Nomenclature is the systematic method for naming chemical compounds, governed by strict rules that communicate a compound’s exact composition and structure. The proposed name “diiron trisulfide” for \(\text{Fe}_2\text{S}_3\) is incorrect because it misapplies the naming conventions for one class of compounds to another. Understanding this common error requires distinguishing between the two fundamental categories of chemical bonding.
Differentiating Ionic and Covalent Compounds
Chemical compounds are broadly categorized based on the type of bond holding their constituent atoms together. Covalent compounds form when atoms, usually two nonmetals, share electrons to complete their outer shells, resulting in discrete molecular units like water (\(\text{H}_2\text{O}\)) or carbon dioxide (\(\text{CO}_2\)).
Ionic compounds typically involve a metal bonding with a nonmetal through the complete transfer of electrons. This transfer creates positively charged cations and negatively charged anions, held together by electrostatic attraction. \(\text{Fe}_2\text{S}_3\) is classified as ionic because it consists of Iron (Fe), a transition metal, and Sulfur (S), a nonmetal, dictating a specific naming procedure.
Why Covalent Prefixes Do Not Apply
Numerical prefixes like di- (two) and tri- (three) are reserved exclusively for naming covalent compounds. These prefixes indicate the exact number of atoms of each element present within a single molecule, such as dinitrogen trioxide (\(\text{N}_2\text{O}_3\)).
Applying “diiron trisulfide” incorrectly borrows this prefix system. Ionic compounds do not exist as discrete molecules but form vast, continuous crystal lattice structures. The formula \(\text{Fe}_2\text{S}_3\) represents the simplest ratio of ions required for electrical neutrality, not the count of atoms in an isolated molecule.
The Correct Naming Convention for \(\text{Fe}_2\text{S}_3\)
The proper naming system for ionic compounds involving transition metals relies on specifying the ion’s charge rather than counting atoms. Iron is a transition metal, meaning it can form ions with multiple positive charges (variable oxidation states). Because both Iron(II) and Iron(III) ions exist, a Roman numeral is necessary to distinguish which form is present. This system is known as the Stock nomenclature.
To determine the correct charge for iron in \(\text{Fe}_2\text{S}_3\), the principle of charge neutrality must be applied. The sulfide ion (\(\text{S}\)) carries a \(-2\) charge, making the total negative charge \(-6\) (\(3 \times -2\)). For the compound to be electrically neutral, the two iron ions must collectively balance this charge with \(+6\). Therefore, each iron ion must possess a \(+3\) charge. The correct name for \(\text{Fe}_2\text{S}_3\) is Iron(III) Sulfide, where the Roman numeral (III) communicates the \(+3\) charge of the iron cation.