Copper sulfide is found written as both CuS and Cu2S. Why are there two forms for writing this chemical formula?

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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.

There are two forms for writing the chemical formula because copper is a transition metal with two possible ionic states (copper I, or 1+, and copper II, or 2+)

Copper is a transition metal with variable oxidation states. The oxidation states of copper is what determines the name of CuS and Cu₂S.

Now, we shall determine the oxidation states of copper in each compound.

For CuS:

CuS = 0

S = –2

Cu =?

Cu + S = 0

Cu + (–2) = 0

Cu – 2 = 0

Collect like terms

Cu = 0 + 2

Cu = +2

For Cu₂S:

Cu₂S = 0

S = –2

Cu =?

2Cu + S = 0

2Cu + (–2) = 0

2Cu – 2 = 0

Collect like terms

2Cu = 0 + 2

Divide both side by 2

Cu = 2/2

Cu = +1

From the calculations made above, we can see that copper has oxidation state of +2 in CuS and +1 in Cu₂S

Thus, we can conclude that Copper is a transition metal with two possible ionic states and this accounts for the different names of CuS and Cu₂S

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