In chemistry, the oxidation number refers to the charge that an atom would have if all of its bonds were 100% ionic. It is a way of keeping track of the distribution of electrons in a molecule or compound.

Looking at the options given, we can evaluate each statement:

1. The oxidation number for hydrogen is always zero.

This statement is incorrect. Hydrogen can have other oxidation numbers besides zero. For example, in molecular hydrogen gas (H2), both hydrogen atoms have an oxidation number of zero because they have no charge. However, in compounds like water (H2O), the oxidation number of hydrogen is +1 because oxygen, being more electronegative, attracts the shared electrons towards itself, giving hydrogen a positive oxidation number.

2. The oxidation number for hydrogen is always +1.

This statement is incorrect. As mentioned above, the oxidation number of hydrogen depends on the compound it is present in. In some compounds, hydrogen can have an oxidation number of +1, while in others, it can have different numbers.

3. The oxidation number for hydrogen is always -1.

This statement is incorrect. Similar to the above statements, the oxidation number of hydrogen can be different in different compounds. It is not always -1.

4. Hydro