The difference in naming between MgCl₂ and CuCl₂ stems from the fact that copper (Cu) is a transition metal, while magnesium (Mg) is an alkaline earth metal.

Here's a breakdown:

* Magnesium (Mg): Magnesium only forms one stable cation, Mg²⁺. Therefore, the name directly reflects its composition: magnesium chloride.

* Copper (Cu): Copper can form two stable cations: Cu⁺ (copper(I)) and Cu²⁺ (copper(II)). To distinguish between the two, Roman numerals are used in the name to indicate the oxidation state of the copper ion. In CuCl₂, copper exists as Cu²⁺, hence the name copper(II) chloride.

In summary:

* For metals that form only one stable cation, the name is straightforward.

* For transition metals that can form multiple cations, Roman numerals are used in the name to specify the oxidation state of the metal ion.