Answer
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The three compounds AlCl₃, CrCl₃, and ICl₃ are named aluminum chloride, chromium(III) chloride, and iodine trichloride, respectively. Although they share a similar formula type (one element combined with chlorine), each follows a different set of naming rules because they belong to different categories of compounds.
AlCl₃ is an ionic compound composed of a metal (aluminum) and a nonmetal (chlorine). Aluminum forms only one common ion, Al³⁺, so its charge does not need to be indicated in the name—hence, it is simply called aluminum chloride.
CrCl₃ is also an ionic compound, but its metal, chromium, can form more than one type of cation (Cr²⁺ and Cr³⁺). Therefore, the Stock system is used to indicate the metal’s oxidation state with a Roman numeral. Since each chloride is Cl⁻ and there are three of them, chromium must be Cr³⁺, giving the name chromium(III) chloride.
ICl₃, on the other hand, is a covalent (molecular) compound formed between two nonmetals—iodine and chlorine. In such cases, prefixes are used to indicate the number of each atom. The compound contains one iodine atom and three chlorine atoms, so it is named iodine trichloride.
In summary, AlCl₃ follows the rule for ionic compounds with metals that have a fixed charge, CrCl₃ follows the rule for ionic compounds with variable oxidation states (Stock system), and ICl₃ follows the covalent naming system using prefixes.
