What is the formula for copper I sulfide?
The correct answer and explanation is:
Correct Answer: The formula for copper(I) sulfide is Cu₂S.
Explanation:
Copper(I) sulfide is a chemical compound formed from copper and sulfur. To determine its formula, one must consider the charges of the ions that make up the compound. Copper(I) refers to copper in the +1 oxidation state. This means each copper ion has a charge of +1, and it is represented as Cu⁺. Sulfide is the name given to the anion of sulfur, which always carries a -2 charge, written as S²⁻.
In order to form a neutral compound, the total positive and negative charges must balance each other. Since each Cu⁺ ion contributes a +1 charge, and the S²⁻ ion contributes a -2 charge, two copper(I) ions are required to balance one sulfide ion:
(+1) + (+1) = +2 from copper
-2 from sulfur
The total charge is +2 and -2, which cancel each other out, resulting in a neutral compound. Therefore, the formula is written as Cu₂S, indicating two copper(I) ions for every one sulfide ion.
It is important not to confuse copper(I) with copper(II). Copper can exist in multiple oxidation states, most commonly +1 and +2. Copper(II) sulfide would have the formula CuS because Cu²⁺ balances with S²⁻ in a 1:1 ratio. In contrast, copper(I) requires a 2:1 ratio to achieve neutrality.
The naming of copper(I) sulfide follows the Stock system, which uses Roman numerals to indicate the oxidation state of a metal that can form more than one ion. Since copper can form both Cu⁺ and Cu²⁺, the numeral I is used to specify that the copper in Cu₂S is in the +1 oxidation state.