Answer
(a) $Cu_2O$
(b) $Cu_2S$
(c) $CuS$
Work Step by Step
Copper (Cu), a transition metal, has several oxidation numbers. To specify which oxidation number this particular atom of copper has, Roman numerals inside parentheses are placed after the name copper. The number of copper atoms in the compound is given by the oxidation number of the anion; conversely, the number of anions present is equal to the oxidation number of the particular atom of copper.
(a) In this compound of copper, we are looking at the copper atom that has an oxidation number of $1+$ because there is a Roman numeral I following copper. If oxygen (O) has an oxidation number of $2-$, then we need two copper atoms for every oxygen atom. Therefore, the formula for this compound is $Cu_2O$.
(b) In this compound of copper, we are looking at the copper atom that has an oxidation number of $1+$ because there is a Roman numeral I following copper. If sulfur (S) has an oxidation number of $2-$, then we need two copper atoms for every sulfur atom. Therefore, the formula for this compound is $Cu_2S$.
(c) In this compound of copper, we are looking at the copper atom that has an oxidation number of $2+$ because there is a Roman numeral II following copper. If sulfur (S) has an oxidation number of $2-$, then we need one copper atom for every sulfur atom. Therefore, the formula for this compound is $CuS$.
