Final answer:
The sulfide ion has a charge of -2, represented as S²-. This negative charge is due to sulfur gaining two electrons, and it is important in forming compounds like ammonium sulfide, where two NH4+ ions balance one S²- ion.
Step-by-step explanation:
The charge on a sulfide ion is -2. This charge arises because when sulfur (with an atomic number Z = 16) gains two electrons, it forms a sulfide ion with a total of 18 electrons and 16 protons, thus having two more negative charges than positive ones. The symbol for a sulfide ion is S²-.
In chemical compounds, such as ammonium sulfide, the sulfide ion is balanced by positively charged ions; for example, it takes two ammonium ions, each with a 1+ charge, to balance the charge on a single sulfide ion, resulting in the compound (NH4)2S. It is essential to distinguish between the sulfide ion's actual charge and its oxidation number, such as in SO2, where sulfur has an oxidation number of +4, which does not reflect an actual 4+ charge but rather an assigned value for bookkeeping in chemical reactions.