Naming Compounds

Now that you have learned how to name the elements, you need to learn how to name compounds.

I. Binary Ionic Compounds

A. Monatomic Ions

Here we are looking at ions combining with ions. Thus we are looking at metals (form cations) combining with nonmetals (form anions).

In these compounds the cation is named first. The monatomic cation has the name of the element and the anion has the root of the element name + ide. Some examples:

NaCl   sodium chloride
KCl    potassium chloride
KI     potassium iodide
CaS    calcium sulfide
CsBr   cesium bromide
MgO    magnesium oxide
Li3N    lithium nitride

Why do we have 3 lithium atoms combining with 1 nitrogen atom to form the compound lithium nitride? Well, compounds have to have a neutral overall charge. Lithium forms a +1 cation (remember Group IA) and nitrogen tends to form a -3 anion. Thus to have a neutral charge we must have three +1 charges and one -3 charge. You will need to memorize the table showing some common monatomic cations and anions (know the elements' names and charges).

Some common names are used for some compounds like H2O is water and NH3 is ammonia.

Unfortunately life is not even this simple. Some atoms form more than one type of cations. You will need to memorize the table below which shows these common cations and their names. Fortunately the names are easy. It used to be that the names were more complicated : the ion with the higher charge has a name ending in -ic, and the on with the lower charge has a name ending in -ous. You will still see this terminology in some literature, but we will use the Roman numeral method as below.


Note that mercury is a bit wierd. It's ionic +1 state is a diatomic.

So how do we name these ionic compounds whose cations can take on more than one form? We just use the naming system above for the particular cation. For example:

CuCl would be copper(I)chloride
HgO would be mercury(II)oxide
Fe2O3 would be iron(III)oxide
(remember that the overall charge has to be zero
so we need two +3 charged cations and three -2 charged anions.)
FeO would be iron(II)oxide
MnO2 would be manganese(IV)oxide

Do not put the Roman numerals after cations which only form one type of ion.

B. Polyatomic ions

Now you have another table to memorize: the table of common polyatomic ions. In that table you will see a number of ions which contain oxygen atoms. These are called oxyanions and their names depend upon the charge, kinda like above. However here we don't use the simpler Roman numeral system for stating which charge the ion has -- I don't know why not. Instead we use a system rather like the older method I talked about above. The polyatomic ion with the small number of oxygen atoms ends in -ite and the name of the one with the larger number of oxygen atoms ends in -ate. Examples would be:

SO32- and SO42- where the first would be called sulfite and the second would be called sulfate. When there are more than two oxyanions in the series, the prefix hypo (meaning less than) and per (meaning more than) are used. For example:

ClO-1  would be hypochlorite
ClO2-1 would be chlorite
ClO3-1 would be chlorate
ClO4-1 would be perchlorate

So let's look at a few naming exercises combining some of the above.

Na2SO4 would be sodium sulfate
Fe(NO3)3 would be iron(III)nitrate
Mn(OH)2 would be manganese(II)hydroxide
Na2CO3 would be sodium carbonate.

II. Covalent Compounds

Now we are looking at the combining of two nonmetals to form a compound. The first element is named first. The second element is named as if it were an anion. Prefixes (mono, di, tri, etc) are used to denote the number of atoms present except mono is never used for the first element. Here are some examples:

N2O would be dinitrogen monoxide or nitrous oxide
NO would be nitrogen monoxide or nitric oxide
NO2 would be nitrogen dioxide
N2O3 would be dinitrogen trioxide
N2O4 would be dinitrogen tetroxide (not tetra oxide)
N2O5 would be dinitrogen pentoxide (not penta oxide)

III. Acids

Acids are substances which produce hydrogen ions (H+ also called protons) in water. We will discuss them in detail later, but here we just look at naming them.

If the anion does not contain O, the acid is named with the prefix hydro- and the suffix -ic. Note that these compounds are given these acid names only when they are in water. Names in and out of water are given below.

Name not in watercompoundName in water
hydrogen chlorideHClhydrochloric acid
hydrogen cyanideHCNhydrocyanic acid
hydrogen sulfideH2Shydrosulfuric acid

If the anion contains oxygen, then we use the following nomenclature:

For example, H2SO4 has a polyatomic ion whose name is sulfate. Therefore the name of this compound in water is sulfuric acid. HNO2 has a polyatomic ion whose name is nitrite so the name of this compound in water is nitrous acid.

Now you must practice, practice, practice and memorize. I recommend that you make some flash cards from the tables that you must memorize. I remember one student who told me that she put them everywhere in her apartment -- on the refrigerator, the mirrors, wherever she might look so that she would be working on this memorization many times a day. Please don't put them on your rearview mirror!

