DrBob222, thank you for answering my question(s). Sorry for asking repeatedly, I am new to this "posting" stuff. I thought I was asking different persons. Let me rephrase my question. How do I determine which roman numeral to place between the cation and the anion of a metal-nonmetal substance? I think that there must be a rule of thumb that would tell me how VCl3 ends up being Vanadium (III) Chloride and how CrO2 ends up being Chromium (IV) oxide. There must be some kind of relationship between the charges of each element. But I just can't figure out what it is and how to apply it. Can you help me, please?

Here is the relationship: The net charge on any molecule is zero.

CrO2 you know O is -2, there are two O, making a -4 charge, so Cr must have a +4 charge. Chromium(IV) oxide.

Check that on VCl3

Here is a site that gives some rules for determining the oxidation state of an element.

http://www.chemteam.info/Redox/Redox-Rules.html

Of course, I'd be happy to help you understand the naming of metal-nonmetal substances using Roman numerals. The Roman numerals in the name of a compound indicate the oxidation state, or charge, of the metal ion. To determine the appropriate Roman numeral, you need to know the charges of the elements involved.

Here's how you can determine the Roman numeral for a metal cation:

1. Identify the metal element: Look up the element on the periodic table and determine its name.

2. Determine the charge of the metal: The charge of the metal cation can usually be found on the periodic table. Many transition metals have multiple possible charges, so you need to determine which charge is present in the particular compound you're naming.

3. Write the Roman numeral: The Roman numeral corresponding to the charge of the metal cation is written in parentheses after the metal's name.

For example, let's use your mentioned compounds:

1. VCl3: The metal is vanadium (V). The charge (oxidation state) of vanadium in this compound is +3, which is determined from its position on the periodic table. Therefore, the name is Vanadium (III) chloride.

2. CrO2: The metal is chromium (Cr). The charge of chromium in this compound is +4, which is also determined from its position on the periodic table. Therefore, the name is Chromium (IV) oxide.

In summary, the Roman numeral in the name of a metal-nonmetal compound represents the charge of the metal cation. To determine this charge, you need to know the element's name and find its charge on the periodic table. Applying this understanding, you can name compounds correctly.