Which of the following rules for oxidation number is not correct?

Options

A. The algebraic sum of all the oxidation numbers in a compound is zero .
B. An element in the free or the uncombined state bears oxidation numbers zero.
C. The oxidation number of hydrogen is always +1.
D. In all its compounds, the oxidation number of fluorine is -1.

Answer

To determine which rule for oxidation numbers is not correct, we will analyze each option provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding the Rules of Oxidation Numbers**:
- Oxidation numbers are assigned to elements in a compound to help determine how electrons are distributed among the atoms.

2. **Evaluating the First Option**:
- **Statement**: The algebraic sum of all the oxidation numbers in a compound is zero.
- **Analysis**: This statement is true for neutral compounds. For example, in water (H₂O), the oxidation states of H (+1) and O (-2) sum to zero.
- **Conclusion**: This option is correct.

3. **Evaluating the Second Option**:
- **Statement**: An element in a free or uncombined state bears 0 oxidation state.
- **Analysis**: This is also true. For instance, in Cl₂ or N₂, the oxidation state of each atom is 0 because they are not combined with other elements.
- **Conclusion**: This option is correct.

4. **Evaluating the Third Option**:
- **Statement**: The oxidation number of hydrogen is always +1.
- **Analysis**: This statement is false. While hydrogen typically has an oxidation state of +1 in most compounds (like H₂O), it can also have an oxidation state of -1 in metal hydrides (e.g., NaH, where Na is +1 and H is -1).
- **Conclusion**: This option is incorrect.

5. **Evaluating the Fourth Option**:
- **Statement**: In all its compounds, the oxidation number of fluorine is -1.
- **Analysis**: This statement is true. Fluorine is the most electronegative element and always has an oxidation state of -1 in its compounds.
- **Conclusion**: This option is correct.

6. **Final Conclusion**:
- The only incorrect statement regarding oxidation numbers is the third option: "The oxidation number of hydrogen is always +1."

### Answer:
The oxidation number of hydrogen is not always +1 (Option 3 is incorrect).

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