The oxidation number for an element is based on the number of   that element has. For example, the oxidation number 2+ means that the element has  electrons that it will donate and, therefore, will become a  charged (+2) ion.

The oxidation number for an element is based on the number of electrons that element has. For example, the oxidation number 2+ means that the element has 2 electrons that it will donate and, therefore, will become a positively charged (+2) ion.

Here are the steps to determine the oxidation number:

1. The oxidation number of an atom in its elemental state is always zero. For example, the oxidation number of oxygen in O2 is 0.

2. The oxidation number of a monatomic ion is equal to its charge. For example, the oxidation number of Na+ is +1.

3. The sum of the oxidation numbers in a neutral compound is zero. For example, in H2O, the oxidation number of oxygen is -2 and that of hydrogen is +1. The sum of the oxidation numbers is 0.

4. The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion. For example, in SO4^2-, the oxidation number of sulfur is +6 and that of oxygen is -2. The sum of the oxidation numbers is -2, which is the charge of the ion.

5. Some elements have specific rules. For example, the oxidation number of oxygen is usually -2, but it is -1 in peroxides (like H2O2) and +2 in compounds with fluorine (like OF2). The oxidation number of hydrogen is +1 in most compounds, but it is -1 in compounds with metals (like NaH).

By following these rules, you can determine the oxidation number of an element in a compound.