2
Formula: FeO

 

Now here's an example of a covalent compound with its Stock name.

Stock name: Oxidation states:
carbon(II) oxide C2+ and O2-  (The Roman numeral tell us the oxidation number of the element directly in front of it, just like in ionic compounds.  The oxidation number of the other element is determined by its position on the periodic table.
Formula: CO

 

You might also have to determine the Stock name based on the formula. Here is an ionic compound as an example.

Formula: Au3N
Oxidation States: Au+ and N3- (Start with the N ion - it should be 3- based on its position on the periodic table. The Au must be 1+ since three Au+ ions are used to balance the one N3- ion.)
Stock name: gold(I) nitride

 

Let's use the same process with a covalent compound.

Formula: CCl4
Oxidation States: C4+ and Cl- (Start with the Cl. Iit should have a 1- oxidation state based on its position on the periodic table. The C must be 4+ since one C is used to balance the four Cl atoms.)
Stock name: carbpn(IV) chloride

 

Try your hand at the following practice problems, also found in exercise 11 in your workbook.

Practice Problems

CO2

N2O4

carbon(IV) fluoride

 

Answers:

CO is carbon(IV) oxide.  (Oxide should be O2- so with a ratio of 2 O to 1 C, the C should be C4+ )

N2O4 is nitrogen(IV) oxide.  (Oxide is O2- again so the nitrogen must be N4+ since the ratio is 2 O to 4 N.)

carbon(IV) fluoride is CF4.  (The Roman numeral shows that we have C4+ and F is F1-, so they must be combined in a 1-to-4 ratio.)

Your lab work this week is to do more nomenclature practice on the computers in the lab. Be sure to work enough examples so that you feel confident naming covalent compounds using both the prefix and Stock names.

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