Answer
(a) Cu$_2$O
(b) Cu$_2$S
(c) CuS
Work Step by Step
(a) Cu$_2$O
Copper (I) means that this copper atom has a valence of 1+. Oxygen has a valence of 2-. Therefore, in order for the charges in the compound to be balanced, we need two copper atoms to balance out the 2- charge of the single oxygen atom.
(b) Cu$_2$S
Copper (I) means that this copper atom has a valence of 1+. Sulfur has a valence of 2-. Therefore, in order for the charges in the compound to be balanced, we need two copper atoms to balance out the 2- charge of the single sulfur atom.
(c) CuS
Copper (II) means that this copper atom has a valence of 2+. Sulfur has a valence of 2-. Copper and sulfur atoms can be combined in a 1:1 ratio. That means we only need one copper atom to balance out the 2- charge of the single sulfur atom.