You must work hard to get good results. Take some of the tests also. The sample tests and the context tests are good practice. But remember that you must take the tests as though you were taking a test which means closed books and notes and no help from anyone else. Then if you can't correctly answer some of the questions you know what you need to work on.

WRITING CHEMICAL FORMULA (partly from the Towson University tutoring site)

I. Writing Formula of Ionic Compounds

When writing a chemical formula from the name of an ionic compound, you must consider the ions contained in the compound. You must recognize the ions which are a part of the compound. These include the cations, as well as the monatomic anions and polyatomic anions. The inability to recognize these ions are the main reason for difficulty in writing chemical formula of inorganic compounds. If you haven't memorized the ions, it is essential you do so immediately.

Recall that some cations will only form one ion (Group Ia, IIA, and IIIA, except Tl). Cations which can form more than one ion will have a Stock number shown as a Roman numeral in parenthesis. For the monatomic anions, the charge is equivalent to the Group number - 8. The polyatomic anions must be memorized.


1. What is the chemical formula for the compound, magnesium oxide?

First, consider the ions involved. Magnesium, found in GroupIIA, can only form a +2 ion. Oxide ion must have a charge of -2, because oxygen is found in GroupVIA. Since the magnitude of the charges are the same, the ions will combine in a 1:1 ratio, and the formula for magnesium oxide is MgO.

2.What happens when the magnitude of the charges are not the same? For instance, what will be the formula of the compound, aluminum chloride?

As before, consider the ions involved. Aluminum, found in GroupIIIA, can only form a +3 ion. Chloride ion must have a -1 charge, because it is found in GroupVIIA. The magnitude of the aluminum ion is greater than the magnitude of chloride ion. One can see that more than one chloride ion will be needed to cancel the +3 charge on aluminum. In fact, three chloride ions at -1 apiece are needed to cancel the +3 on aluminum ion. To show three chloride ions in the formula a subscript of three is used. Incidently, by convention, the cation is always written first: AlCl3.

3. Let's consider a compound containing a polyatomic anion. Polyatomic anions, with few exceptions, will have name ending in -ite or -ate, so they should be easy to recognize. What is the formula for the compound, sodium sulfate?

Consider the ions. Sodium, found in GroupIA, can only form a +1 ion. Sulfate ion is SO42-, and has a -2 charge. The magnitude of the sulfate ion is greater than the magnitude of the sodium ion. One can see that more than one sodium ion will be needed to cancel the -2 charge on the sulfate ion. Since two sodium ions are needed, a subscript of 2 will be found in the formula: Na2SO4.

4. Let's consider a compound involving a Roman numeral. What is the formula for the compound, chromium(III) nitrate?

Consider th ions. The Roman numeral, III, for chromium(III), indicates the charge of the chromium ion is +3. Nitrate ion you have memorized to be NO3-, and has a -1 charge. The magnitude of the chromium(III) ion is greater than the magnitude of the nitrate ion. Three nitrate ions at -1 apiece are needed to cancel the +3 on the chromium ion. In order to show three nitrate ions in the formula, one must use parenthesis around the nitrate ion and a subscript of 3 outside the parenthesis: Cr(NO3)3

5. Here is a compound which is a little more difficult. What is the formula for iron(III) carbonate?

Consider the ions. Iron(III) has a +3 charge, as indicated by its Stock number. Carbonate ions has the formula, CO32-, and thus has a -2 charge. The magnitudes of the charges are not equal, and they are not multiples of each other. One needs to consider a common multiple, in this case 6, and adjust the number of each ion to this multiple. This means that two iron(III) ions, totaling +6, and three carbonate ions, totaling -6 are needed to write the formula: Fe2(CO3)3.

II. Writing Formula of Binary Molecular Compounds

Writing the formula of a binary molecular compound is actually quite easy. The number of each element in the compound is indicated by a prefix. Apply the prefix when writing the formula. Thus, the formula for carbon dioxide is CO2, since the name indicates one carbon (no prefix), and two oxygens (prefix di- means 2).

The compound, disulfur tetrafluoride, would have the formula, S2F4, since the di- prefix indicates two sulfurs and the tetra- prefix indicates four fluorines..

help Here is an Tutoring Site for naming compounds.

Now take a practice quiz to help you understand if you understand the basic concepts.
You must use your real name when it asks for a name.
The test will only submit when you have answers all of the questions correctly.
If you are not taking this course for credit please do not answer all the questions correctly for I don't want to be flooded with email answers to the tests.

Web Author: Dr. Leon L. Combs
Copyright 2000 by Dr. Leon L. Combs - ALL RIGHTS RESERVED