### Binary Compounds of Cations with Variable Charges Given Name, Write the Formula The Stock System

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 SnBr4.

This lesson shows you how to write the formula of a binary compound from the word name when a cation of variable charge is involved. The four formulas above are all examples of this type.

The cations involved in this lesson have AT LEAST TWO charges. The anions involved have only one charge.

Your teacher will hold you responsible for the cations you must learn. The ChemTeam holds their students responsible for: Cu, Fe, Hg, Pb, Sn, Mn, Co, Au, and Cr.

The type of naming you will learn about is called the Stock system or Stock's system. It was designed by Alfred Stock (1876-1946), a German chemist and first published in 1919. In his own words, he considered the system to be "simple, clear, immediately intelligible, capable of the most general application."

In 1924, a German commission recommended Stock's system be adopted with some changes. For example, FeCl2,which would have been named iron(2)-chloride according to Stock's original idea, became iron(II) chloride in the revised proposal. In 1934, Stock approved of the Roman numerals, but felt it better to keep the hyphen and drop the parenthesis. This suggestion has not been followed, but the Stock system remains in use world-wide.

Example #1 - Write the formula for: copper(II) chloride

Step #1 - the first word tells you the symbol of the cation. In this case it is Cu.

Step #2 - the Roman numeral WILL tell you the charge on the cation. In this case it is a positive two.

Step #3 - the anion symbol and charge comes from the second name. In this case, chloride means Cl¯.

Step #4 - remembering the rule that a formula must have zero total charge, you write the formula CuCl2.

On writing the formula from the ions, you may want to review Charge-Crossing.html or Least-Common-Multiple.html for more information.

This graphic summarizes example #1:

The ChemTeam is often asked by students, "But how do you know that chloride means Cl¯?" That type of question is usually answered with a question, as in "How do you know the name and face of your best friend?" That's right, you've spent time in their company, to the point where you have memorized the connection between name and face. Chloride is the name and Cl¯ is the face.

Example #2 - Write the formula for: copper(I) oxide

Step #1 - the first word tells you the symbol of the cation. It is Cu.

Step #2 - the Roman numeral WILL tell you the charge on the cation. It is a positive one.

Step #3 - the anion symbol and charge comes from the second name. Oxide means O2¯.

Step #4 - since a formula must have zero total charge, you write the formula Cu2O.

This graphic summarizes example #2:

Example #3 - Write the formula for: iron(III) sulfide

Step #1 - the symbol of the cation is Fe.

Step #2 - the charge on the cation is a positive three. remember, that comes from the Roman numeral.

Step #3 - Sulfide (the anion) means S2¯.

Step #4 - since a formula must have zero total charge, you write the formula Fe2S3.

Look at Least-Common-Multiple.html if you're not sure about how the subscripts in iron(III) sulfide came to be.

This graphic summarizes example #3:

Example #4 - Write the formula for: tin(IV) phosphide

First symbol is Sn from the name tin.

The Roman numeral IV gives +4 as tin's charge.

Phosphide give P3¯.

This compound's formula is Sn3P4.

This graphic summarizes example #4:

Here is an extra graphic for gold(III) bromide:

Practice Problems

Write the correct formula for:

1) iron(II) chloride

2) copper(I) sulfide

4) tin(II) fluoride

5) mercury(I) bromide

6) tin(II) oxide

7) chromium(III) oxide

8) gold(I) iodide

9) manganese(II) nitride

10) cobalt(III) phosphide

11) iron(III) chloride

12) copper(II) sulfide

14) tin(IV) iodide

15) mercury(II) fluoride

16) tin(IV) oxide

17) manganese(III) chloride

18) chromium(II) nitride

19) gold(III) oxide

20) cobalt(II) phosphide Answers to Set Two

21) tin(II) sulfide

22) mercury(I) sulfide

23) gold(III) bromide

24) manganese(II) oxide

25) chromium(II) chloride