Why is the chemical formula of iron (III) sulfide Fe2S3 and not Fe2SO3?
The correct answer and explanation is:
The chemical formula for iron (III) sulfide is Fe₂S₃ and not Fe₂SO₃ because it is a compound formed from iron and sulfur atoms, not involving oxygen. Iron (III) sulfide consists of iron ions with a +3 charge and sulfide ions with a -2 charge. The formula reflects the ratio needed to balance these charges and create a neutral compound.
Iron (III) means iron is in the +3 oxidation state, so each iron ion is Fe³⁺. Sulfide is the sulfur ion with a charge of S²⁻. To balance the total positive and negative charges, the smallest whole-number ratio of ions is two Fe³⁺ ions (total +6 charge) to three S²⁻ ions (total -6 charge). This results in the formula Fe₂S₃.
On the other hand, Fe₂SO₃ contains sulfur, iron, and oxygen, which is not just a sulfide but includes the polyatomic ion sulfite (SO₃²⁻). Sulfite is a separate ion made of one sulfur atom and three oxygen atoms with a -2 charge. This changes the chemical nature of the compound entirely. Fe₂SO₃ would imply iron combined with the sulfite ion rather than just sulfur, and the compound would be iron (III) sulfite instead of iron (III) sulfide.
In summary, iron (III) sulfide is Fe₂S₃ because it is composed only of iron and sulfide ions, balancing the charges +3 and -2 respectively. The presence of oxygen in Fe₂SO₃ indicates a different compound containing the sulfite ion, not a simple sulfide. Therefore, the correct formula for iron (III) sulfide excludes oxygen and is Fe₂S₃.