Binary Compounds of Cations with Variable Charges

Given Formula, Write the Name

Common Name System

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A binary compound is one made of two different elements. There can be one of each element such as in CuCl or FeO. There can also be several of each element such as Fe2O3 or CuBr2.

This lesson shows you how to name binary compounds (using the common naming system) from the formula when a cation of variable charge is involved. The four formulas above are all examples of this type. Important point to remember: the cations involved in this lesson have variable charges. The anions involved have only one charge.

Antoine Laurent Lavoisier (1743-94) reformed chemistry in the late 1700's with his publication of Méthode de nomenclature chimique in 1787 (along with three co-authors) and Traité élémentaire de Chimie in 1789. He is known as the "Father of Modern Chemistry."

Two typical names of chemicals up to this point in history are "foliated earth of tartar" and "phlogisticated vitriolic acid." There were hundreds of such names. One goal of the Méthode was to create chemical names based on the chemical composition.

Lavoisier's solution, which will be studied in this lesson, was to use different suffixes to indicate differences in composition. Specifically, the use of "-ous" and "-ic" will be studied.

Here is what the IUPAC currently says about this naming system: "The following systems are in use but not recommended: The system of indicating valence by means of the suffixes -ous and -ic added to the root of the name of the cation may be retained for elements exhibiting not more than two valences."

By the way, this picture of Lavoisier is a detail from a larger painting. In it, he is looking up toward his wife.

In everything that follows, remember this overall guiding principle: the total positive charge MUST equal the total negative charge.

All names such as "ferrous" or "plumbic" are given in the Nomenclature Data Sheet. This is what is handed out in the ChemTeam's classroom. Feel free to download it, clean it up in your word processor and use it. Print off a few extra to share with others. The ChemTeam thanks you.

In the past, some students have asked for a list of roots. Here is a small list.

Elementroot Elementroot
iron"ferr-" chromium"chrom-"
lead"plumb-" tin"stann-"
copper"cupr-" cobalt"cobalt-"
gold"aur-" manganese"mangan-"

Example #1: FeO

Step #1 - the first part of the name is the root of the first element in the formula plus a suffix. For iron the root to use is "ferr-". The suffix will be either "-ous" or "-ic."

Here is how to determine the suffix.

  1. multiply the charge of the anion (the O) by its subscript. Ignore the fact that it is negative.
  2. divide the result by the subscript of the cation (the Fe). This gives the positive charge on the cation.
  3. the lower of the two values for a given cation is assigned the ending "-ous" and the higher uses the ending "-ic."

The result from (1) and (2) just above is two. (As you memorize the various charges, you will also internalize the above three steps.)

That last part merits a repeat: the lower of the two values will use the "-ous" ending and the higher will use "-ic." I can see you saying to yourself "How in the world do I know which one is the lower and which one is the higher?" Answer - you will know from your studies which one is lower and which is higher.

For example, iron takes on a +2 value and a +3 value. As you begin to learn these values, the question of lower and higher becomes much easier. Trust me!

Step #2 - the anion is named in the usual manner of stem plus "ide."

The answer to this example is ferrous oxide.

Example #2: Fe2O3

When you multiply the anion's charge (negative two) by its subscript (three) and drop the sign, you get six for an answer.

Then you divide the six by two (the iron's subscript) and you get three. This means the charge on each iron is positive three.

Since this is the higher of the two charges, the term "ferric" is used.

The answer to this example is ferric oxide.

Example #3: CuCl2

The first part of the name comes from the first element's root: cupr-.

Two chlorides equal -2, so the Cu must be +2. "-ic" is used because +2 is the HIGHER of the two charges copper is known to have.

The second part of the name comes from the root of the second symbol plus 'ide,' therefore chlor + ide = chloride.

This compound is named cupric chloride.

Example #4: SnO

The first part of the name comes from the first element's root: stann-.

One oxygen = negative 2, so the one tin equals +2. The tin must equal +2 because you must create a formula with zero total charges. "-ous" is used because +2 is the LOWER of the two charges tin is known to have.

Second element is oxygen (from the symbol O), so the name is ox + ide = oxide.

This compound is named stannous oxide.

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Practice Problems
Decide on a common name and then click the answer links to check your work.

Write the correct name for:

1) CrS

2) PbBr4

3) Pb3N2

4) Fe2S3

5) FeI2

6) Hg2F2

7) Cu2S

8) SnCl2

9) HgO

10) Sn3P4 Answers to Set One

Write the correct name for:

11) Cr2S3

12) PbCl2

13) Sn3N4

14) FeS

15) FeBr3

16) HgF2

17) CuS

18) SnI4

19) Hg2O

20) Pb3P4 Answers to Set Two

Write the correct name for:

21) MnO

22) SnO2

23) PbO2

24) Fe2O3

25) CuI

26) Hg2Cl2

27) CuO

28) Sn3N2

29) Hg2O2 [Hint: the answer to this is different from #9 and #19. Hint: look up peroxide]

30) CuCl2 Answers to Set Three

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