Which of the following statement regarding the rules to calculate the oxidation number is incorrect?
| 1. | The oxidation number of hydrogen is always +1. |
| 2. | The algebraic sum of all the oxidation numbers carried by elements in a compound is zero. |
| 3. | An element in its free or uncombined state has an oxidation number of zero. |
| 4. | Generally, in all its compounds, the oxidation number of fluorine is -1. |
