Copper sulfide is found written as both CuS and Cu2S. Why are there two forms for writing this chemical formula? SELECT AN ANSWER This is an example of a combination reaction: CuS is the halfway stage, before the second copped atom joins the molecule. Copper is a transition metal with two possible ionic states (copper I, or 1+, and copper II, or 2+). This is a synthesis reaction and so the formula will continue changing until all the atoms have conglomerated. One of the versions is a mistake, the correct form should be Cu2S.
Copper sulfide is found written as both CuS and Cu2S. Why are there two forms for writing this chemical formula? SELECT AN ANSWER This is an example of a combination reaction: CuS is the halfway stage, before the second copped atom joins the molecule. Copper is a transition metal with two
The Correct Answer and Explanation is:
Correct Answer: Copper is a transition metal with two possible ionic states (copper I, or 1+, and copper II, or 2+).
Explanation
Copper sulfide can exist in two distinct forms — Cu₂S and CuS — because copper is a transition metal that can exhibit more than one oxidation state. Specifically, copper commonly forms ions in the +1 oxidation state (called copper(I) or cuprous) and the +2 oxidation state (called copper(II) or cupric). These different oxidation states allow copper to combine with sulfur in more than one ratio, resulting in different compounds.
In Cu₂S, each sulfur ion (S²⁻) pairs with two Cu⁺ ions. This compound is called copper(I) sulfide. Since each copper ion has a +1 charge, two of them are needed to balance the -2 charge from a single sulfide ion, giving the formula Cu₂S.
On the other hand, CuS is copper(II) sulfide, where each Cu²⁺ ion combines with a single S²⁻ ion. Here, the +2 and -2 charges cancel out in a 1:1 ratio, resulting in the formula CuS.
This phenomenon is not unusual among transition metals, many of which can form multiple stable ionic forms. The multiple oxidation states of copper arise from the variable occupation of its d-electrons, a hallmark trait of transition metals. These different oxidation states lead to the formation of compounds with distinct physical and chemical properties.
Therefore, Cu₂S and CuS are not mistakes or stages of a reaction — they are two distinct chemical compounds with unique structures and uses, both correctly described by their respective formulas. This is why it’s essential to specify the oxidation state in the name when dealing with transition metals, to avoid confusion between compounds like copper(I) sulfide and copper(II) sulfide.
