Question

Why do you use Roman numerals in an ionic compound formula?

a) To indicate the number of atoms
b) To represent the charge on ions
c) To denote the molecular weight
d) To specify the chemical element

Answer 1

Roman numerals in ionic compounds represent the charge of ions, specifically for metals that can have multiple ionic charges, to help correctly determine the compound's formula.

Step-by-step explanation:

In ionic compounds, Roman numerals are used to represent the charge on ions, particularly for metals that can form more than one possible charge. This is vital for determining the correct formula of the compound. For example, iron can form +2 or +3 charged ions, so the Roman numerals II and III would specify which ion is present in a compound like iron(II) sulfate or iron(III) chloride. The ionic compound formula reflects the lowest whole-number ratio of cations to anions that results in an electrically neutral substance.

The crisscross method is commonly used to write the correct formula. This involves taking the magnitude of the charge of one ion and using it as the subscript for the other ion and vice versa, omitting the charge signs. For instance, in the compound aluminum oxide (Al2O3), the formula indicates two aluminum ions (Al³+) for every three oxide ions (O²−) based on the charges. Hence, the use of subscripts in the formula indicates the number of each type of ion, not the overall charge.