Question

What is the name of `Cu_2S`?

Answer 1

Copper sulfide or dicopper sulfide

Explanation:

Copper has two common oxidation numbers + 1 and +2.
Sulfur has a common oxidation number of -2

A more precise name would be Copper I sulfide.

A common name would be Cuprous sulfide. This is an old name for compounds with a +1 charge on Copper. Cupric Sulfide would have the formula `CuS` with a +2 charge on Copper.

Answer 2

The name is copper(I) sulfide.

Explanation:

Copper is in the `"d"`-block of elements, so it is a transition metal.

Here are the steps for deriving the name of a compound of a transition metal.

Step 1. Write the name of the transition metal

This gives: "copper".

Step 2. Add the name of the nonmetal with the ending "-ide"

This gives: "copper sulfide".

Step 3. Use the charge on the non-metal to find the charge on the transition metal

The charge on `"S"` is -2.

The total charge on the `"Cu"` must be +2. Hence, each `"Cu"` has a charge of +1.

Step 4. Write the charge on the non-metal as Roman numerals in parentheses immediately after (no spaces) the name of the metal.

This gives the name: copper(I) sulfide.