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 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 fluroine is -1

Explanation

To determine which rule regarding oxidation numbers is not correct, we will analyze each of the provided options based on the established rules of oxidation numbers.

### Step-by-Step Solution:

1. **Option 1: Oxidation number of hydrogen is always +1.**
- This statement is **not correct**. While hydrogen typically has an oxidation number of +1 in most compounds (like HCl), it can have an oxidation number of -1 in metal hydrides (e.g., NaH, where Na is +1 and H is -1). Additionally, in its elemental form (H2), the oxidation number is 0. Therefore, this rule is not universally applicable.

2. **Option 2: The algebraic sum of all oxidation numbers in a compound is zero.**
- This statement is **correct**. In a neutral compound, the sum of the oxidation numbers of all the atoms must equal zero. For example, in NaCl, Na has an oxidation number of +1 and Cl has -1, summing to 0. Similarly, in H2SO4, the sum of oxidation numbers also equals zero.

3. **Option 3: An element in the free or uncombined state bears an oxidation number of 0.**
- This statement is **correct**. Elements in their elemental form (like O2, N2, H2, etc.) have an oxidation number of 0. This is a fundamental rule in determining oxidation states.

4. **Option 4: In its compound, the oxidation number of fluorine is -1.**
- This statement is **correct**. Fluorine is the most electronegative element and always has an oxidation number of -1 in its compounds (e.g., in HF, F is -1).

### Conclusion:
The only incorrect rule among the options provided is **Option 1**, which states that the oxidation number of hydrogen is always +1.

### Final Answer:
The rule that is not correct is: **The oxidation number of hydrogen is always +1.**

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