In inorganic chemistry, what is the oxidation number of sulfur in hydrogen sulfide (H2S)?

Introduction / Context:
Oxidation numbers are a formal way of tracking electron gain and loss in chemical compounds. They help in balancing redox reactions, understanding oxidation states of elements and predicting reaction behaviour. This problem focuses on determining the oxidation number of sulfur in hydrogen sulfide, a simple but important inorganic compound with formula H2S.

Given Data / Assumptions:
- The compound is hydrogen sulfide with formula H2S.
- Hydrogen is bonded to sulfur, and there is no overall charge on the molecule.
- We use standard oxidation number rules, including the usual oxidation state of hydrogen in covalent compounds.

Concept / Approach:
In most covalent compounds where hydrogen is bonded to non metals, hydrogen has an oxidation number of +1. The sum of oxidation numbers of all atoms in a neutral molecule must equal zero. Therefore, if we assign +1 to each hydrogen and let x represent the oxidation number of sulfur, we can solve a simple algebraic equation to find x. This method is widely applicable to many binary compounds and is an essential basic skill in inorganic chemistry.

Step-by-Step Solution:
Step 1: Write the chemical formula H2S and note that the molecule is neutral (overall charge is 0). Step 2: Use the rule that hydrogen usually has an oxidation number of +1 in compounds with non metals. Step 3: Assign +1 to each hydrogen atom. With two hydrogen atoms, the total oxidation contribution from hydrogen is 2 * (+1) = +2. Step 4: Let the oxidation number of sulfur be x. The sum of oxidation numbers must equal zero for a neutral compound, so we write the equation: 2 + x = 0. Step 5: Solve the equation: x = -2. Step 6: Conclude that the oxidation number of sulfur in hydrogen sulfide is -2.

Verification / Alternative check:
You can cross check this by considering related compounds of sulfur. In sulfide ions (S^2-), sulfur has an oxidation number of -2. Hydrogen sulfide can be thought of as the molecular form of hydrogen ions (H+) combining with sulfide ions (S^2-), giving an overall neutral molecule. This supports the result that sulfur is in the -2 oxidation state in H2S. Furthermore, in many metal sulfides like FeS or ZnS, sulfur is also in the -2 state, reinforcing the consistency of this assignment.

Why Other Options Are Wrong:
2: A positive value of +2 for sulfur would make the sum of oxidation numbers equal +4, which cannot be correct for a neutral molecule H2S.
1: An oxidation number of +1 for sulfur would give a total of +3 (+2 from hydrogen and +1 from sulfur), again inconsistent with overall neutrality.
-1: If sulfur were -1, the total sum would be +1, not zero, so this is also not possible.

Common Pitfalls:
Students sometimes forget to multiply the oxidation number of hydrogen by the number of hydrogen atoms and therefore get incorrect totals. Another mistake is to confuse peroxides or metal hydrides, where hydrogen oxidation states can differ, with typical covalent hydrides like H2S. To avoid such errors, always apply the rule that hydrogen is usually +1 with non metals, multiply by its subscript and ensure the total oxidation number equals the overall charge of the species.

Final Answer:
The oxidation number of sulfur in H2S is -2.