What is the formula for Iron(III) sulfide? And why?
The correct answer and explanation is:
The formula for Iron(III) sulfide is Fe₂S₃.
Iron(III) indicates that iron is in the +3 oxidation state, written as Fe³⁺. Sulfide comes from sulfur, which typically forms an ion with a -2 charge, written as S²⁻.
To write the formula for a compound, the total positive charge must balance the total negative charge, resulting in an electrically neutral compound. Iron(III) sulfide combines Fe³⁺ and S²⁻ ions in a ratio that balances these charges.
Since iron has a charge of +3 and sulfur has a charge of -2, the goal is to find the smallest whole-number ratio of Fe³⁺ and S²⁻ ions that make the net charge zero. To balance +3 and -2 charges, find the least common multiple of 3 and 2, which is 6.
- Iron(III) contributes +3 charge per ion. To reach +6 total positive charge, 2 Fe³⁺ ions are needed: 2 × +3 = +6.
- Sulfur contributes -2 charge per ion. To reach -6 total negative charge, 3 S²⁻ ions are needed: 3 × -2 = -6.
Thus, combining 2 iron(III) ions with 3 sulfide ions balances the charges: (2 × +3) + (3 × -2) = 0.
Therefore, the chemical formula is Fe₂S₃, showing 2 iron atoms and 3 sulfur atoms.
This reflects the fundamental principle in ionic compounds where the total positive charge from cations equals the total negative charge from anions, making the compound electrically neutral.
In summary, the formula Fe₂S₃ is derived by balancing the charges of Fe³⁺ and S²⁻ ions in the smallest whole number ratio, ensuring electrical neutrality in Iron(III) sulfide.