The oxidation number of an element in a compound is evaluated on the basis of certain rules. Which of the following rules is not correct in this respect ?

Options

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

Explanation

To determine which rule regarding oxidation numbers is incorrect, we will evaluate each option provided in the question based on the established rules of oxidation states.

### Step-by-Step Solution:

1. **Evaluate the first option**: "The oxidation number of hydrogen is always +1."
- This statement is **not correct**. Hydrogen can have different oxidation states:
- In metal hydrides (e.g., NaH), hydrogen has an oxidation number of -1.
- In molecular hydrogen (H₂), it has an oxidation number of 0.
- In compounds like HCl, it has an oxidation number of +1.
- Therefore, this rule is incorrect.

2. **Evaluate the second option**: "The algebraic sum of all the oxidation numbers in a compound is 0."
- This statement is **correct**. In a neutral compound, the sum of the oxidation states of all atoms must equal zero. For example:
- In NaCl: Na is +1 and Cl is -1, so the sum is 0.
- In NH₃: N is -3 and each H is +1 (total +3), so the sum is also 0.

3. **Evaluate the third option**: "The element in free state has an oxidation number of 0."
- This statement is **correct**. Elements in their free or uncombined state (e.g., O₂, N₂, H₂, Cl₂) have an oxidation number of 0.

4. **Evaluate the fourth option**: "The oxidation number of fluorine is always -1."
- This statement is **correct**. Fluorine is the most electronegative element and always has an oxidation number of -1 in its compounds.

### Conclusion:
After evaluating all options, the only incorrect statement is the first one: "The oxidation number of hydrogen is always +1."

Thus, the answer is **A**.