Final answer:
Oxygen and sulfide ions have negative charges due to the gain of extra electrons. The sulfide ion becomes S²⁻ upon gaining two electrons, having more electrons than protons. Electronegativity affects partial negative charges in polar covalent bonds.
Step-by-step explanation:
The oxygen and sulfide ions both have negative charges because they gain electrons. An element's oxidation state is different from its formal charge. The oxidation state is a hypothetical charge based on an uneven distribution of electrons in chemical bonds whereas the formal charge is calculated from the number of valence electrons an atom owns versus the electrons it's assigned in a molecule.
When sulfur (with atomic number Z = 16) gains two extra electrons, it becomes a sulfide ion (S²⁻) with a negative two charge, resulting from having more electrons (18) than protons (16). This makes it an anion, which is an atom or molecule that has gained electrons. In the case of oxygen (O²⁻), it typically has six valence electrons, and gaining two more gives it a full octet, resulting in a negative two charge as well.
Electronegativity also plays a role in partial charges observed in covalent bonds, like in water (H₂O), where oxygen has a partial negative charge due to its stronger pull on the shared electrons.