how do you know what/which roman numeral to place between the cation and the anion of metal-nonmetal substances?

if the sountance is hard that is roman numeral.

Tammy is incorrect with that answer. The correct answer is that the Roman numeral is the oxidation state of the cation. For example, in CuSO4 it is copper(II) sulfate. The old name was cupric sulfate.

Another example. Cu2O is copper(I) oxide. CuO is copper(II) oxide.
FeSO4 is iron(II) sulfate.
Fe2(SO4)3 is iron(III) sulfate.

To determine the Roman numeral to place between the cation and the anion in metal-nonmetal substances, you need to understand the concept of oxidation states or charges of the elements involved in the compound.

Here's a step-by-step guide to determine the Roman numeral:

1. Identify the elements: Determine the cation (metal) and the anion (nonmetal) in the compound. Let's take an example compound, Iron(III) oxide (Fe2O3), where Iron is the cation and oxygen is the anion.

2. Determine the oxidation state of the metal: Each element has an oxidation state or charge that indicates how many electrons it has gained or lost. In our example, Iron has an oxidation state of +3, indicated by the Roman numeral III in the compound name, Iron(III) oxide.

3. Identify the charge of the anion: For nonmetals, it is common to use the suffix "-ide" to indicate ions. In our example, oxygen has an oxidation state of -2, which means it gained two electrons.

4. Balance the charges: To form a neutral compound, the total positive charge of the cation must be equal to the total negative charge of the anion. In our example, since each Fe ion has a +3 charge, and there are two Fe ions, the total positive charge is +6. Since there are three O ions, each with a -2 charge, the total negative charge is -6. Hence, Fe2O3 is neutral.

In summary, the Roman numeral in metal-nonmetal compounds indicates the oxidation state of the cation, which helps in determining the correct charge balance between the cation and anion. It is important to know the oxidation states of the elements involved to properly write the compound name.