Question

What is the stock system name for ferric sulfide?
A. Iron (III) sulfide
B. Iron sulfide
C. Iron trisulfide
D. Iron monosulfide

Answer 1

The system for naming an ionic compound is different from that for naming a covalent compound, so before a compound can be named, it must be classified as ionic or covalent. Classifying a compound is not an easy task, but for the purposes of naming them, we can check the presence of metal or polyatomic ions.

Complete step by step solution:
A chemical formula indicates the relative number of atoms of each kind in a chemical compound. It also reveals the number of atoms of each element.
The stock system is introduced by Alfred Stock. In the stock system, Roman letters are used to indicate the oxidation number of the metal.
The given compound is an ionic compound. Ionic compounds are named simply by naming the ions present. There are some complicating factors:
1.Some metals form more than one ion.
2.Identifying polyatomic ions.
Monoatomic ions are the ions formed from a single atom. In naming the anions, drop the elements ending and add “ide”. Binary compounds are the compounds composed of two different elements. The naming system of binary ionic compounds involves combining the names of the compound’s positive and negative ions.
Near the cation name, Roman numeral is inserted which indicates the charge. The structure of ferric sulfide is ${\text{F}}{{\text{e}}_2}{{\text{S}}_3}$. In the molecule, the oxidation number of iron is $3$. So the Roman number is kept near the metal atom. Since sulfur anion is formed, it is named as sulfide. Thus the name is given as Iron (III) sulfide.

Hence the correct option is A.

Note: The sum of the oxidation numbers of a compound is zero. The oxidation number of sulfide is $- 2$. Thus the oxidation number can be calculated by:
$2\times{\text{x} + 3\times{(-2)}} = 0$
$\Rightarrow$ $2\times{\text{x}} = 6$
$\Rightarrow$ $\text{x} = +3$
Thus the oxidation number of iron is $+ 3$.