Which of the following is the correct chemical formula for cobalt(II) sulfide?

• A. Co2S
• B. Co2S3
• C. CoS2
• D. CoS

Answer: (D)

Verified step by step guidance

1. Identify the oxidation state of cobalt in cobalt(II) sulfide. The Roman numeral II indicates that cobalt has a +2 charge, so Co is Co^{2+}.
2. Determine the charge of the sulfide ion. Sulfide is the anion of sulfur with a charge of 2-, so S is S^{2-}.
3. To write the correct formula, balance the total positive and negative charges so the compound is electrically neutral. Since Co^{2+} and S^{2-} have charges of equal magnitude but opposite sign, they combine in a 1:1 ratio.
4. Write the formula by placing the symbols for cobalt and sulfide with their respective subscripts. Because the charges balance one-to-one, the formula is CoS.
5. Verify that the formula CoS is neutral by confirming that the total positive charge (+2 from Co) equals the total negative charge (-2 from S).