Question

Why is there Roman numerals (II) in copper(II) chloride, but no (III) in Aluminum chloride?

Answer 1

This is because copper is a transition metals.

Transition metals don't have a definite charge, you can see this on the periodic table. However, Alkali Earth, Alkali, Metalloids, Non-metals, Halogens, Noble Gases, etc, all have a specified charge.

Although, there are exceptions: Zinc, Gold, and Cadmium.

Aluminum is a poor metal, but it has it's own charge (3+).

Therefore, Copper has roman numerals due to the fact that it's a transition metal and Aluminum is a poor metal.

Hope this helps!

Answer 2

There are Roman numerals in Copper Chloride because those elements are transitional metals. Aluminum is a metal. Elements that are metals and non-metals don’t use Roman numerals, but elements that are transitional metals such as Copper(II) Chloride must use Roman numerals.