Final answer:
Copper (Cu) requires a Roman numeral in its chemical name because it is a transition metal with multiple possible ionic charges, unlike sulfur (S), calcium (Ca), and carbon (C) which have only one common ionic charge each.
Step-by-step explanation:
The element that would need a Roman numeral when naming a chemical with it is Cu, copper. This is because copper is a transition metal that can have more than one possible ionic charge. In ionic compounds, such as copper(I) ion or the cuprous ion and copper(II) ion or the cupric ion, the charge needs to be specified in the name using Roman numerals. For example, CuCl is copper(I) chloride, and CuCl2 is copper(II) chloride. The other elements, sulfur (S), calcium (Ca), and carbon (C), generally do not require Roman numerals in their names because they have only one common charge in their ionic forms, the sulfide ion (S2-), the calcium ion (Ca2+), and carbon does not form ions that require the use of a Roman numeral in naming.